U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5015

Prepared in cooperation with the Colorado Water Institute and 

Colorado Water Conservation Board

Evaluation of Groundwater Levels in the South Platte River 

Alluvial Aquifer, Colorado, 1953–2012, and Design of Initial 

Well Networks for Monitoring Groundwater Levels
Cover. The South Platte River near Kersey, Colorado, and surrounding area. Photographs by Nancy Bauch and Jim Collins (retired), 

U.S. Geological Survey.
Evaluation of Groundwater Levels in 

the South Platte River Alluvial Aquifer, 

Colorado, 1953–2012, and Design of 

Initial Well Networks for Monitoring 

Groundwater Levels

By Tristan P. Wellman

Prepared in cooperation with the Colorado Water Institute and Colorado Water 

Conservation Board

Scientific Investigations Report 2015–5015

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Wellman, T.P., 2015, Evaluation of groundwater levels in the South Platte River alluvial aquifer, Colorado, 1953–2012, 

and design of initial well networks for monitoring groundwater levels: U.S. Geological Survey Scientific Investigations 

Report 2015–5015, 67 p., http://dx.doi.org/10.3133/sir20155015.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope .......................................................................................................................................4

Previous Work ................................................................................................................................................4

Description of Study Area ............................................................................................................................4

Geographic Setting...............................................................................................................................4

Climate....................................................................................................................................................5

Geology...................................................................................................................................................5

Hydrology and Water Management ..................................................................................................8

Methods of Investigation..............................................................................................................................8

Data Compilation...................................................................................................................................8

Groundwater Levels ...........................................................................................................................10

Trend Analysis ............................................................................................................................10

Frequency of High Groundwater Levels ................................................................................11

Spatial Grouping of High Groundwater Levels .....................................................................12

Correlation of Attributes to High Groundwater Levels ........................................................12

Optimization of Monitoring Well Network Design.........................................................................13

Analysis of Groundwater Levels................................................................................................................14

Overview Statistics of Groundwater Levels in Wells....................................................................14

Frequency of High Groundwater Levels across Subwatersheds...............................................19

Spatial Grouping of Subwatersheds by High Groundwater Levels............................................27

Change in Groundwater Levels ........................................................................................................28

Wells .........................................................................................................................................28

Subwatersheds ..........................................................................................................................31

Correlation of System Attributes to High Groundwater Levels...................................................35

Precipitation, River Discharge, and High Groundwater Levels .........................................39

Geographic Characteristics.....................................................................................................39

Administrative Structures ........................................................................................................40

Possible Solutions to Address Rising and Declining Water Levels............................................44

Monitoring Well Network ...........................................................................................................................44

Benefits of a Monitoring Network ...................................................................................................44

Monitoring Network Design Components ......................................................................................45

Unstressed Subnetwork ...........................................................................................................45

Targeted Subnetwork................................................................................................................45

Baseline Monitoring..................................................................................................................45

Surveillance Monitoring ...........................................................................................................45

Trend Monitoring........................................................................................................................45

South Platte River Alluvial Aquifer Network Design.....................................................................45

Frequency of Groundwater-Level Measurements ..............................................................46

Monitoring Well Candidates ....................................................................................................46

Monitoring-Well Network Optimization .................................................................................47

Summary........................................................................................................................................................56
iv

Acknowledgments.......................................................................................................................................58

References Cited..........................................................................................................................................58

Appendix 1.....................................................................................................................................................61

Appendix 2.....................................................................................................................................................62

Figures

1. Map showing location of the South Platte River alluvial aquifer in 

Water Districts 1, 2, and 64..........................................................................................................3

2. Graphs showing time series of average annual, average decadal, and median

decadal precipitation and discharge of the South Platte River, averaged monthly

from five gaging stations for U.S. Geological Survey water years 1953 to 2012 ................6

3. Map showing estimated thickness of the South Platte River alluvial aquifer....................7

4. Map showing locations of active decreed administrative structures near the South

Platte River alluvial aquifer in Water Districts 1, 2, and 64....................................................9

5. Map showing complete set of 1,669 wells used to evaluate groundwater levels,

identified by primary use type ..................................................................................................15

6. Graphs showing decadal summary of shallow groundwater levels and 

well counts...................................................................................................................................16

7. Graphs showing average and standard deviation of depths to water by decade

using all observations, observations of depth to water less than or equal to 20 feet,

and maximum depths to water per decade using all observations ...................................17

8. Map showing well locations with at least one recorded occurrence of shallow

groundwater between 1953 and 2012......................................................................................18

9. Map showing subwatershed areas used for analysis that intersect the South Platte

River alluvial aquifer...................................................................................................................20

10. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 1953 to 1962............................................................21

11. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 1963 to 1972............................................................22

12. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 1973 to 1982............................................................23

13. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 1983 to 1992............................................................24

14. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 1993 to 2002............................................................25

15. Map showing frequency of high groundwater levels within subwatersheds of the

South Platte River alluvial aquifer from 2003 to 2012............................................................26
v

16. Map showing groupings of subwatersheds using Local Moran’s I autocorrelation

statistics to show regional patterns of high groundwater levels from 1963 to 1972 .......29

17. Map showing groupings of subwatersheds using Local Moran’s I autocorrelation

statistics to show regional patterns of high groundwater levels from 2003 to 2012 .......30

18. Map showing groundwater-level change (trend) at wells with acceptable records

from 1953 to 1962.........................................................................................................................32

19. Map showing groundwater-level change (trend) at wells with acceptable records

from 1963 to 1972.........................................................................................................................33

20. Map showing groundwater-level change (trend) at individual wells and effective

change per subwatershed from 2003 to 2012 ........................................................................34

21. Map showing groundwater-level change (trend) at individual wells and effective

change per subwatershed in areas for the complete period of record from 1953 

to 2012...........................................................................................................................................36

22. Map showing net difference in average groundwater levels per subwatershed

between decades 1963–1972 and 2003–2012.........................................................................37

23. Map showing net difference in variability (standard deviation) of groundwater

levels per subwatershed between decades 1963–1972 and 2003–2012............................38

24. Graphs showing quarterly summary of high groundwater levels and river 

discharge per decade................................................................................................................40

25. Graph showing correlation coefficients between geographic attributes at well

locations and frequency of observed high groundwater levels .........................................41

26. Graph showing correlation coefficients between attributes of administrative

structures and frequency of observed high groundwater levels in wells at

optimized search distances ......................................................................................................42

27. Graph showing optimized search distances yielding maximum correlations

between attributes of administrative structures and frequency of observed high

groundwater levels in wells......................................................................................................43

28. Graph showing required frequency of groundwater-level measurements based on

system conditions.......................................................................................................................47

29. Map showing candidate monitoring wells in the South Platte River alluvial aquifer

considered for analysis .............................................................................................................48

30. Map showing kriging interpolation errors using depth to water in the 1,669 wells

considered ...................................................................................................................................51

31. Graph showing optimization results for the “unstressed” monitoring subnetwork........52

32. Map showing proposed “unstressed” monitoring subnetwork composed of 

96 monitoring wells.....................................................................................................................53

33. Map showing proposed target monitoring subnetwork to evaluate hydrologic

stresses ........................................................................................................................................54

34. Map showing proposed target monitoring subnetwork to evaluate influences of

administrative structures ..........................................................................................................55
vi

Tables

1. Description of Local Moran’s I statistics used to examine regional patterns of high

groundwater levels.....................................................................................................................28

2. Summary of well statistics for groundwater-level trend analysis......................................31

3. All monitoring wells, trend monitoring wells, and monitoring subnetworks

(unstressed, administrative structures, and hydrologic), by agency. “Other” sites

indicate miscellaneous sources, and “tentative” sites have no known data history

and require additional evaluation ............................................................................................49

Appendix Tables

2–1. Candidate wells for the “unstressed” monitoring subnetwork. Proposed candidates

to be evaluated during the implementation phase...............................................................62

2–2. Candidate wells for the hydrologic target subnetwork to examine trends and high

groundwater levels. Proposed candidates to be evaluated during the

implementation phase................................................................................................................65

2–3. Candidate wells for the administrative structures target subnetwork to examine

influences of structures on wells. Proposed candidates to be evaluated during the

implementation phase................................................................................................................66
vii

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

million gallons (Mgal) 3,785 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft]

 0.2070 liter per second per meter 

[(L/s)/m]

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2

/d) 

Leakance

foot per day per foot [(ft/d)/ft] 1 meter per day per meter

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter 

[(mm/yr)/m]
viii

Supplemental Information

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Water Year (WY) is defined as beginning October 1 and continuing through September 30 of the 

following year.

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
Evaluation of Groundwater Levels in the South Platte River 

Alluvial Aquifer, Colorado, 1953–2012, and Design of Initial 

Well Networks for Monitoring Groundwater Levels

Tristan P. Wellman

Abstract

The South Platte River and underlying alluvial aquifer 

form an important hydrologic resource in northeastern Colorado 

that provides water to population centers along the Front Range 

and to agricultural communities across the rural plains. Water is 

regulated based on seniority of water rights and delivered using 

a network of administration structures that includes ditches, reservoirs, wells, impacted river sections, and engineered recharge 

areas. A recent addendum to Colorado water law enacted during 

2002–2003 curtailed pumping from thousands of wells that 

lacked authorized augmentation plans. The restrictions in pumping were hypothesized to increase water storage in the aquifer, 

causing groundwater to rise near the land surface at some locations. The U.S. Geological Survey (USGS), in cooperation with 

the Colorado Water Conservation Board and the Colorado Water 

Institute, completed an assessment of 60 years (yr) of historical 

groundwater-level records collected from 1953 to 2012 from 

1,669 wells. Relations of “high” groundwater levels, defined as 

depth to water from 0 to 10 feet (ft) below land surface, were 

compared to precipitation, river discharge, and 36 geographic 

and administrative attributes to identify natural and human 

controls in areas with shallow groundwater.

Averaged per decade and over the entire aquifer, depths 

to groundwater varied between 24 and 32 ft over the 60-yr 

record. The shallowest average depth to water was identified 

during 1983–1992, which also recorded the highest levels 

of decadal precipitation. Average depth to water was greatest (32 ft) during 1953–1962 and intermediate (30 ft) in the 

recent decade (2003–2012) following curtailment of pumping. 

Between the decades 1993–2002 and 2003–2012, groundwater 

levels declined about 2 ft across the aquifer. In comparison, in 

areas where groundwater levels were within 20 ft of the land 

surface, observed groundwater levels rose about 0.6 ft, on 

average, during the same period, which demonstrated preferential rise in areas with shallow groundwater.

Approximately 29 percent of water-level observations 

were identified as high groundwater in the South Platte River 

alluvial aquifer over the 60-yr record. High groundwater 

levels were found in 17 to 33 percent of wells examined by 

decade, with the largest percentages occurring over three 

decades from 1963 to 1992. The recent decade (2003–2012) 

exhibited an intermediate percentage (25 percent) of wells 

with high groundwater levels but also had the highest percentage (30 percent) of high groundwater observations, although 

results by observations were similar (26–29 percent) over 

three decades prior, from 1963 to 1992. Major sections of 

the aquifer from north of Sterling to Julesburg and areas near 

Greeley, La Salle, and Gilcrest were identified with the highest 

frequencies of high groundwater levels.

Changes in groundwater levels were evaluated using 

Kendal line and least trimmed squares regression methods 

using a significance level of 0.01 and statistical power of 

0.8. During 2003–2012, following curtailment of pumping, 

88 percent of wells and 81 percent of subwatershed areas 

with significant trends in groundwater levels exhibited rising water levels. Over the complete 60-yr record, however, 

66 percent of wells and 57 percent of subwatersheds with 

significant groundwater-level trends still showed declining 

water levels; rates of groundwater-level change were typically 

less than 0.125 ft/yr in areas near the South Platte River, with 

greater declines along the southern tributaries. In agreement, 

58 percent of subwatersheds evaluated between 1963–1972 

and 2003–2012 showed net declines in average decadal 

groundwater levels. More areas had groundwater decline 

in upgradient sections to the west and rise in downgradient 

sections to the east, implying a redistribution of water has 

occurred in some areas of the aquifer.

Precipitation was identified as having the strongest 

statistically significant correlations to river discharge over 

annual and decadal periods (Pearson correlation coefficients 

of 0.5 and 0.8, respectively, and statistical significance defined 

by p-values less than 0.05). Correlation coefficients between 

river discharge and frequency of high groundwater levels were 

statistically significant at 0.4 annually and 0.6 over decadal 

periods, indicating that periods of high river flow were often 

coincident with high groundwater conditions. Over seasonal periods in five of the six decades examined, peak high 

groundwater levels occurred after spring runoff from July to 

September when administrative structures were most active. 

Between 1993–2002 and 2003–2012, groundwater levels rose 
2 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

while river discharge decreased, in part from greater reliance 

on surface water and curtailed pumping from wells without 

augmentation plans.

Geographic attributes of elevation and proximity to 

streams and rivers showed moderate correlations to high 

groundwater levels in wells used for observing groundwater 

levels (correlation coefficients of 0.3 to 0.4). Local depressions and regional lows within the aquifer were identified as 

areas of potential shallow groundwater. Wells close to the 

river regularly indicated high groundwater levels, while those 

within depleted tributaries tended to have low frequencies of 

high groundwater levels. Some attributes of administrative 

structures were spatially correlated to high groundwater levels 

at moderate to high magnitudes (correlation coefficients of 

0.3 to 0.7). The number of affected river reaches or recharge 

areas that surround a well where groundwater levels were 

observed and its distance from the nearest well field showed 

the strongest controls on high groundwater levels. Influences 

of administrative structures on groundwater levels were in 

some cases local over a mile or less but could extend to several miles, often manifesting as diffuse effects from multiple 

surrounding structures. 

A network of candidate monitoring wells was proposed 

to initiate a regional monitoring program. Consistent monitoring and analysis of groundwater levels will be needed for 

informed decisions to optimize beneficial use of water and to 

limit high groundwater levels in susceptible areas. Finalization of the network will require future field reconnaissance 

to assess local site conditions and discussions with State 

authorities.

Introduction

The South Platte River Basin contains essential water 

resources for municipalities, agriculture, industry, and energy 

exploration in northeastern Colorado. The South Platte River 

alluvial aquifer is an important hydrologic resource in Water 

District 1, Water District 2, and Water District 64, which are 

areas defined by the Colorado Division of Water Resources. 

The districts compose a portion of the basin extending from 

the Front Range of the Rocky Mountains between Denver and 

Greeley to the eastern plains near Julesburg (fig. 1). The South 

Platte River system and underlying alluvial aquifer, together 

with transbasin water diversions, serve water to 70 percent of 

the State’s population (U.S. Census Bureau, 2005). The region 

supports about 830,000 acres of the State’s most productive 

irrigated agricultural land with an economic impact of more 

than 2 billion dollars per year (Thorvaldson and Pritchett, 

2005). Fertile alluvial soils produce irrigated corn, alfalfa, 

sugar beets, beans, and market vegetables. The prevalence of 

feed and proximity to large markets encourages the development of large feedlot and dairy operations.

The South Platte River Basin will be facing recurring 

challenges to satisfy water needs of a population that is 

expected to increase by about 35 percent between 2000 and 

2030 (U.S. Census Bureau, 2005). The South Platte River is 

over-appropriated, in that the amount of water granted to existing water rights exceeds the available supply, and is governed 

by senior administrative calls, whereby senior water users can 

request authorities to shut off fulfillment to junior water rights 

and thus enable the senior users’ access to their entitled water 

(Waskom, 2013). River water can be freely available in the 

spring runoff months and following large storm events, when 

senior water users typically do not make administrative calls. 

For most of the irrigation season, however, the river serves 

water rights with priority dates before 1900, and in the winter 

months the river is dedicated to filling reservoirs with priority 

dates before 1915. 

Through the 1950s, agricultural producers were not 

apprised of the potential effects of well pumping on surface 

flows in the South Platte River. Since the 1950s, however, 

with increases in development and associated demand, water 

resources have become increasingly important. In 1966, the 

Colorado General Assembly funded a study of the interaction between the South Platte River alluvial aquifer and the 

surface-water flows in the South Platte River. The general 

assembly considered the findings of the study in passing the 

Water Right Determination and Administration Act of 1969 

(Colo. Sess. Laws ch. 373 § 1 [1969], codified as amended, 

Colo. Rev. Stat. § 37-92-101 [1999]). Wells were integrated 

into a prior appropriation system through the Act of 1969, 

which allowed groundwater to be used if depletions to 

the South Platte River could be mitigated through a court 

approved augmentation plan. Augmentation plans authorized 

by the 1969 act are still operating on a large scale in the South 

Platte River Basin. 

By 2002–2003, water use in the South Platte River 

alluvial aquifer had grown substantially in both magnitude 

and complexity with approximately 8,200 high-capacity wells. 

Concurrent with a period of drought and extensive water use 

in 2002–2003, Colorado water authorities began curtailment 

of pumping for thousands of wells that did not have courtapproved augmentation plans (Waskom, 2013). A few years 

later, farmers in Weld County, Colo., requested that the curtailment on wells be lifted to meet the needs of the agricultural 

community. As justification, they referred to areas in the South 

Platte River alluvial aquifer with accounts of “high groundwater” and locations where flooding had occurred in the 

basements of residences, particularly near Gilcrest, La Salle, 

and Sterling, Colo. In response to this request, Governor John 

Hickenlooper produced a legal memorandum from the Office 

of the Colorado Attorney General stating that the Governor 

did not have the authority to permit the resumption of pumping from the aquifer below farmland in the South Platte River 

Basin (Fryar, 2012). 

Although many scientific and technological advances 

have occurred since the 1960s, no comprehensive study has 

been conducted thereafter regarding the effect of alluvial 

groundwater usage on the South Platte River. Colorado House 

Bill 12-1278 (General Assembly of the State of Colorado, 
Introduction 3

Figure 1. Location of the South Platte River alluvial aquifer in Water Districts 1, 2, and 64.

Box Elder Creek

Kiowa Creek

Julesburg Reservoir

Riverside Reservoir

Jackson Reservoir

Empire Reservoir

South Platte River

South Platte River

Pawnee Creek

North

Sterling

Reservoir

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Fort

Morgan

Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

6759500

6754000

6721000

6720500

6764000

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

COLORADO

WYOMING

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

South Platte River

South Platte tributary

Ditch

Gaging station and number

105° 104° 103°

41°

40°

6759500

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

NEBRASKA

COLORADO

Map area

Water Division 1

boundary
4 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

2012) is a legislative response action to examine areas of 

high groundwater and other related issues relevant to water 

allocations in the South Platte River Basin. In response to the 

need for technical assistance to support Colorado House Bill 

12-1278, the U.S. Geological Survey (USGS), in cooperation 

with the Colorado Water Institute and the Colorado Water 

Conservation Board, conducted a study of groundwater levels 

in the South Platte River alluvial aquifer in Water Districts 1, 

2, and 64.

Purpose and Scope

The primary purpose of this report, which addresses 

a subsection of the goals outlined in Colorado House Bill 

12-1278, is to evaluate a record of groundwater-level data 

representative of the South Platte River alluvial aquifer 

in Water Districts 1, 2, and 64 over a 60-year (yr) record 

inclusive of calendar years 1953 through 2012 (fig. 1). The 

record includes data collected by multiple Federal, State, and 

local organizations. Analysis was focused on regions within 

the basin adversely affected by high groundwater levels, 

which will provide information that can form the basis for 

implementing measures to mitigate adverse effects of high 

groundwater levels. The analyses looked at historical changes 

in groundwater levels. Results from these activities were 

synthesized to (a) identify areas affected by high groundwater 

levels; (b) identify changing groundwater levels; (c) develop 

hypotheses regarding the causes of high groundwater levels; 

and (d) develop a groundwater monitoring plan that focuses on 

characterizing groundwater levels in the aquifer but can also 

be used to evaluate causes of high groundwater levels.

Previous Work

A comprehensive report on groundwater in the Denver 

Basin and extended areas including the South Platte River 

alluvial aquifer is provided in Paschke (2011). An extensive 

collection of studies has examined the alluvial aquifer of 

the South Platte River and its tributaries. Stratigraphy of the 

alluvial deposits was originally described by Hunt (1954) and 

Scott (1960) and later by Scott (1963). Several workers developed maps of the extent, thickness, and depth to water of the 

South Platte alluvial aquifer beginning in the 1950s (Bjorklund 

and Brown, 1957; Smith and others, 1964; Duke and Longenbaugh, 1966; Nelson, Haley, Patterson, & Quirk, Inc., 1967; 

Hurr and others, 1972 a, b, c; Hurr and others, 1975; Konikow, 

1975; Nadler and Schumm, 1981; Robson and others, 2000 a, 

b). The South Platte Decision Support System (SPDSS, http://

cdss.state.co.us/basins/Pages/SouthPlatte.aspx) compiled 

selected maps of these features into geographic information 

system (GIS) datasets (Colorado Water Conservation Board, 

2006b). Robson (1989) describes the interconnection between 

bedrock and alluvial aquifers in the study area.

Senate Bill 96-74 (SB96-74), implemented in 1996, provided funds for a study of the Denver Basin and South Platte 

aquifer systems in anticipation of developing the SPDSS 

(Graham and Van Slyke, 2004). Development of the SPDSS 

is under the direction of the Colorado Water Conservation 

Board (CWCB) and the Colorado Division of Water Resources 

(DWR) and addresses many areas of water management in 

the South Platte River Basin, including surface-water diversions, consumptive water use, irrigated lands, land use, and 

groundwater (Colorado Water Conservation Board, 2004a). 

The SPDSS groundwater studies focus primarily on the alluvial aquifer of the South Platte River and its tributaries; this 

study used data compilations from the SPDSS as the basis for 

aquifer characterization (Colorado Water Conservation Board, 

2004b, 2006 a, b, c).

Description of Study Area

The South Platte River originates in the Rocky Mountain 

Front Range of Colorado and flows north-northeast through 

metropolitan areas of Denver, then east-northeast across the 

rural eastern plains of Colorado, and finally over the State 

line where it joins the North Platte River near North Platte, 

Nebraska. The South Platte River alluvial aquifer underlies the South Platte River and its tributaries from the south 

and spans an area exceeding 3,000 square miles (mi2). The 

area under investigation includes regions of the South Platte 

River alluvial aquifer located within Water District 1, Water 

District 2, and Water District 64 in the middle interior of Water 

Division 1, all of which are management areas designated by 

the Colorado Division of Water Resources (fig. 1).

Geographic Setting

The South Platte River Basin covers an area of 

27,660 mi2. The western section of the South Platte River 

Basin is dominated by the Rocky Mountain Front Range interspersed with 14,000-foot (ft) peaks. The plains begin at about 

6,000 ft in elevation and slope downward to the east, lowering 

to 3,400 ft in elevation at the eastern border with Nebraska. 

Major terraces a few miles in width have formed across the 

plains from past episodes of lateral cutting and brief periods 

of equilibrium when streams crossed the threshold between 

aggradation and degradation (Lindsey and others, 2005). The 

modern-day flood plain averages about a mile in width and 

has an irregular surface that consists of swamps, oxbow lakes, 

abandoned meander scars, and other minor terraces (Smith and 

others, 1964).

The South Platte River rises in the Rocky Mountains 

southwest of Denver, enters the Colorado Piedmont (base 

of the Rocky Mountain Front Range) about 25 miles (mi) 

south-southwest of Denver, and then flows in a general northnortheastward direction toward Greeley, where it alters course 

to the east, ultimately reaching Julesburg and extending into 
Description of Study Area 5

Nebraska. The major perennial tributaries of the South Platte 

River along the Front Range are Clear Creek, Big Dry Creek, 

St. Vrain Creek, Big Thompson River, Cache la Poudre River, 

Lone Tree Creek, and Crow Creek. Tributaries and canals, 

some of which are ephemeral, are located from east of the 

Front Range to Julesburg, including Kiowa, Bijou, Badger, 

Beaver, Pawnee, and Cedar Creeks (fig. 1).

Land use in the region is quite diverse. The population 

in forested mountain areas lives primarily in single-family 

residences and small residential communities. Between the 

mountains and the plains is a transition zone where the largest 

population centers of the basin are located, including Denver. 

The basin extends eastward across the plains, where agriculture is the predominant land use and water is used primarily 

for irrigation of crops. Following from Dennehy and others 

(1998), in terms of area, rangeland is the largest land use 

(41 percent) but has a relatively small effect on water quality 

because of the lack of water and minimal water use. Irrigated 

agriculture comprises only 8 percent of the basin but accounts 

for 71 percent of the water use. Urban lands comprise only 

3 percent of the basin but account for 12 percent of the water 

use (or 27 percent if power generation is considered an urban 

water use). 

Climate

The South Platte River Basin is characterized by a 

continental-type climate with moderately cold winters, warm 

summers, and irregular patterns of seasonal and annual precipitation. Precipitation across the basin is strongly influenced 

by differences in elevation. The greatest rates of precipitation in the basin fall in the mountains as snow, in excess of 

30 inches (in.) of water per year (Dennehy and others, 1998). 

The majority of the annual water supply is derived from 

the mountain snowpack, which melts during the spring and 

becomes spring runoff. In contrast, annual precipitation on the 

flat plains is generally less than 15 in. (Dennehy and others, 

1993). Transbasin water mainly from the Colorado River 

Basin supplements natural inflow to the South Platte River.

For the Colorado House Bill 12-1278 (HB-1278) study, 

monthly precipitation rates were determined by the Colorado 

Climate Center using SNOTEL (snow telemetry) and COOP 

(Cooperative Observer Program) weather stations located 

along the Front Range and plains (Waskom, 2013). All station 

records were averaged per month. The record indicates that 

annual precipitation ranged from about 10 to 21 in/yr from 

1953 to 2012 using water years following USGS convention 

(October 1–September 30) (fig. 2A). Over decadal periods, 

average and median precipitation were less variable and 

ranged from 14 to 16 in/yr. After the peak decade 1983–1992, 

decadal precipitation decreased from 1993 to 2002 and 

remained relatively constant from 2003 to 2012. Precipitation during the last two decades (1993–2012), despite having 

declined substantially from peak levels, remained higher than 

the earliest three decades of record (1953–1982).

Overall, about 70 percent of the total precipitation on 

the plains falls during the 6-month period from April through 

September, whereas the remaining 30 percent of precipitation 

occurs as winter snowfall (Hansen and others, 1978). Approximately 3 percent of total precipitation that falls on native vegetation in the basin becomes recharge to the alluvial aquifer 

system (Waskom, 2013). Phreatophyte evapotranspiration in 

the riparian zone of the South Platte River has increased to a 

minor degree from 1990 to 2010, whereas evapotranspiration 

from groundwater for years 1990 and 2010 was about 115,438 

and 156,601 acre-ft, respectively (Waskom, 2013). The latter finding shows a net increase in evapotranspiration from 

groundwater of 35 percent over the last 20 years, although the 

trend is nonlinear over the duration of record.

Geology

Multiple glaciations in the Rocky Mountains during the 

Pleistocene are reflected in the South Platte River valley by 

successive stages of degradation and aggradation (Bjorklund 

and Brown, 1957). Across the valley, the ancestral South Platte 

River and its large tributaries incised deep channels in the landscape, which nearly conform to present drainage patterns. Alluvium in the valley of the South Platte River consists of poorly 

sorted mixtures of unconsolidated gravel, sand, silt, and clay or 

interlayered beds of relatively well-sorted sand, gravel, or silty 

clay. Clay layers are fairly common throughout the basin, both 

laterally and vertically. Beds of gravel and cobbles occur at the 

base of the alluvium near Denver. Large deposits of dune sand 

and loess were deposited mainly during late Pleistocene time. 

The eolian deposits were derived from the wide flood plains of 

the South Platte River and its larger tributaries and from deflation of the Ogallala and Laramie Formations, Fox Hills Sandstone, and Pierre Shale. Prevailing winds probably originated 

from the northwest, given the alignment and topography of the 

dunes, and caused local southeastward migration of dune sand 

over terrace deposits (Bjorklund and Brown, 1957). 

The South Platte River alluvial aquifer overlies the 

Denver Basin aquifer system along the valley of the South 

Platte River from Denver to Greeley. East of Greeley, alluvium 

along the South Platte River is held within an ancestral valley 

eroded into Cretaceous Pierre Shale. Underlying bedrock is 

generally much less permeable than surficial alluvium and 

forms a low permeability boundary to the alluvial aquifer. In 

the plains, which are areas east of the Rocky Mountain Front 

Range, sedimentary rocks underneath the alluvium often 

consist of shale and sandstone of Cretaceous age that form 

fairly broad, mildly sloping valleys. At the headwaters, the 

South Platte River follows narrow valleys of the Rocky Mountains. The alluvial aquifer in the plains ranges substantially 

in thickness with an upper limit on the order of 200 to 300 ft 

and a width of 1 to 10 mi with an average width of about 

5 mi (fig. 3). Comparatively smaller deposits of alluvium or 

windblown sand extend up the valleys of numerous tributaries 

(Robson and Banta, 1995).
6 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Recent investigations within the Colorado Piedmont 

along the western edge of the South Platte River alluvial 

aquifer from Denver to Greeley reveal complex sediment 

structure and depositional history (Lindsey and others, 2005). 

Terrace systems carved from fluvial processes show gradual 

and progressive incision with periods of aggradation during 

glaciations in stream valleys. The South Platte River and major 

tributaries have eroded valleys and deposited the late middle 

Pleistocene Louviers Alluvium about 130 to 200 thousand 

years ago followed by the late Pleistocene Broadway Alluvium 

about 12 to 30 thousand years ago (Lindsey and others, 2005). 

These deposits represent aggradation during increased sediment production, followed by incision when sediment production diminished. Laterally extensive units of sand, silt, and clay 

reside within and on Broadway and Louviers terraces of the 

South Platte River. They formed when streams left fine-grained 

sediment on flood plains during periods of high discharge. 

When floodwater receded, winds swept sediment across the 

valley surface and onto adjacent uplands. Increased sediment 

production, terrace aggradation, and extensive deposition of 

fine-grained sediment on flood plains might be a response to 

glacier retreat in headwater valleys of major streams (Lindsey 

and others, 2005). At the end of Pleistocene time, the South 

Platte River cut through Broadway Alluvium and into bedrock of the Denver Formation. The South Platte River and its 

tributaries then formed the low terraces of present-day valleys. 

More recently in the Holocene, part of the gravel fill of the 

South Platte River was reworked by lateral cutting and infilling 

as the river tended toward equilibrium. Along some channels, 

the grand ancestral South Platte River formed bedding structures with relatively large clast sediment that have not been 

reworked by the diminished modern river (Robson, 1989). 

Annual

Average of decade

Median of decade

EXPLANATION

5

10

15

20

1952 1962 1972 1982 1992 2002 2012

1952 1962 1972 1982 1992 2002 2012

Annual precipitation, in inches per year

0

500

1,000

1,500

2,000

2,500

Discharge, in cubic feet per second

Water year

A

B

Figure 2. Time series of average annual, average decadal, and median decadal (A) precipitation and (B) discharge of the South Platte 

River, averaged monthly from five gaging stations for U.S. Geological Survey water years 1953 to 2012. (See fig. 1 for station numbers 

and locations.)
Description of Study Area 7

Figure 3. Estimated thickness of the South Platte River alluvial aquifer. (From Colorado Decision Support Systems, 

http://cdss.state.co.us)

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Poudre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

Denver

Fort Collins

Fort

Morgan Fort

Morgan Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Thickness of South Platte River 

alluvial aquifer, in feet

High: 300

Low: 10

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

DENVER

BROOMFIELD

Water Division 1

boundary
8 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Hydrology and Water Management

The South Platte River alluvial aquifer is an important 

water resource in Colorado, having a capacity to store approximately 10 million acre-ft of water (Colorado Water Conservation Board, 2007). The degree of surface water–groundwater 

interaction is controlled by several factors, including local 

hydraulic conditions and lithology. Transmissivity of deposits 

along the South Platte River range from less than 1,000 square 

feet per day (ft2/d) along the margins of the aquifer where the 

aquifer is thin to more than 100,000 ft2/d in a few areas near 

the central part of the lower valley where the aquifer is thick 

(Robson and Banta, 1995). Transmissivity generally increases 

downstream along the river valley, primarily because deposits 

of alluvium typically thicken downstream. Native water is 

supplied to the aquifer from direct precipitation on the plains 

and spring runoff originating from the Rocky Mountains that 

infiltrates into the aquifer when streamflows are high. Transbasin water augments the native water supply and is sourced 

primarily from the Upper Colorado River Basin, which is west 

of the study area and extends over parts of California, New 

Mexico, Utah, Wyoming, and Colorado. Precipitation and 

transbasin diversions are the primary inputs to the local water 

budget and control the quantity and, to a degree, the timing 

of water that enters the South Platte River and underlying 

alluvial aquifer.

Decade averaged discharge of the South Platte River at 

five gaging stations (fig. 1) increased from the 1950s through 

the early 1990s but decreased thereafter such that the recent 

decade (2003–2012) is near the historic low observed during 

1953–1962 (fig. 2B) (National Water Information System, 

http://waterdata.usgs.gov/nwis; Colorado Decision Support 

Systems, http://cdss.state.co.us/onlineTools/Pages/

StreamflowStations.aspx). The 60-yr record indicates a 

mixture of periods with relatively high and low annual river 

discharge rates ranging from about 150 to 2,350 cubic feet 

per second (ft3/s). Years of greatest discharge occurred mainly 

between 1973 and 2002. When examined by decade, river 

discharge increased over the first 40 years of record from 1953 

to 1992 and decreased over the last 20 years of record from 

1993 to 2012. The decadal peak of river discharge is coincident with the decadal peak of precipitation during 1983–1992 

(fig. 2A, B).

Water management in the South Platte River alluvial 

aquifer, including the allocation and timing of authorized 

water use, is administered by the State of Colorado through 

a network of 18,600 decreed diversions according to seniority of the water right (Waskom, 2013). The complete network 

of administrative structures with water-right identifications 

(WDID) include diversion and recharge structures. About 

14,000 active decreed administrative structures (that is, reservoirs, engineered recharge areas [mainly recharge ponds], 

impacted river reaches [areas of depletions and accretions 

along the river], ditches [includes ditches and canals], wells, 

and well fields) are located near the South Platte River alluvial 

aquifer in Water Districts 1, 2, and 64 (fig. 4). The use of 

diversion structures to store, route, and administer water contributes a large anthropogenic influence on the natural cycling 

of water through the aquifer.

The South Platte River Basin has experienced nearly successive cycles of over-appropriation followed by adjustment 

and supply enhancement (Waskom, 2013). Canals, reservoirs, 

transbasin diversions, and wells were constructed in past 

decades to address water shortages and refine water rights 

and irrigable acreage. Following various responsive actions 

to improve water availability, the aquifer quickly returned to 

a fully appropriated system, requiring additional development 

to accommodate societal needs. Water development generally 

followed an upstream to downstream progression. Development occurred at a rapid pace, and by the late 1890s to early 

1900s, a more stable water supply based on return flows 

was available in the lower river, leading to the development 

of a more extensive irrigation network. Cycles of changing 

regulations, increased diversions, and development followed 

in later years. Prior to 2002, groundwater pumping from 

high-capacity alluvial wells located along the South Platte 

River had reached nearly 500,000 acre-ft/yr from approximately 8,200 wells (Waskom, 2013). In the recent decade it is 

estimated that around 450,000 acre-ft/yr were pumped in the 

basin from 6,500 high-capacity wells. Decreed water rights in 

the basin from return flows of upstream diversions of native 

water, reservoir deliveries, and imported transbasin water 

sum to about 4,000,000 acre-ft/yr. In 2005, 60 percent of the 

830,000 irrigated acres in the basin were watered exclusively 

with surface water, 18 percent exclusively with groundwater, 

and 22 percent with a mix of surface water and groundwater. 

Canal seepage ranges from 10 to 50 percent of water transported through canals with an average loss of 23 percent. 

Aquifer recharge from canals and ditches is estimated to be in 

excess of 500,000 acre-ft/yr.

Methods of Investigation

This section describes the compilation of data, analysis of 

groundwater levels, and procedure used to design a monitoring 

well network. The techniques adopted for each method were 

selected to minimize the effects of data irregularity and uncertainty and employ conservative thresholds of significance.

Data Compilation

Multiple tasks related to data compilation were performed as components of the project. The study focuses 

on examining groundwater-level data from calendar years 

1953 through 2012. Calendar years were selected to meet 

the agreement with project cooperators for examining data 
Methods of Investigation 9

Figure 4. Locations of active decreed administrative structures near the South Platte River alluvial aquifer in Water Districts 1, 2, 

and 64.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek Wildcat Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

Denver

Fort Collins

Fort

Morgan

Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

105° 104° 103°

41°

40°

Administrative water management 

structures

Ditch

Impacted river reach

Engineering recharge area

Reservoir

Well

Well field

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

DENVER

BROOMFIELD

Water Division 1

boundary
10 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

through the end of 2012. The collection of additional data 

or performing ground truths of historical data were beyond 

the scope of the project. Non-Federal groundwater-level data 

(Colorado Decision Support System, http://cdss.state.co.us/

onlineTools/Pages/GroundWaterWaterLevels.aspx; Takis 

Oikonomou, HB-1278 team, oral commun., 2012) were collected primarily from (a) the South Platte Decision Support 

System (SPDSS), (b) the Central Colorado Water Conservancy 

District (CCWCD), (c) the Lower South Platte Water Conservancy District (LSPWCD), and (d) Colorado State University 

(CSU). Federal groundwater-level data were collected from 

U.S. Geological Survey (USGS) National Water Information 

System (NWIS) records (http://waterdata.usgs.gov/nwis). 

Inspection revealed there were different protocols of record 

keeping, data collection, and quality control, resulting in a 

mixture of data quality and accuracy. Some wells have been 

instrumented with pressure transducers (for example, SPDSS), 

whereas groundwater levels were measured manually at 

other wells. Members of the HB-1278 team identified many 

wells by State identification codes from personal communication with agency members during the initial inspection of 

the datasets. Several groundwater-level measurements were 

corrected based on these discussions. The USGS compared 

records of each agency to records in HydroBase (State of 

Colorado data repository, http://water.state.co.us/DataMaps/

DataSearch/Pages/DataSearch.aspx) and NWIS. Portions of 

agency records were found to exist in HydroBase. In addition, 

some records in HydroBase were identified to originate from 

NWIS. Comparisons between multiple datasets with partially 

overlapping records were performed using a hierarchical 

system of concatenating data. Records were assembled as 

either new wells needing to be established in NWIS or original 

groundwater-level data appending to existing wells in NWIS. 

In comparing data, NWIS records were considered senior to 

HydroBase records, such that original HydroBase records suitable for basic groundwater analyses were appended to NWIS 

records. HydroBase records were grandfathered above other 

non-Federal agency records in the same manner. Wells without 

verifiable identification or possessing data having irresolvable 

disparities were disregarded. Wells were further evaluated by 

lithology, record length, and well depth. Lithology and use 

type were identified by State and Federal records and personal 

communication with the HB-1278 team and individual agencies. Wells were selected if open to the South Platte alluvium 

and less than 200 ft in depth. Groundwater levels were considered if below the land surface to avoid examining conditions 

of surface-water ponding or springs. Moreover, to prevent use 

of data collected immediately after drilling or from arbitrary 

measurements without an established history or purpose, wells 

were required to have at least three observations spanning at 

least a one-month duration. The compiled dataset from multiple agencies includes 1,669 wells having about 150,000 waterlevel observations. Nearly 400 wells are used specifically for 

monitoring groundwater levels as indicated in USGS records 

or described by non-Federal agencies. Other wells serve different purposes but are mainly used for irrigation. 

Other data records of precipitation, river discharge, and 

State-managed water diversions were collected from existing 

resources. Monthly precipitation data were compiled from 

National Oceanic and Atmospheric Administration records 

by the Colorado Climate Center as part of the HB-1278 

project. Records of river discharge of the South Platte River 

at five USGS gaging stations near Fort Lupton, Fort Morgan, 

Henderson, Julesburg, and Kersey (site numbers: 6721000, 

6759500, 6720500, 6764000, 6754000, respectively) were 

downloaded from NWIS. The Julesburg gaging station has 

complete monthly data during 1953–2012, but collectively for 

all five stations there is about 50 percent data coverage. State 

discharge records at these locations (PLALUPCO.06721000, 

PLAMORCO.06759500, PLAHENCO.06720500, 

PLAJUCCO.06764000, and PLAKERCO.06754000) were 

used to fill missing records where available, which improved 

data coverage to about 80 percent over the 60-yr period. 

Administrative water management structures and other water 

management records were downloaded from SPDSS online 

resources using the bulk data exporter, accessed during the 

data collection phase of the project in 2012.

Groundwater Levels

“High groundwater” is not explicitly defined in Colorado 

House Bill 12-1278. A depth to water below land surface 

of 10 ft or less was used as the primary metric in this report 

to delineate high groundwater levels based on discussions 

with the Colorado Division of Water Resources. Analyses of 

groundwater levels were performed using data from individual 

wells and data collected from groups of wells located within 

subwatersheds of the South Platte River alluvial aquifer that 

range in area from several to tens of square miles. Subwatersheds were used because they provided a balance between 

defining areas where sufficient data could be aggregated 

while remaining sufficiently detailed to resolve spatial differences. Choropleth (thematic) maps, in which areas are shaded 

according to the measurement of the statistic, were defined by 

subwatershed delineations.

Trend Analysis

Changes in groundwater levels were examined using 

nonparametric (Kendall line) and iterative multiple regression 

(least trimmed squares) linear trend analyses using methods 

and programs developed by Hawkins (1994), Hawkins and 

Olive (1999), Press and others (1999), and Helsel and Hirsch 

(2002). The decision to use two statistical approaches rather 

than a single approach was justified as a way to verify results 

and identify cases where discrepancies exist, either from 

artifacts in the datasets or assumptions inherent to the method. 

The Kendall line is a simple and widely recognized nonparametric method used to fit a linear trend to the data (Helsel 

and Hirsch, 2002). The slope of the Kendall line is computed 

by comparing each data pair (time, groundwater levels) to 
Methods of Investigation 11

all others in a pairwise fashion. A dataset of n data pairs will 

result in n(n−1)/2 pairwise comparisons. For each of these 

comparisons, groundwater-level change is computed. The 

median of all possible pairwise slopes is taken as the nonparametric slope estimate and the trend is then applied to a linear 

fit relation. Least trimmed squares is a more advanced method, 

both mathematically and computationally, and was recently 

developed for studies in data mining. It involves a criterion for 

analyzing multiple regression datasets in which there might be 

a few outliers. The method consists of finding a subset of cases 

whose deletion from the dataset would lead to the regression 

with the smallest residual sum of squares (Hawkins, 1994; 

Hawkins and Olive, 1999). It is used as a general-purpose high 

breakdown method and gives the maximum likelihood estimator of the regression under an outlier model. 

To test whether a trend is significant, both type I and 

type II errors were evaluated. The significance, α, is used to 

evaluate type I error. Type I error is the probability of rejecting the null hypothesis when it is true. The significance level 

for the current study was set to a value of 0.01 to produce a 

conservative assessment of trends. Type I errors were evaluated using p-values and the Kendall Tau correlation coefficient 

to evaluate monotonic relations in the water-level data (Press 

and others, 1999; Helsel and Hirsch, 2002). Methods of 

nonparametric trend analysis such as those based on Kendall’s 

coefficient are widely used to test for the presence of monotonic trends in environmental time series data (Helsel and 

Hirsch, 2002). The type II error, β, is a measure of statistical 

power (1–β). Type II error is the probability of correctly rejecting the null hypothesis when it is false. The power threshold 

used to define acceptable accuracy was a value of 0.8, which is 

commonly used in statistical studies. Power was approximated 

through Monte Carlo simulation described in Yue and others 

(2002). Two thousand independent normally distributed time 

series were generated numerically and evaluated for a range 

of different sample sizes to estimate Type II errors. Power can 

be calculated as the number of numerical experiments that 

fall in the confidence region in relation to the total number of 

experiments conducted. Power equal to 0.80 implies 80 percent of cases meet this criterion. Usually, the smaller the trend 

and (or) the higher the groundwater level variability (noise-tosignal ratio), the more data will be required to verify predictions. In some instances, hundreds of observations might be 

needed to identify a significant trend. Considering both Type 

I and II errors adds reliance to rejecting the null hypothesis 

when it is either true or false. 

In addition to examining for Type I and II errors, trend 

predictions from the Kendall line and least trimmed squares 

regression approaches were compared. In about 95 percent of 

cases, predicted trends of the 1,669 wells showed agreement in 

trend magnitude within 1 ft/yr. A threshold of 1 ft/yr was used 

as a metric to identify anomalies in the data in order to avoid 

cases that were overly dependent on the chosen method. For 

the remaining 5 percent of cases, differences in trend magnitudes were greater than 1 ft/yr, but the majority of these cases 

were also flagged because of other quality control issues. In 

some instances, particularly with small trend magnitudes, the 

signs of predicted trends were opposed. Results in this case 

were considered statistically insignificant (inconclusive). 

Choropleth (subwatershed) maps were also used to 

examine trends of high groundwater levels in regions defined 

by subwatersheds of the South Platte River alluvial aquifer 

from the National Hydrography Dataset (http://nhd.usgs.gov). 

A hybrid approach was adopted to indicate subwatershed areas 

suitable for trend analysis and areas which showed a significant trend. Preprocessing was first performed to spatially 

detrend the water-level data using the median groundwater 

level recorded for each well. Although significant fluctuations 

arose in evaluating groundwater levels using a group of wells 

within each subwatershed, aggregating the incomplete well 

record enabled a broadened perspective of aquifer conditions. This includes instances where aggregating well data 

over a subwatershed yielded a sufficient record in areas where 

individual well records were not suitable for analysis and 

areas where individual wells showed different responses than 

the subwatershed they are contained within because of local 

influences (such as diversion structures) or spatial variation 

(geologic heterogeneity).

Predicted trends in groundwater levels were ultimately 

evaluated based on several criteria: (a) Type I and Type II 

errors, (b) sufficient trend agreement between methods, and 

(c) sufficient data record over the defined period. For a time 

series to be considered as having a sufficient data record, there 

must be at least 70 percent data coverage over the evaluated 

time range (that is, data range of at least 7 of 10 years per 

decade) and 50 percent data coverage using biannual time 

divisions (that is, at least 10 of 20 biannual periods per decade 

must have data observations). For records of groundwater 

levels that meet these conditions, the average trend estimated 

between the Kendall line and least trimmed squares regression was used for further analysis. The procedures allowed 

for three possible scenarios for a given well or subwatershed: 

(a) data coverage was insufficient to support a prediction and 

results were not presented, (b) data coverage was sufficient 

but the prediction was statistically insignificant, or (c) data 

coverage was sufficient and the prediction was statistically 

significant. 

Frequency of High Groundwater Levels

Frequency of high groundwater levels was chosen as the 

variable to evaluate because it is an extensive property that 

can be defined over an area (subwatershed). Other measures 

such as counts of high groundwater levels would be affected 

by subwatershed size and total number of measurements and 

would be biased accordingly. When calculating frequency of 

high groundwater levels aggregated over each defined area 

(subwatershed) of the South Platte River alluvial aquifer, a set 

of required conditions was applied to data collected from multiple wells. Only those subwatersheds with combined datasets 

having at least 20 observations over the evaluated time period, 

encompassing at least 70 percent of the evaluated time range, 
12 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

and filling at least half of the biannual time divisions were 

considered representative. The requirement for 20 combined 

observations over the time period examined was deemed minimally sufficient to sample effective high groundwater levels 

over the subwatershed area, and the requirement that data have 

at least 50 percent biannual sampling and at least 70 percent 

coverage of the range of time period examined was deemed 

minimally sufficient to sample major temporal changes in the 

dataset. In addition, subwatersheds were required to contain 

at least 3 wells with water-level data. Subwatersheds with 

water-level data that did not meet all data requirements were 

not considered in the analysis. The areas with insufficient data 

coverage are shown by spatial gaps in the choropleth maps. 

Spatial Grouping of High Groundwater Levels

Local Moran’s I is a local spatial autocorrelation statistic based on the global Moran’s I statistic. The method was 

developed by Anselin (1995) as a local indicator of spatial 

association (LISA) statistic. LISA statistics serve two purposes. First, they can be interpreted as indicators of local 

pockets of nonstationarity, or hot spots, similar to statistics 

proposed by Getis and Ord (1992). Secondly, they can be used 

to assess the influence of individual locations on the magnitude of the global anomalies. Given a set of weighted features, 

the method identifies clusters of features with values similar 

in magnitude and spatial outliers. To accomplish this goal, the 

method calculates a Local Moran’s I value, a Z score, and a 

p-value and adopts a naming convention representing the cluster type for each feature (High-High, High-Low, Low-Low, 

Low-High, or insignificant). Features for this investigation 

used choropleth maps defined by decadal frequencies of high 

groundwater levels aggregated over each subwatershed of the 

South Platte River alluvial aquifer. The Z scores and p-values 

are measures of statistical significance which denote whether 

to reject the null hypothesis, feature by feature (subwatershed 

by subwatershed), weighted by area. The statistics indicate 

whether the apparent similarity or dissimilarity in values for a 

feature and its neighbors is greater than expected in a random distribution. The Z score is based on the null hypothesis 

computation. A high, positive Z value for Local Moran’s I 

indicates that the feature is surrounded by features with similar 

values (High-High or Low-Low). Features of this class were 

considered as part of a “cluster.” A low, negative Z value 

for Local Moran’s I indicates that the feature is surrounded 

by other features with dissimilar values (High-Low or LowHigh). Features of this class were considered “outliers.” A 

p-value less than a confidence level of 0.05 was considered 

statistically significant. 

Correlation of Attributes to High Groundwater 

Levels

Pearson correlation coefficients and confidence intervals 

were used to examine relations between high groundwater 

levels observed in wells and system attributes of the South 

Platte River alluvial aquifer. Correlation coefficients provide 

a simple measure of the interdependence of two variables that 

range in value from –1 to +1. A value of –1 indicates perfect 

negative correlation, and a value of +1 indicates perfect positive correlation. An absence of correlation falls between these 

values at zero. Correlation magnitudes between about 0.3 and 

0.5 are typically considered moderate for natural data, while 

those above and below this range are considered high and 

low, respectively. Statistical significance of each correlation 

was determined using p-values. The p-value is computed by 

transforming the correlation to create a t-statistic having N–2 

degrees of freedom, where N is the number of data samples. 

Data with a p-value less than 0.05 were considered statistically significant. The 95-percent confidence intervals for each 

correlation were approximated based on an asymptotic normal 

distribution, which is most applicable for moderate to large 

datasets. 

Geographic and administrative structures were examined 

for correlations to high groundwater levels. The geographic 

attributes examined for correlations were measures of land 

surface elevation, distance of wells from the South Platte 

River and its tributaries, and distance of wells from administrative structures. The administrative structures examined for 

correlations were production wells and well fields, ditches, 

reservoirs, impacted river reaches, and recharge areas. Attributes of administrative structures were examined using nearest 

distances, decree rates, and group statistics within defined 

search radii. For the evaluation of diversion structures, search 

distances (radii) from wells were incrementally increased, 

incorporating more structures at larger distances until the maximum correlation between an attribute and frequency of high 

groundwater levels could be identified. This was considered 

to be the scale (search area) where an attribute had greatest 

influence that can be supported by the data. Minimum search 

distances used to determine correlation coefficients were 

controlled by sample size. A sample of sufficient size is beneficial to support estimation of 95-percent confidence intervals 

and representation of the aquifer, and it reduces the effects 

from a single outlier observation. Estimates of correlation 

coefficients were considered reasonable at search distances 

where at least 10 samples were available and p-values were 

less than or equal to 0.05. The minimum scale to evaluate 

correlation coefficients corresponded to the approximate scale 

where 10 samples could be evaluated. The correlations were 

determined using MATLAB (MathWorks, 2008). Distances 

of wells to selected attributes were determined using ArcGIS 

layers and geographic feature information provided by SPDSS 

and MATLAB using diversion information provided by the 

Colorado DWR (MathWorks, 2008).
Methods of Investigation 13

Optimization of Monitoring Well Network 

Design

A simple optimization approach was applied to the design 

of the proposed monitoring network. The optimization was a 

first-tier analysis to identify representative candidate monitoring wells for delineating regional water levels based on 

hydrologic conditions and nearby administrative structures. 

Candidate wells must be further evaluated through direct 

inspection of the field site, review of additional well records, 

and discussions with State authorities during the implementation phase of the network. 

The monitoring network was created for identifying 

depths to groundwater, but it will also provide utility in characterizing the potentiometric surface of the aquifer. Kriging 

was performed to interpolate water levels given as depth to 

water below land surface in order to develop a monitoring 

network most applicable for defining areas of high groundwater. Characterizing depths to groundwater usually requires 

more monitoring locations than needed for characterizing 

the potentiometric surface. The potentiometric surface has 

a longer correlation scale and requires fewer observation 

wells because the water table is smoothed under an irregular 

land surface. The proposed monitoring network will also be 

useful for characterizing the potentiometric surface because 

topographic variations along the South Platte River alluvial 

aquifer are generally moderate and well spacing is sufficient. 

Candidate monitoring wells were selected through an iterative 

selection process that included an evaluation of kriging output. 

The kriging procedure entailed first detrending the data and 

identifying a suitable model variogram, which was used in the 

interpolation procedure. Kriging variance is an outcome of the 

interpolation and a measure of uncertainty (error), which is 

detailed in appendix 1. Kriging variance was used to evaluate 

the benefit of incorporating a candidate monitoring well into 

the monitoring network design during each iteration in the 

optimization process. 

The monitoring network is composed of three subnetworks: a primary subnetwork and two targeted subnetworks. 

Subnetworks were designed in part based on kriging variance 

and in part based on local hydrologic conditions and characteristics of administrative structures surrounding each well 

site. The primary (“unstressed”) subnetwork was designed to 

measure ambient groundwater levels. This subnetwork was 

developed by considering well location in the kriging process 

and limiting the degree to which a well site can be affected by 

surrounding administrative structures. Administrative structures were avoided to minimize effects on hydrologic processes that would introduce artifacts into the dataset. Weighting interpolations of water levels along with the primary 

constraint was beneficial to limit or prevent the selection of 

wells in isolated clusters and to better sample across the aquifer extent, depending on the weighting that was used. One of 

the secondary subnetworks targeted wells with high groundwater conditions and those which showed substantial trends in 

water-level change. Monitoring wells were selected based on 

their location in delineating water levels and had greater frequencies of high-water conditions or water-level change. The 

other secondary subnetwork targeted the effects of administrative structures. Monitoring wells were selected based on their 

location in delineating water levels and proximity to administrative structures and (or) areas with high average decree rates 

of administrative structures in the local search neighborhood, 

defined as structures within a 10-mi radius from each well 

location. 

Ranking criteria were used to evaluate the candidate pool 

of monitoring wells and optimize the design of the intended 

monitoring networks. The ranking was done in a manner that 

balanced the importance of well location as determined by 

the kriging variance of each well location in reducing spatial 

uncertainties along with other defined criteria. For simplicity, 

attributes considered in selecting monitoring wells were normalized over the interval 0–1, ensuring that the weight (importance), ω, given to each attribute was applied to the same 

range of values. The associated weights and group-normalized 

attributes from multiple criteria were used to produce ranks, 

R, and also ranged over the interval 0–1. The highest rank 

under the proposed convention indicates the best candidate 

well in the candidate pool. 

In the initial procedure, each attribute examined, ε, was 

normalized using a simple minimum-maximum (min-max) 

division and offset in the following form:

	 ψε = (εi – min (ε)) (max (ε) – min (ε))–1 (1)

where ψ indicates the normalized attribute ε. An exponential 

distance relation, X, was also used when normalizing distances 

of the closest features to a specified well, given as

Xdist = (1 – exp (–3))–1 (1 – exp (3 (ψdist))) (2)

which skewed the linear normalization to be more heavily 

weighted at close distances. This was beneficial in giving 

additional weight to structures closer to a well where beyond a 

given distance there was no discernible difference in influence. 

In addition to the normalized attributes, accuracy of well location was considered using a unit step function:

δ loc = { } 0

1

,

,

else

gps (3)

which functioned as a 1–0 switch to indicate if a site had 

been located using the Global Positioning System (GPS). It 

was useful to prioritize well sites located with greater location accuracy with a defined weight because spatial relations 

to surrounding structures were known with greater certainty. 
14 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

The combined weight function for system attributes of the 

unstressed monitoring network, β*, was given as

β* = (ωdistXdist + ωdecr (1 – ψdecr) + ωlocδloc) (4)

which grouped criteria weighting of the minimum distance 

to a diversion structure (dist), average decree rate within the 

search distance to a well (decr), and whether the site had been 

positioned with GPS (loc). The weights used in the analysis 

for diversion distance, decree rate, and GPS location accuracy 

were 0.5, 0.3, and 0.2, respectively. The combined weight 

function for wells near administration structures, βadm, was 

given as

βdiv = (ωdist(1 – Xdist) + ωdecrψdecr + ωlocδloc) (5)

which grouped criteria weighting of proximity to a diversion 

structure (dist), average decree rate within a 10-mi radius 

of a well (decr), and whether the site had been positioned 

with GPS (loc). Note that high-low weighting priorities are 

switched for diversion distance and decree rate between equations 4 and 5. The weights used in the analysis for diversion 

distance, decree, and GPS location were 0.5, 0.3, and 0.2, 

respectively.

Rank coefficients, R, were used to select monitoring well 

candidates for the unstressed monitoring network (unstress), 

structural target monitoring networks for each diversion type 

(div), and hydrologic target monitoring networks emphasizing 

high groundwater levels (hw) and trends in groundwater levels 

(trend) given as

Runstress

well

= ωσ 2 ψσ 2 + (1 – ωσ 2)(ψβ*), (6)

Radm

well

= ωσ 2 ψσ 2 + (1 – ωσ 2)(ψβadm), (7)

Rhw

well

= ωσ 2 ψσ 2 + (1 – ωσ 2)(ψβhw  ), and (8)

Rtrend

well

= ωσ 2 ψσ 2 + (1 – ωσ 2)(ψ|θ|), (9)

respectively. Rank coefficients were a function of each normalized attribute and normalized kriging variance derived 

from the interpolation of depth to water corresponding to each 

well, σ 2, and used a prescribed weight, ωσ 2, of 0.6 for all cases. 

The selected weight allowed partial emphasis on the normalized attribute but also promoted sampling across the aquifer. 

Attribute ranks of system characteristics remained constant 

through the optimization procedure. Selection of wells added 

to the monitoring network was performed iteratively, however, 

because kriging variances across the spatial grid evolved with 

an expanding dataset. Automation of the kriging spatial interpolation, boundary clipping to a specified optimization region, 

assessment of well candidates, and updating of the candidate 

pool and monitoring network were performed in ArcGIS using 

Python commands.

Analysis of Groundwater Levels

This section of the report provides a general statistical 

overview of wells and groundwater levels and includes an 

examination of frequency of high groundwater levels defined 

over subwatershed areas, differences in high groundwater 

levels between subwatersheds, changes (trends) in groundwater levels, and factors that influence high groundwater levels in 

the South Platte River alluvial aquifer.

Overview Statistics of Groundwater Levels in 

Wells

Groundwater levels from 1,669 wells were evaluated over 

60 years of record from 1953 to 2012 (fig. 5). Well coverage 

across the South Platte River alluvial aquifer was generally 

dense in areas near the South Platte River and relatively sparse 

along tributaries. The number of wells with data observations 

varies by decade (fig. 6A). At a maximum, there were more 

than 1,100 wells with data suitable for analysis from 1963 to 

1982. The fewest wells (300–400) were available for decadal 

periods between 1953–1962, 1983–1992, and 1993–2002. 

Since 1983–1992, the number of available wells has increased, 

reaching 572 in the recent decade (2003–2012). Basic decadal 

statistics were selected for analysis to attenuate potential 

biases caused by different sample sizes and provide insight on 

general patterns in groundwater levels.

Over the 60-year record, average depths to water varied 

between 24 and 32 ft, depending on decade (fig. 7A). The 

shallowest (smallest) average depth to water was identified 

during 1983–1992, which also recorded the highest levels of 

decadal precipitation (fig. 2A). Average depth to water was 

greatest during 1953–1962 and intermediate in the recent 

decade (2003–2012) following curtailment of pumping. 

Average groundwater levels generally rose from 1953–1962 

to 1983–1992 and declined from 1983–1992 to 2003–2012. 

Between 1993–2002 and 2003–2012 groundwater levels 

declined 2 ft when considering all areas of the aquifer. In 

addition, standard deviations of groundwater levels were 

determined per decade to examine variability of groundwater 

levels above and below decadal averages. The standard deviation of depths to water ranged from 21 to 27 ft and generally 

increased over the 60-yr record by about 0.1 ft/yr, in part 

caused by groundwater depletion in some areas. Maximum 

depth to water generally increased over the 60-yr record by 

about 1 ft/yr on average but fluctuated by decade (fig. 7C). 

The majority of change was largely incremental and occurred 

between 1953–1962 and 1963–1972 and between 1983–1992 

and 1993–2002. For other periods, maximum depths to 
Analysis of Groundwater Levels 15

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

Denver

Fort Collins

Fort

Morgan Fort

Morgan Gilcrest

Greeley

Julesburg

LaSalle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

105° 104° 103°

41°

40°

Well network (all) by use type

Monitoring

Irrigation

Stock

Other

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

DENVER

BROOMFIELD

Water Division 1

boundary

Figure 5. Complete set of 1,669 wells used to evaluate groundwater levels, identified by primary use type.
16 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

water decreased toward recent time, but the differences are 

less substantial.

Changes in basic decadal statistics were identified when 

examining observed depth to water under different levels of 

censoring. Portions of the data were extracted using specified maximum limits of depth to water and compared to the 

complete dataset as a baseline. Using a depth to water less than 

or equal to10 ft, averages and standard deviations of depth 

to water were similar between decades at about 6 ft and 2 ft, 

respectively. The analysis indicates that depths to water less or 

equal to 10 ft were fairly constant, on average, over six decades 

of record. When increasing the prescribed limit to 20 ft or less 

below land surface, average depths to water varied between 9 

and 12 ft depending on decade (fig. 7B). The shallowest average depths to water occurred over two decades from 1973 to 

Figure 6. Decadal summary of shallow groundwater levels and well counts. Shallow groundwater levels are defined as “near 

surface” (depth to water of 5 feet or less) and “high” (depth to water of 10 feet or less). A, Number of wells with groundwater-level 

data and number of wells eligible for trend analysis. B, Percentage of wells with at least one shallow groundwater-level observation. 

C, Percentage of shallow groundwater-level observations.

1992. Average depth to water was greatest during 1953–1962 

and intermediate in the recent decade (2003–2012) relative to 

the complete record. Observed groundwater levels rose about 

0.6 ft, on average, between 1993–2002 and 2003–2012 in areas 

where groundwater levels were within 20 ft of the land surface, 

despite an overall decline across the aquifer using all depthto-water observations (fig. 7A, B). By extending the censoring 

to depths to water less than or equal to 30 ft, decadal averages more closely resemble results from the complete dataset. 

Although the rise in shallow groundwater levels over the last 

20 years of record is not reflected aquifer-wide, average depth 

to water during the recent decade was intermediate over the 

period of record, which supports that recent conditions are not 

unprecedented for the aquifer as a whole. Moreover, shallow 

water-level fluctuations (decadal standard deviations) were 

0

250

500

750

1,000

1,250

1,500

Number of wells

A

0

5

10

15

20

25

30

35

40

Percentage of observations Percentage of wells

B

0

5

10

15

20

25

30

35

1953–1962 1963–1972 1973–1982 1983–1992 1993–2002 2003–2012

C

Year

EXPLANATION

Wells with groundwater-level

data

Wells eligible for trend analysis

High groundwater levels

Near-surface groundwater

levels

EXPLANATION

Wells with high groundwater

levels

Wells with near-surface

groundwater levels

EXPLANATION
Analysis of Groundwater Levels 17

fairly constant over six decades of record (fig. 7B). In considering there has been a history of change in water policy in 

the region, such policies either have not substantially affected 

natural variations of shallow water levels or have consistently 

helped to moderate water-level variability.

As recommended by the Colorado Division of Water 

Resources, a depth to water of less than or equal to 10 ft 

below land surface was used to define locations with “high” 

groundwater levels. The 10 ft censoring limit was used as 

the primary measure to evaluate shallow groundwater levels 

during the remainder of the investigation. A depth to water of 

less than or equal to 5 ft was also used in the current assessment to evaluate “near-surface” groundwater levels nearest to 

the land surface and enhance the analyses using the primary 

measure of shallow groundwater. Over the course of 60 years 

there have been several areas with shallow groundwater levels 

along the main stem of the South Platte River (fig. 8). The 

western and eastern sections of the South Platte River alluvial 

aquifer had the broadest spatial extent of shallow groundwater 

observations orthogonal to the river channel up to a few miles, 

whereas in the central section, shallow groundwater was generally more concentrated near the South Platte River.

Figure 7. Average and standard deviation of depths to water by decade using (A) all observations, 

(B) observations of depth to water less than or equal to 20 feet, and (C) maximum depths to water per 

decade using all observations.

20

30

40

Depth to water, in feet

A

100

125

150

175

200

1952 1962 1972 1982 1992 2002 2012

Maximum depth to water, in feet

Calendar year

C

0

10

20

Depth to water, in feet

B

EXPLANATION

Average

Standard deviation

EXPLANATION

Average

Standard deviation
18 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 8. Well locations with at least one recorded occurrence of shallow groundwater between 1953 and 2012.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

Denver

Fort Collins

Fort

Morgan

Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Shallow groundwater

105° 104° 103°

41°

40°

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Depth to water less than or equal to 5 feet 

below land surface

Depth to water from 5 to 10 feet below 

land surface

DENVER

BROOMFIELD

Water Division 1

boundary
Analysis of Groundwater Levels 19

Over the 60-yr record, summary statistics of groundwater 

levels indicate clear differences between decades (fig. 6). The 

percentage of wells with shallow groundwater varied between 

5 to 33 percent, depending on the decade examined and depth 

to water being considered (fig. 6B). High groundwater levels 

occurred in 17 to 33 percent of wells with the largest percentages occurring over a three-decade period from 1963 to 1992. 

The most recent decade (2003–2012) shows that 25 percent of 

wells had high groundwater levels, which is below the threedecade maximum and an intermediate frequency relative to the 

60-yr record. Near-surface groundwater levels occurred in 5 to 

17 percent of wells, depending on decade. The largest percentages of wells with near-surface groundwater also occurred in 

a three-decade period from 1963 to 1992. In each case, the 

percentage of wells with shallow groundwater in the recent 

decade was below the observed maximum and intermediate 

over the period of record. 

Shallow groundwater levels were also evaluated using 

percentage of observations as a comparison to the percentage 

of wells with shallow groundwater (fig. 6C). High groundwater levels accounted for 13 to 30 percent of observations 

depending on decade. Corresponding near-surface groundwater levels accounted for 3 to 11 percent of observations, 

roughly one-third the frequency of high groundwater levels. 

The highest percentage of near-surface groundwater levels 

occurred over three decades from 1963 to 1992 and is similar 

to the analysis by wells with the most recent decade having an 

intermediate percentage of observations as compared to the 

complete record (fig. 6C). Overall, the frequency of shallow 

groundwater levels showed little change from 1963 to 1992. 

Comparison also revealed that although the highest frequency 

of high groundwater levels occurred in the most recent decade, 

there was only an intermediate percentage of wells with 

high groundwater levels during this period (fig. 6B, C). The 

expected change in near-surface groundwater levels between 

wells and observations is generally consistent with observed 

values, which implies there were proportionally more observations of depth to water between 5 and 10 ft during 2003–2012. 

Although high groundwater levels were found to be elevated 

at 30 percent of all observations during 2003–2012, they still 

approximated previous levels recorded over three decades during 1963–1993 within a 4 percent difference (26–29 percent) 

(fig. 6C).

Frequency of High Groundwater Levels across 

Subwatersheds

Choropleth (thematic) maps, in which areas are shaded 

according to the frequency of high groundwater levels, were 

developed using regions defined by subwatersheds of the 

South Platte River alluvial aquifer. Divisions of subwatersheds 

were based on surface topography and clipped to regions that 

intersected the South Platte River alluvial aquifer boundary 

(fig. 9). Choropleth maps were examined chronologically per 

decade over subwatersheds with acceptable data to provide a 

first-order assessment of regional patterns in high groundwater 

levels (figs. 10–15).

During the initial decade, 1953–1962, the large majority of subwatersheds either possessed insufficient data to be 

examined or showed a low frequency (between 0 and 20 percent) of high groundwater levels (fig. 10). Greater frequencies 

of high groundwater levels occurred along a small section 

near Julesburg (20–40 percent), near Greeley and Gilcrest 

(20–60 percent), and east of Sterling (60–100 percent). The 

area east of Sterling had the highest frequencies of high 

groundwater levels in the aquifer during this period.

During the second decade, 1963–1972, there was a 

broader spatial distribution in frequencies of high groundwater levels and better coverage of subwatersheds with 

acceptable data (fig. 11). Similar to the pattern which occurred 

in the previous decade, the majority of subwatersheds along 

the tributaries south of the South Platte River had a relatively 

low frequency of high groundwater levels (0–20 percent). 

Areas with moderate frequency of high groundwater levels (20–40 percent) extended intermittently from Denver to 

Julesburg along the main stem of the South Platte River. Areas 

around Greeley, La Salle, and Gilcrest, west of Fort Morgan, 

and between Sterling and Julesburg showed greater frequencies of high groundwater levels (40–80 percent). One small 

region midway between Sterling and Julesburg had pervasive 

high groundwater levels (80–100 percent) from 1963 to 1972.

During the third and fourth decades, 1973–1982 and 

1983–1992, coverage of subwatersheds with acceptable data 

is sparse (figs. 12–13). During 1973–1982, areas near Greeley, 

La Salle, and Gilcrest and west of Fort Morgan had moderate frequencies of high groundwater levels (20–60 percent) 

(fig. 12). There were no areas with frequent (60–80 percent) 

or pervasive high groundwater levels (80–100 percent) from 

1973 to 1982. During the fourth decade, 1983–1992, areas 

near and east of La Salle and Gilcrest showed moderate 

frequencies of high groundwater levels (40–60 percent), while 

areas near the mouth of Lost Creek and the South Platte River 

near Riverside Reservoir and about 10 mi east of Sterling at 

the mouth of Cedar Creek and the South Platte River had pervasive high groundwater levels (80–100 percent) (fig. 13).
20 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS

ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort

Collins

Fort

Collins

Fort

Morgan Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Subwatershed intersections with 

South Platte River alluvial aquifer

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary

Figure 9. Subwatershed areas used for analysis that intersect the South Platte River alluvial aquifer.
Analysis of Groundwater Levels 21

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Fort

Morgan

Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

105° 104° 103°

41°

40°

Frequency of high water levels from 

1953 to 1962, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary

Figure 10. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 1953 to 1962. 

Spatial gaps between subwatersheds possess inadequate data to examine.
22 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Fort

Morgan Fort

Morgan

Gilcrest

Greeley

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Frequency of high water levels from 

1963 to 1972, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary

Figure 11. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 1963 to 1972. 

Spatial gaps between subwatersheds possess inadequate data to examine.
Analysis of Groundwater Levels 23

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Frequency of high water levels from 

1973 to 1982, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary

Figure 12. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 1973 to 1982. 

Spatial gaps between subwatersheds possess inadequate data to examine.
24 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 13. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 1983 to 1992. 

Spatial gaps between subwatersheds possess inadequate data to examine.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Frequency of high water levels from 

1983 to 1992, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 25

Figure 14. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 1993 to 2002. 

Spatial gaps between subwatersheds possess inadequate data to examine.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Frequency of high water levels from 

1993 to 2002, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
26 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 15. Frequency of high groundwater levels within subwatersheds of the South Platte River alluvial aquifer from 2003 to 2012. 

Spatial gaps between subwatersheds possess inadequate data to examine.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Frequency of high water levels from 

2003 to 2012, in percent

0.0000000 to 20.000000

20.000001 to 40.000000

40.000001 to 60.000000

60.000001 to 80.000000

80.000001 to 100.000000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 27

During the fifth decade, 1993–2002, areas with moderate frequency of high groundwater levels were present 

intermittently along the corridor from Denver to La Salle 

(20–60 percent) (fig. 14). Three of the subwatershed regions 

between Greeley and Fort Morgan and 10 mi east of Sterling 

had pervasive high groundwater levels (80–100 percent).

During the sixth decade, 2003–2012, pumping administered by the State of Colorado was curtailed beginning 

around 2002–2003 (Waskom, 2013). Areas with moderate to pervasive frequencies of high groundwater levels 

(20–100 percent) were intermittent along the main stem of 

the South Platte River (fig. 15). Subwatersheds with moderate to pervasive high groundwater levels occurred in a region 

near Greeley, La Salle, and Gilcrest and up to 20 mi east and 

in a second cluster from about 20 mi east of Fort Morgan to 

Julesburg. The region with the overall highest frequencies of 

high groundwater levels was between Sterling and Julesburg. 

It bears noting, however, that in comparison to the previous 

decade (1993–2002), in areas that can be compared, high 

groundwater conditions are generally similar but exhibit some 

differences. For locations near Julesburg, the majority of subwatersheds show greater frequency of high groundwater levels 

compared to the previous decade.

Spatial Grouping of Subwatersheds by High 

Groundwater Levels

The identification of local patterns of spatial association 

is an important point of interest with respect to evaluating 

water levels in the South Platte River alluvial aquifer. Because 

high groundwater levels were a common occurrence in many 

subwatersheds, differences in high-water conditions between 

subwatersheds are not easily ascertained from direct visual 

inspection. Statistical methods can be used to methodically 

identify groups of subwatersheds where differences in high 

groundwater levels are statistically significant. The Local 

Moran’s I statistic was calculated for each subwatershed using 

the percent frequency of high groundwater levels as the evaluated variable. The Local Moran’s I method was used to identify statistical distinctions between regions (table 1). Two indicators form each classification (for example, High-High). If a 

subwatershed had neighboring features with similarly high or 

low attribute values, then the feature was part of a “cluster” of 

subwatersheds with relatively low (Low-Low) or high (HighHigh) values. A region was an “outlier” when attribute values 

were dissimilar to surrounding regions, resulting in different 

classification indicators (High-Low or Low-High). If relative 

differences fell below statistical criteria, as described in the 

methods section, results were indicated to be insignificant.

Local Moran’s I was used to indicate subwatershed areas 

with anomalously frequent or infrequent high groundwater 

levels at a scale of about 60 mi. Local Moran’s I is appropriate from a statistical perspective to evaluate regions in the 

choropleth map with greater than approximately 30 subwatersheds (features). Preliminary analyses were conducted 

to select the final search scale by maximizing the Z score, 

which revealed that the strongest distinctions in subwatershed 

groupings occurred at the regional scale. According to the Z 

score analysis, there was less than 1 percent likelihood that 

the observed regional patterns could have formed by random 

selection. Choropleth maps of spatial anomalies of frequency 

of high groundwater levels were evaluated chronologically 

per decade and for the complete period of record from 1953 

to 2012. After inspection, two time periods (1963–1972 and 

2003–2012) were identified as having sufficient data to provide reliable results, which were useful in comparing spatial 

characteristics across four decades. 

During the second decade, 1963–1972, four types of 

spatial anomalies were present, which can be categorized into 

four major spatial groups (fig. 16). In the first group, high 

outlier anomalies (High-Low) were present near and east of 

Greeley, La Salle, and Gilcrest and west of Fort Morgan. This 

group indicates areas where subwatersheds showed higher 

frequencies of high groundwater levels as compared to the 

majority of surrounding subwatersheds. In the second group, 

high clustered anomalies (High-High) extended from Sterling 

to Julesburg. This group indicates areas where subwatersheds show relatively high frequencies of high groundwater 

levels and the majority of surrounding subwatersheds have 

similar properties. In the third group, many of the tributaries 

south of the South Platte River had clustered low anomalies 

(Low-Low). This group indicates areas where subwatersheds 

show relatively low frequencies of high groundwater, similar 

to nearby watersheds. In the fourth group, areas along the 

main stem of the South Platte River between Denver, Greeley, 

and Sterling reveal insignificant results. The two main pockets 

of high statistical anomalies of high groundwater levels 

occurred near Greeley, La Salle, and Gilcrest and along the 

section from Sterling to Julesburg.

During the sixth decade 2003–2012, all four major types 

of spatial anomalies were once again present (fig. 17). Large 

scale patterns of statistical anomalies emerged that were 

similar, although not identical, to 1963–1972, where comparisons could be made. As in previous results, a few isolated 

High-Low statistical anomalies were present near Greeley, 

La Salle, and Gilcrest, although the main affected subwatersheds had extended more eastward toward Fort Morgan. 

Clustered High-High anomalies covered areas from Sterling 

to Julesburg in essentially the same manner as four decades 

prior in 1963–1972, but with greater continuity in coverage. 

Some areas near and west of Fort Morgan, however, that 

formerly showed insignificant relationships during 1963–1972 

changed to clustered Low-Low statistical anomalies, which 

are relatively low frequencies of high groundwater conditions 

as compared to other major sections of the aquifer.

Although some differences were identified between the 

1963–1972 and 2003–2012 periods, spatial anomalies at the 

60-mi scale were generally consistent. As a generalization, 

two dominant sections with high anomalous frequencies of 

high groundwater levels occurred intermittently between Greeley and Fort Morgan and continuously between Sterling and 
28 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Julesburg. Subwatersheds between Greeley and Fort Morgan 

showed relative “hot spots” of high groundwater conditions 

with lower frequencies of high groundwater in surrounding 

areas, while subwatersheds between Sterling and Julesburg 

were areas with pervasive high groundwater. Thus, there is a 

stronger likelihood for a subwatershed between Greeley and 

Fort Morgan to have larger differences in frequency of high 

groundwater in surrounding areas than a subwatershed located 

between Sterling and Julesburg. Most of the tributaries to the 

south of the South Platte River show low statistical anomalies, 

indicating infrequent high groundwater conditions. Subwatersheds between Fort Morgan and Sterling and near Gilcrest, 

La Salle, and Greeley show a mixture of frequencies of high 

groundwater that do not indicate relative highs or lows at a 

regional scale and were found statistically insignificant.

Change in Groundwater Levels

Changes in groundwater levels were examined chronologically per decade and for the complete period of record 

from 1953 to 2012. At frequently sampled wells, the ability to 

resolve decadal or multidecadal changes in groundwater levels 

was substantially improved. When a trend was determined, the 

data record was required to be sufficiently complete to confirm 

that trend predictions were representative of the time period 

examined, as described in the methods section. Trends of 

moderate to large magnitude were easier to resolve than trends 

of negligible to small magnitude and required less data to be 

statistically significant within assigned levels of confidence. 

The majority of wells have sparse data records that were not 

sufficient to estimate trends. Spatial coverage of wells with 

sufficient data records across the South Platte River alluvial 

aquifer was found to be irregular in the majority of decades 

considered, which caused uncertainty of conditions in some 

areas depending on the time period examined. Moreover, 

individual wells are “point” observations affected by local 

conditions that are not apparent on a regional scale. This rendered the cause of groundwater-level change at wells difficult 

to resolve in the absence of detailed local information. To 

average potential influences from individual wells, changes in 

groundwater levels were also examined within subwatersheds 

where acceptable data were available.

Wells

General statistics of wells and groundwater levels for the 

trend analyses are provided in table 2. Wells may be clustered 

spatially or positioned manually and non-uniquely at centers 

of map sections, which allows multiple wells with short 

separation distances to be represented at the same regional 

location. The number of individual wells eligible for trend 

analysis varies by decade from 7 to 256. For decades 1973–

1982 and 1983–1992, there are relatively few wells eligible 

for trend analysis (7 and 22, respectively), bringing to question 

the reliability of results for these periods. In other decades, 

there are at least 100 eligible wells to evaluate, and two 

decades (1963–1972 and 2003–2012) have substantially larger 

numbers of available wells (256 and 214, respectively). Up to 

39 percent of wells have validated trends at the decadal time 

scale, with the recent decade, 2003–2012, having the highest 

percentage of wells with significant trends. Overall, the majority of wells in each decade with sufficient data to assess trends 

do not show statistically significant trends in groundwater 

levels using conservative significance values.

Rates of groundwater-level change ranged from insignificant to a couple of feet per year over six decades of 

record. During the initial decade, 1953–1962, 84 percent (16 

of 19) of eligible wells with significant trends had declining 

groundwater levels (table 2, fig. 18). Rates of change in water 

levels varied between –2.9 and 3.0 ft/yr. Wells exhibiting 

Table 1. Statistics used to examine regional patterns of high groundwater levels.

[Statistically significant (0.05 level) classifications are described using two designations (for example, High-High). The first designation describes whether a 

subwatershed has relatively frequent (High) or infrequent (Low) occurrences of high groundwater levels. In areas where surrounding subwatersheds are similar 

to the subwatershed analyzed, the subwatershed is considered part of a “cluster” of subwatersheds and the two designations are equal. A subwatershed is an 

“outlier” where frequencies of high groundwater levels are dissimilar to surrounding subwatersheds and the two designations are opposed.]

Local Moran’s I

classifications

Frequent high groundwater levels in current 

subwatershed

High

Infrequent high groundwater levels in current 

subwatershed

Low

Frequent high groundwater levels 

in surrounding subwatersheds

High

High-High classification:

Cluster—high groundwater levels more 

frequent in current subwatershed and 

surrounding subwatersheds

Low-High classification:

Outlier—high groundwater levels less 

frequent in current subwatershed and more 

frequent in surrounding subwatersheds

Infrequent high groundwater 

levels in surrounding 

subwatersheds

Low

High-Low classification:

Outlier—high groundwater levels more 

frequent in current subwatershed and less 

frequent in surrounding subwatersheds

Low-Low classification:

Cluster—high groundwater levels less 

frequent in current subwatershed and 

surrounding subwatersheds
Analysis of Groundwater Levels 29

Figure 16. Groupings of subwatersheds using Local Moran’s I autocorrelation statistics to show regional patterns of high groundwater 

levels from 1963 to 1972. Spatial gaps between subwatersheds possess inadequate data to examine. (See table 1 for a description of the 

High and Low classifications.)

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Grouping of subwatersheds, 

1963–1972

Statistically insignificant

High-High

High-Low

Low-High

Low-Low

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
30 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 17. Groupings of subwatersheds using Local Moran’s I autocorrelation statistics to show regional patterns of high groundwater 

levels from 2003 to 2012. Spatial gaps between subwatersheds possess inadequate data to examine. (See table 1 for a description of the 

High and Low classifications.)

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Grouping of subwatersheds, 

2003–2012

Statistically insignificant

High-High

High-Low

Low-High

Low-Low

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 31

groundwater-level declines were located within tributaries of 

the South Platte River including Lost Creek, Kiowa Creek, 

Bijou Creek, Badger Creek, and Buck Creek. A cluster of 

three other wells showed groundwater rise near Lost Creek. 

No wells along the main stem of the South Platte River had 

significant trends during this decade.

During the second decade, 1963–1972, all (17 of 17) 

eligible wells with significant trends had declining groundwater 

levels (fig. 19). Trends in groundwater levels varied between 

–2.1 and –0.7 ft/yr. Wells with groundwater decline were found 

mainly within tributaries of the South Platte River including 

Box Elder Creek, Kiowa Creek, and Bijou Creek. Two other 

wells had declining groundwater levels near the South Platte 

River west of Fort Morgan near Empire Reservoir between 

Greeley and Fort Morgan, but they appear to be part of the 

cluster of wells with groundwater decline along the tributaries 

(for reference, major reservoirs are labeled in figure 1).

During the fifth decade, 1993–2002, only seven wells 

show groundwater decline; six wells are located along Box 

Elder Creek in a cluster pattern similar to past decades 

(1953–1962 and 1963–1972), and one well is located about 

5 mi southeast of Empire Reservoir along Kiowa Creek. Rates 

of change in groundwater levels varied between –2.4 and 

–0.7 ft/yr. 

During the recent decade, 2003–2012, 88 percent (73 

of 83) of eligible wells with significant trends show rising 

groundwater levels (fig. 20). Rates of change in groundwater levels varied between –1.2 and 2.8 ft/yr. Wells with 

groundwater decline were concentrated along Lost Creek and 

at locations a few miles east of Greeley and La Salle. The 

majority of remaining areas indicating groundwater rise were 

observed near Gilcrest and La Salle, along Box Elder Creek, 

west of Fort Morgan, and intermittently from Fort Morgan to 

Julesburg. The highest density of rising groundwater levels 

was observed near Gilcrest, La Salle, and Julesburg.

Over the complete 60-yr record, 1953–2012, about 

65 percent of wells with significant trends (32 of 49) had a 

significant trend. Of those wells, about 66 percent (21 of 32) 

had declining groundwater levels (fig. 21). Trends in groundwater levels varied between –1.2 and 0.44 ft/yr. Wells with 

the greatest groundwater decline were observed along Kiowa 

Creek and Bijou Creek. Other areas with groundwater decline 

occurred at an isolated number of locations along the South 

Platte River. Wells with the greatest groundwater rise were 

concentrated mainly along Box Elder Creek and Lost Creek.

Subwatersheds

Changes in groundwater levels were also determined 

over subwatersheds for the recent decade (2003–2012) and 

complete record (1953–2012) where there were sufficient data 

histories. Subwatershed results were combined with corresponding results from individual wells for direct comparison 

in figures 20 and 21. In cross-validating the results, there is 

agreement between trend analyses at individual wells and 

those taken over subwatershed areas. In certain instances, 

a well had a trend in groundwater levels, but the watershed 

where it was located showed no significant trend, or vice 

versa. This was not considered a disagreement, but rather 

a difference between a subwatershed (area) response and a 

well (“point”) response within the subwatershed or a difference in data availability. Although trend magnitudes differ 

between individual wells and subwatersheds, there were only 

2 of 83 wells (2 percent) during 2003–2012 and 0 of 32 wells 

(0 percent) during 1953–2012 where a disagreement in trend 

sign (positive or negative) was identified. Areas of disagreement, while rare, occurred near Empire Reservoir and Riverside Reservoir (fig. 20). 

In the recent decade 2003–2012, groundwater levels 

rose in about 81 percent (26 of 32) of subwatersheds, mainly 

Table 2. Summary of well statistics for groundwater-level trend analysis.

Well statistics 1953–1962 1963–1972 1973–1982 1983–1992 1993–2002 2003–2012 1953–2012

Number of available wells 

(all types)

354 1,203 1,125 317 382 572 1,669

Number of eligible wells with 

sufficient data for trend 

analyses

111 256 7 22 142 214 49

Number of wells with a trend 19 17 0 5 7 83 32

Number of wells with rising 

water levels

3 0 0 0 0 73 11

Number of wells with declining 

water levels

16 17 0 5 7 10 21

Percentage of wells with trend 17.1 6.6 0.0 22.7 4.9 38.8 65.3
32 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 18. Groundwater-level change (trend) at wells with acceptable records from 1953 to 1962. Positive values indicate rising water 

levels and negative values indicate declining water levels.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Rate of groundwater-level change in wells 

from 1953 to 1962, in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 33

Figure 19. Groundwater-level change (trend) at wells with acceptable records from 1963 to 1972. Positive values indicate rising water 

levels and negative values indicate declining water levels.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Rate of groundwater-level change in wells 

from 1953 to 1962, in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
34 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 20. Groundwater-level change (trend) at individual wells and effective change per subwatershed from 2003 to 2012. Positive 

values indicate rising water levels and negative values indicate declining water levels. Spatial gaps between subwatersheds possess 

inadequate data to examine.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial 

aquifer

Rate of groundwater-level change 

in wells from 2003 to 2012, 

in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

Rate of groundwater-level change

for subwatersheds from 2003 to 

2012, in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 35

focused in areas near the South Platte River (fig. 20). The 

majority of tributaries south of the main stem (Lost Creek, 

Kiowa Creek, and Bijou Creek) with significant trends had 

groundwater declines. Areas near Greeley, La Salle, and Gilcrest, along Box Elder Creek, and west of Fort Morgan along 

the main stem had the most pronounced rise in groundwater 

levels. Other areas of groundwater-level rise occurred over 

a section extending from Sterling to about 25 mi southwest 

of Fort Morgan and at Julesburg. Of these locations, areas 

near Greeley, La Salle, Gilcrest, Sterling, and Julesburg have 

recently shown frequent shallow groundwater conditions. 

Nearly all subwatersheds near the South Platte River with 

identifiable trends reflected rising groundwater levels. 

Over the complete 60-yr record, there has been a mixture 

of areas where groundwater levels were rising or declining 

(fig. 21). The areas showing greatest groundwater decline 

occurred south of the South Platte River along tributaries 

(Kiowa Creek and Bijou Creek). Areas near Gilcrest, La Salle, 

and Fort Morgan and to a lesser extent in small pockets east 

of Fort Morgan along the main stem of the river also exhibited 

groundwater decline, generally less than 0.125 ft/yr. The main 

areas of groundwater rise generally were less than 0.125 ft/yr 

in magnitude between Fort Morgan and Sterling and to a lesser 

extent in small pockets between Sterling and Julesburg and 

along some tributaries. Overall, less than half of the watersheds intersecting the South Platte River indicate significant 

trends in groundwater levels over the complete period of 

record. In reference to the 70 subwatersheds examined across 

the aquifer with sufficient records, 15 subwatersheds had 

rising groundwater levels and 20 subwatersheds had declining groundwater levels. This amounts to about 57 percent 

(20 of 35) of subwatersheds with trends that had declining 

groundwater levels. The majority of subwatersheds west of 

Fort Morgan had declining water levels, while the majority 

of subwatersheds east of Fort Morgan had rising groundwater 

levels, implying there was a redistribution of groundwater 

from upgradient to downgradient sections of the aquifer over 

the last several decades that was spatially variable. The results 

also demonstrate that changes in groundwater levels in the 

recent decade had disparities to the complete 60-yr record 

(figs. 20–21).

Net differences in average groundwater levels and 

standard deviation of groundwater levels were determined in 

subwatersheds with sufficient data coverage for the decadal 

periods 1963–1972 and 2003–2012 (figs. 22–23). Decadal 

differences of less than 0.5 ft were considered moderately 

constant and were differentiated from gains or losses of 

greater magnitudes. When determining differences in standard 

deviations of groundwater levels, the data were first detrended 

to limit trends from altering the results; differences in averages 

used the original dataset. Between 1963–1972 and 2003–2012, 

average groundwater levels rose in areas north of Sterling to 

Julesburg, in areas around Fort Morgan, and in areas just east 

of La Salle and extending south along the Box Elder Creek 

and Lost Creek tributaries (fig. 22). Average groundwater 

levels declined between these periods over regions near and 

about 20 mi west of Fort Morgan, extending south along the 

Kiowa and Bijou Creek tributaries; near and about 10 mi south 

of Sterling; and along the corridor from Denver to La Salle. 

Overall, about 58 percent of subwatersheds show a net decline 

in average groundwater levels, which is generally consistent 

with predicted trends over the 60-yr record from 1953–2012, 

where 57 percent of subwatersheds and 66 percent of individual wells with sufficient data coverage show groundwater 

declines. There were more declining water levels west of Fort 

Morgan and more rising water levels east of Fort Morgan. 

Regionally, the pattern implies that groundwater has been 

redistributed from upstream to lower downstream sections of 

the aquifer.

In terms of differences in decadal standard deviation, 

most areas beginning from about 10 mi west of Fort Morgan 

and extending to Julesburg indicate that variability in groundwater levels amplified between 1963–1972 and 2003–2012 

(fig. 23). Upon initial inspection, the increase in variability of 

groundwater levels was not explained by changes in precipitation or river discharge, however both decreased in variability 

between periods.

Correlation of System Attributes to High 

Groundwater Levels

To explore potential causes of high groundwater conditions in the South Platte River alluvial aquifer, correlations 

were examined between the frequency of high groundwater 

levels observed in wells with GPS locations used for observing 

groundwater levels over the recent decade (2003–2012) and 

system features that include precipitation and river discharge, 

geographic characteristics, and attributes of administrative 

structures. In the case of geographic characteristics and administrative structures, only the most recent decade (2003–2012) 

was selected for analysis primarily because high groundwater 

levels were relatively frequent and secondly because resulting 

correlations, available GIS coverages, and diversion records 

are most representative of current conditions. Correlations 

between system attributes and high groundwater levels were 

used to identify potential controls on high groundwater levels, 

which may not be analogous to identifying direct causes of 

high groundwater levels.
36 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 21. Groundwater-level change (trend) at individual wells and effective change per subwatershed in areas for the complete 

period of record from 1953 to 2012. Positive values indicate rising water levels and negative values indicate declining water levels. 

Spatial gaps between subwatersheds possess inadequate data to examine.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial 

aquifer

Rate of groundwater-level change 

in wells from 1953 to 2012, 

in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

Rate of groundwater-level change

for subwatersheds from 1953 to

 2012, in feet per year

-3.000 to -1.001

-1.000 to -0.501

-0.500 to -0.251

-0.250 to -0.126

-0.125 to -0.002

Statistically insignificant

0.002 to 0.125

0.126 to 0.250

0.251 to 0.500

0.501 to 1.000

1.001 to 3.000

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 37

Figure 22. Net difference in average groundwater levels per subwatershed between decades 1963–1972 and 2003–2012.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Difference in average groundwater levels 

from 1963–1972 to 2003–2012

Increase greater than 0.5 feet

Change between -0.5 to 0.5 feet

Decrease less than -0.5 feet

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
38 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 23. Net difference in variability (standard deviation) of groundwater levels per subwatershed between decades 1963–1972 and 

2003–2012.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Difference in standard deviation of 

detrended groundwater levels 

from 1963–1972 to 2003–2012

Increase greater than 0.5 feet

Change between -0.5 to 0.5 feet

Decrease less than -0.5 feet

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Analysis of Groundwater Levels 39

Precipitation, River Discharge, and High 

Groundwater Levels

Time series of river discharge and precipitation were used 

to identify relations between climatic and hydrologic conditions over the 60-yr record using water years following USGS 

convention (October 1–September 30) (fig. 2). For those 

correlations between precipitation and river discharge that 

are significant (p-values less than or equal to 0.05), correlation coefficients are about 0.6 annually and 0.8 over decadal 

periods, which is considered high for natural data. Significant 

correlations between discharge and percent frequency of high 

groundwater levels have correlation coefficients that are about 

0.4 annually and 0.6 over decadal periods. The results imply 

that periods of high river flow were often coincident with high 

groundwater conditions. Correlation coefficients between 

precipitation and percent frequency of high groundwater levels 

range from about 0.2 to 0.6 but are statistically insignificant on 

both annual and decadal time scales, which implies that other 

factors influence high groundwater levels in the region. 

A pattern reversal occurred between decadal periods 

1993–2002 and 2003–2012, where although average precipitation was fairly constant and river discharge decreased 

between decades, there was a distinct increase in decadal 

frequency of high groundwater levels, which followed the 

curtailment of pumping from wells without augmentation 

plans beginning in 2002–2003 (figs. 2, 6B, C). Frequencies 

of high groundwater levels were examined subannually over 

quarter water years (October–December, January–March, 

April–June, and July–September) following USGS convention (October 1–September 30) and compared to average 

river discharge to identify subannual relations across the 

60-yr record (fig. 24). Confidence intervals of two standard 

deviations were determined per decade using a bootstrap 

method (Efron, 1982). The analysis showed that peak river 

discharge from April to June did not correlate to the period 

with greatest frequency of high groundwater levels. In 5 of 

the 6 decades examined, excluding 1973–1982, peaks of high 

groundwater levels occurred from July to September, after 

spring runoff. There were greater differences in high groundwater levels between April–June and July–September in early 

decades (1953–1962 and 1963–1972) as compared to recent 

decades (1983–1992, 1993–2002, and 2003–2012). During 

2003–2012, following curtailment of pumping of wells by 

the State, the highest frequency of high groundwater levels 

occurred between July and September, a common decadal 

pattern identified over the last 60 yr. High groundwater levels 

were observed during all periods examined but were highly 

variable, ranging between approximately 10 and 40 percent of 

observations over quarter-year durations.

The effects of time lags, which can include a delayed 

exchange of water between the river and the alluvial aquifer, were not considered in the analysis. Previous analyses 

indicate that flow and direction of alluvial exchange for the 

South Platte River region varies by river section, time of 

year, and rate of discharge (Sjodin and others, 2001). During 

1983–1993, the exchange of water from the river to the alluvial aquifer as a proportion of total flow was estimated seasonally to be between about 3 and 8 percent from April to June 

and near 3 percent from July to September (Sjodin and others, 

2001). Equivalent or in some cases greater bank storage during April–June does not clearly explain peak high groundwater 

levels observed from July to September.

Geographic Characteristics

Correlation coefficients were evaluated between high 

groundwater levels observed in wells and geographic attributes that pertain to well position, corresponding subwatershed, and distance from surface-water features. The attributes 

describing location are (a) land surface elevation at a well, 

(b) subwatershed area, (c) maximum elevation in a subwatershed, (d) minimum elevation in a subwatershed, (e) average elevation in a subwatershed, (f) range of elevations in a 

subwatershed, (g) standard deviation of elevations in a subwatershed, (h) relative elevation of a well at the land surface 

within the surrounding subwatershed (that is, relative position 

between the lowest and highest elevations over an interval of 

0 to 1, respectively), and (i) aquifer thickness at a well. The 

attributes for examining distances from wells to surface-water 

features include (j) closest distance from a ditch, (k) closest 

distance from a major tributary, and (l) closest distance from 

the South Platte River. 

Of the 12 geographic attributes evaluated, 8 attributes 

showed statistically significant correlations to frequency 

of high groundwater levels (fig. 25). The majority of attributes with significant correlations (6 of 8) are measures of 

land-surface elevation. The strongest negative correlation 

coefficient is for land-surface elevation at a well, implying 

that wells higher in elevation within the aquifer system more 

often exhibit lower frequencies of high groundwater levels, 

and vice versa. The majority of statistics used to describe land 

surface elevation (minimum, maximum, average, and relative elevation) have a negative correlation to frequency of 

high groundwater levels. In terms of the minimum, average, 

and maximum elevations, which show similar magnitudes 

of correlation coefficients, there is little distinction between 

which measure is used as the representative elevation in a 

subwatershed. This was interpreted to be due to the tendency 

for average elevation differences between subwatersheds to 

be greater than elevation differences within subwatersheds, 

on average. The negative correlation for relative elevation 

of a well within a subwatershed implies that topographic 

depressions are expected to have greater frequencies of high 

groundwater levels. The only measure of elevation with a 

positive correlation to high groundwater levels is the standard 

deviation of elevation within a subwatershed, which suggests 

that subwatersheds with greater topographic variation show 

higher frequencies of high groundwater levels, although the 

correlation coefficient magnitude (relation) is small. This 

might result from the channeling of water toward drainages 

in the subwatershed, causing depression-focused recharge, 
40 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

as compared to a subwatershed with flat topography where 

water would be more evenly dispersed. The effect of elevation 

change is not merely differences between high and low landsurface elevations, because the range (maximum to minimum) 

of elevations in a subwatershed was shown to be statistically 

insignificant. In terms of surface water, wells located closer to 

the South Platte River had a greater frequency of high groundwater levels, while those closer to tributaries of the South 

Platte River had a lower frequency of high groundwater levels, 

on average.

Figure 24. Quarterly summary of high groundwater levels and river discharge per decade. A, Percentage of observations with 

high groundwater levels per quarter water year by decade. The brackets represent confidence intervals of two standard deviations. 

B, Corresponding river discharge averaged monthly from five gaging stations over the period of record 1953–2012. (See fig. 1 for station 

numbers and locations.)

Administrative Structures

Correlations were also identified between high groundwater levels observed in wells and attributes of administrative 

structures. It is important to conceptualize that administrative 

structures that leak water to the aquifer might not be directly 

associated to observations of high groundwater levels. A 

monitoring well, for instance, might be located upgradient 

from a leaking administrative structure or within an area near 

leaking administrative structures that was not affected by 

B

EXPLANATION

1953–1962

1963–1972

1973–1982

1983–1992

1993–2002

2003–2012

A

0

200

400

600

800

1,000

1,200

1,400

Quarter water year

0

10

20

30

40

50

October–December January–March April–June July–September

October–December January–March April–June July–September

Quarter water year by decade

South Platte River discharge, in cubic feet per second High groundwater levels, in percent

Average discharge of five gaging sites, 1953–2012

EXPLANATION
Analysis of Groundwater Levels 41

high groundwater levels. The analysis of structure attributes 

was designed to identify a search distance (radius) from wells 

with GPS locations used to observe groundwater levels that 

yielded the largest correlation to high groundwater levels, up 

to a maximum distance of 20 mi. The procedure provided a 

measure of the distance of influence of multiple structure attributes in addition to the corresponding correlation coefficient 

value. The examined administrative structures are (a) ditches, 

(b) engineered recharge areas, (c) reservoirs, (d) wells, 

(e) well fields, and (f) impacted river reaches. Structure type 

names reflect the physical structure in the aquifer, except for 

impacted river reaches, which are sections of the river where 

depletions and accretions occur from water diversions and 

where owed water may be repaid to the river. The following 

measures were determined for each type of diversion structure: 

(1) closest distance from a structure, (2) decree rate of closest structure, (3) number of structures in a search radius, and 

(4) average decree rate from structures within a search radius. 

In total, the analysis included 24 structure attributes for which 

correlations to high groundwater levels were examined. 

Of the 24 attributes of administrative structures selected 

for examination, 19 attributes show statistically significant 

maximum correlations (fig. 26). Generally, correlation coefficients are moderate in magnitude with a range of 0.3 to 0.7. 

The four greatest positive correlations are associated with 

the number of impacted river reaches, number of engineered 

Figure 25. Correlation coefficients between geographic attributes at well locations and frequency of observed high groundwater 

levels. The brackets represent confidence intervals of two standard deviations.

recharge areas, closest distance from a well field, and average decree of wells. The results indicate that areas with more 

impacted river reaches or engineered recharge areas experienced a higher frequency of high groundwater levels as 

a cumulative influence. Locations closer to pumping wells 

or well fields tend to experience lower frequencies of high 

groundwater levels because of local drawdown of the water 

table. Wells or well fields with higher decreed pumping rates, 

however, tend to be located in areas of the aquifer with higher 

frequencies of high groundwater levels.

Other significant positive correlations are associated with 

reservoirs and ditches but at smaller (small to moderate) magnitudes. At locations where nearby reservoirs have relatively 

high decree rates or where average decree rates are larger 

over the search area, there tend to be greater frequencies of 

observed high groundwater levels, implying that groundwater 

levels can be affected by water that is leaked or released by 

reservoirs. The relations for ditches are sensitive to the number 

of structures and decree rate, implying that some ditches lose 

water to the aquifer, causing groundwater levels to rise, but the 

effect in part is cumulative to multiple decreed ditch locations 

in an area. In addition to engineered recharge areas, structures 

with high decree rates are positively correlated to areas with 

high groundwater levels. 

For other relations, the greatest negative correlations to 

high groundwater levels occur for number of wells and well 

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

Elevation of well at land surface

Average land-surface elevation in subwatershed

Minimum land-surface elevation in subwatershed

Position (low to high) of well in subwatershed

Maximum land-surface elevation in subwatershed

Distance of well from South Platte River

Variability of land-surface elevation in subwatershed

Distance of well from tributary

Correlation of geographic attribute to frequency of high groundwater levels
42 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

fields and distance from an engineered recharge area. At observation locations where there are more wells or well fields in 

the defined search area, the cumulative effect may be to lower 

groundwater levels, while at locations nearer to engineered 

recharge areas that release water, there tend to be higher 

groundwater levels. Decree rates of engineered recharge areas 

that reflect capacity of storage are negatively correlated to frequency of high groundwater levels. The negative correlation 

could be the result of differences between decreed capacities 

and actual volumes of water that drain to the aquifer but were 

thought to be caused by regional influences. 

Corresponding search distances (radii) that provided the 

maximum correlations to high groundwater levels indicate 

whether influences were local or regional (fig. 27). Minimum 

search distances from wells where correlation coefficients 

could be resolved statistically vary for each attribute but 

Figure 26. Correlation 

coefficients between attributes 

of administrative structures 

and frequency of observed high 

groundwater levels in wells at 

optimized search distances. The 

brackets represent confidence 

intervals of two standard 

deviations.

average to about 1 mi. Although correlations can be calculated 

in every case, generalizing representative correlation coefficients over shorter search distances is expected to require 

additional data and improved location accuracies, perhaps 

attainable from small-scale pilot studies. Overall, search 

distances that correspond to maximum correlation coefficients 

ranged from less than 1 mi to about 18 mi. Particularly for 

search distances of many miles, correlation coefficients might 

be reflective of regional averages or diffuse influences from 

multiple surrounding structures. Well attributes and number 

of engineered recharge areas and impacted river reaches had 

the shortest ranges of maximum correlation to high groundwater levels of the attributes examined. Number of reservoirs 

and well fields had some of the longest ranges of correlation, 

between 16 and 18 mi, respectively. Both the distance from 

a recharge area and average decree of recharge areas also 

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

Number of wells

Number of well fields

Distance from recharge area

Distance from ditch

Decree of closest recharge area

Number of reservoirs

Average decree of recharge areas

Average decree of ditches

Number of ditches

Average decree of reservoirs

Decree of closest reservoir

Distance from impacted river reach

Average decree of well fields

Decree of closest well

Distance from well

Average decree of wells

Distance from well field

Number of recharge areas

Number of impacted river reaches

Maximum correlation of attributes of administrative structures

to frequency of high groundwater levels
Analysis of Groundwater Levels 43

Figure 27. Optimized search distances yielding maximum correlations between attributes of administrative structures 

and frequency of observed high groundwater levels in wells. The minimum search distance is the scale where sufficient 

data was available to determine correlations.

Minimum distance evaluated

Distance of maximum correlation

EXPLANATION

0 5 10 15 20

Number of well fields

Number of reservoirs

Average decree of recharge areas

Distance from recharge area

Decree of closest recharge area

Average decree of ditches

Average decree of well fields

Distance from ditch

Number of ditches

Distance from well field

Average decree of reservoirs

Decree of closest reservoir

Distance from impacted river reach

Number of impacted river reaches

Number of recharge areas

Distance from well

Number of wells

Decree of closest well

Average decree of wells

Search distance yielding maximum correlation of attributes of administrative structures to

frequency of high groundwater levels, in miles 
44 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

revealed long ranges of correlation, between 15 and 17 mi. 

Although the number of surrounding recharge areas was 

shown to affect high groundwater levels locally, visual inspection indicates that recharge areas were not always located in 

areas with high groundwater levels. In general, the largest 

correlations to high groundwater levels occurred over small 

ranges of correlations, while correlations over long ranges 

were low to moderate. A large break in maximum correlation scale occurred between the distance from well field and 

number of ditches, between 4 and 8 mi, which perhaps distinguishes local from regional influences. Analysis shows that 

administrative structures can be associated with high groundwater levels in wells over distances between less than 1 mi 

to nearly 20 mi, specific to the type of structure and measure 

examined.

Possible Solutions to Address Rising and 

Declining Water Levels

Regardless of the cause(s) of observed long-term change 

in groundwater levels, greater knowledge and control of 

aquifer conditions would allow for improved water management and more efficient use of water resources. One solution 

would be to update existing groundwater-level monitoring by 

using real-time recordings and to develop a set of contingency 

procedures of water administration and allocation based on 

present aquifer conditions. Alternatively, large water storage 

facilities could be built to better regulate groundwater levels 

and allow increased water use. Diversions could be constructed at the downstream end of the aquifer near Julesburg to 

return excess water back upstream for recirculation or to transport water downstream across the State line to Nebraska if the 

resource were dispensable. Pumping wells help to remediate 

high groundwater levels by removing water from the aquifer 

and could be an important administrative tool to lower groundwater levels in desired areas in the future. Implementing these 

tools would enable more precise water management, provide 

real-time management decisions using real-time monitoring, 

and offer additional administrative structures at key locations 

for greater flexibility in distributing and storing water.

It is also important to understand that management 

decisions require high-quality and easily accessible data. 

Providing a quality, continuous data record at key well locations would improve characterization of groundwater levels 

and inform decisions that could allow more efficient use of 

Colorado’s water.

Monitoring Well Network

Groundwater-level measurements from monitoring 

wells are a primary source of information to evaluate hydrologic conditions in an aquifer. Measurements can be used to 

identify changes to groundwater levels affected by natural and 

human processes. Long-term, systematic measurements of 

water levels provide essential data that are critical to evaluate changes in the aquifer over time, to develop and calibrate groundwater models, to forecast trends, and to design, 

implement, and monitor the effectiveness of groundwater 

management and resource protection programs (Advisory 

Committee on Water Information, 2009).

Projects that measure water levels orchestrated by 

multiple agencies can involve different objectives, monitoring designs, protocols, and reporting requirements. In some 

instances, wells used as observation sites are not fully devoted 

to monitoring groundwater-levels and reflect, at least in part, 

other influences such as local domestic water use or irrigation. 

In other instances, wells are devoted primarily to monitoring 

groundwater levels and instrumented with transducers for high 

precision measurements. Although groundwater levels in the 

South Platte River alluvial aquifer are monitored by several 

agencies, there is no unified and dedicated monitoring network 

at present sufficient to characterize areas of shallow groundwater or the water table surface (potentiometric surface), and 

there is no expansive network in place that targets the influence of administrative structures on groundwater levels or 

areas of hydrologic interest. In aggregate, the existing well 

networks do not have unifying objectives or reporting requirements needed for a comprehensive aquifer monitoring plan. 

A regional groundwater-level monitoring network for aquiferscale characterization might include examining influences 

from (a) administrative structures and (b) areas of hydrologic 

interest with changing groundwater levels and frequent high 

groundwater levels. The network could provide a foundation for informed decision making and hydrologic analysis in 

future studies performed through partnerships at the Federal, 

State, and local levels.

Benefits of a Monitoring Network

Monitoring wells are designated specifically for the 

collection of groundwater-level and (or) water-quality data. 

Decisions made about the quantity and locations of monitoring 

wells are crucial to any data collection program. Ideally, wells 

chosen for a monitoring network will provide data that are 

representative of various physiographic and land-use environments. The primary purposes of a groundwater-level monitoring network are to measure (1) ambient groundwater conditions (effects of natural, climatic related hydrologic stresses) 

and (2) other influences on the aquifer often related to human 

activities. Groundwater-level monitoring programs for complex or multilayer aquifer systems might also require measurements in wells completed at multiple depths and perhaps in 

different geologic units.

To successfully manage groundwater resources and 

ensure effective planning, an understanding of the processes 

and properties of the groundwater systems is required, including detailed information on groundwater levels because 

groundwater-level measurements are the only direct measure 

available to evaluate aquifer hydraulic conditions. A rise in 

groundwater levels indicates greater water stored within the 
Monitoring Well Network 45

pore space of an aquifer, and declines in water levels indicate 

decreased quantities of water in storage. Uses of groundwaterlevel monitoring data are critical to evaluate (a) changes in 

groundwater recharge and storage, (b) effects from climate 

variability (for example, floods or droughts), (c) groundwaterlevel (potentiometric) surface of the water table or confined 

aquifers, (d) alterations in groundwater flow directions, 

(e) interactions between groundwater and surface water, and 

(f) groundwater flow control on contaminant transport (Taylor 

and Alley, 2001; Advisory Committee on Water Information, 

2009).

Monitoring Network Design Components

The design of the proposed monitoring network is 

described using terminology and definitions proposed by the 

Subcommittee on Ground Water (SOGW) (Advisory Committee on Water Information, 2009). The SOGW was founded in 

2007 by the Federal Advisory Committee on Water Information to develop a framework that establishes and encourages 

implementation of long-term groundwater assessments. The 

major components of a monitoring network are classified 

as “unstressed” and “targeted.” Within these groupings are 

subcomponents to develop baseline historical data records 

(“baseline monitoring”), synoptic assessments (“surveillance monitoring”), and water-level trend monitoring (“trend 

monitoring”). Further technical considerations applicable to 

the design of a groundwater-level monitoring network are discussed in several technical documents (Winter, 1972; Heath, 

1976; Taylor and Alley, 2001; Advisory Committee on Water 

Information, 2009).

Unstressed Subnetwork

The unstressed component of a monitoring network 

includes monitoring wells that provide data from unstressed 

(or least stressed) parts of an aquifer. Under optimal protocols and design, the unstressed subnetwork ensures that a 

consistent group of wells is regularly monitored to generate 

water-level data from areas of the aquifer reflecting ambient 

conditions. It is, however, expected that total network-wide 

isolation from land use, diversions, and development is not 

possible. In practice, “unstressed” regions are those that either 

have limited stress or have been least affected by human 

activities. 

Targeted Subnetwork

The targeted component of a monitoring network 

includes monitoring points that provide data from aquifers 

that are affected by human activities of some form. This 

includes areas that are known to be heavily pumped or to have 

undergone substantial land-use change or areas with managed groundwater resources. Effects of managed groundwater 

resources include artificial recharge or enhanced storage and 

recovery. The targeted subnetwork also includes monitoring 

points in an area expected to be developed. 

Baseline Monitoring

In the event that historical records do not exist, then an 

initial baseline monitoring period for up to 5 years is recommended for new monitoring wells to define hydrologic conditions and to account for natural variability. Once baseline data 

are available, data should be reviewed to determine whether 

the monitoring well should be assigned to the surveillance or 

trend monitoring classifications, or whether the baseline phase 

should be extended. When baseline monitoring is completed, 

wells are available for surveillance and trend monitoring. Over 

time, as conditions change, wells should be critically evaluated 

to ensure they are assigned to the proper subnetwork.

Surveillance Monitoring

Surveillance monitoring is used to periodically report on 

the overall water-level conditions in the aquifer at points in 

time. It might not be possible to regularly monitor all surveillance wells because of cost limitations, but an aquifer census 

could be taken in a rotating program over different areas. Over 

time, surveillance monitoring can be thought of as a series 

of discrete snapshots of aquifer conditions. The frequency 

of surveillance monitoring generally is much less than trend 

monitoring. 

Trend Monitoring

Trend monitoring requires frequent water-level measurements for a manageable number of wells given budgetary 

constraints and aquifer requirements. A subset of the wells 

used for trend analyses of groundwater levels are designated 

as the “backbone” of the monitoring network. These sites are 

carefully selected and are fully supported for continued data 

collection over the duration of the program. Every consideration must be given to continuing the long-term record from 

the “backbone” of the monitoring network for a continuous 

historical record. Measurement frequencies for trend monitoring must be appropriate to determine long-term trends and 

seasonal variability in water levels at selected locations. 

South Platte River Alluvial Aquifer Network 

Design

The proposed water-level monitoring network for the 

South Platte River alluvial aquifer consists of three subnetworks: an unstressed subnetwork and two targeted subnetworks. The unstressed subnetwork will focus on defining 

baseline conditions and performing surveillance and trend 

monitoring. One of the targeted subnetworks will focus on 

potential locations where administrative structures might 
46 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

affect water levels (structural target). Another targeted subnetwork will focus on areas of notable “high water” conditions or 

those with appreciable changes in groundwater levels occurring over the recent decade (2003–2012) (hydrologic target). 

Subnetworks should include a subset of available monitoring wells sufficient for the intended purpose. Well sites best 

suited for trend analyses include those with water-level data 

recorded on at least a seasonal frequency. Subnetworks might 

be composed of wells managed by different agencies that 

will require shared strategies and guidelines for data collection. The term “network of networks” can be used to describe 

combining well networks of different agencies operated over 

smaller areas to form an inclusive network (Advisory Committee on Water Information, 2009). Small-scale pilot studies 

in Gilcrest, La Salle, Sterling, and other areas along the 

South Platte River alluvial aquifer are being conducted by the 

Colorado Division of Water Resources and other agencies and 

would complement the targeted subnetworks. 

Frequency of Groundwater-Level Measurements 

The frequency of groundwater-level measurements is 

among the most important components of a groundwater-level 

monitoring program (Taylor and Alley, 2001; Advisory Committee on Water Information, 2009). Although often influenced 

by economic constraints, the frequency of measurements 

should be determined according to the anticipated variability 

of groundwater-level fluctuations in the monitoring wells and 

the data resolution or degree of detail needed to fully characterize the hydrologic behavior of the aquifer. Systematic, 

long-term collection of groundwater-level data offers the 

greatest likelihood that groundwater-level fluctuations caused 

by variations in climatic conditions and groundwater-level 

trends caused by changes in land-use or water-management 

practices will be sampled. Moreover, long-term groundwaterlevel records greatly enhance the ability to forecast future 

water levels. Multiple factors considered for the South Platte 

River alluvial aquifer point to the need for frequent groundwater-level measurements (fig. 28). For one, the aquifer is 

unconfined in most locations and composed mainly of permeable sediment with moderate to large variations in thickness 

(fig. 3). There are also some areas with clay and silt lenses 

that, if laterally continuous enough, could impede groundwater flow and cause local aquifer conditions to become 

semi-confined; it might be important to have sufficient spatial 

sampling to distinguish these areas. Another factor is that Colorado is projected to be heavily influenced by climate change 

as compared to other States far removed from a climate 

transition zone (Milly and others, 2008). There are also local 

societal influences that could affect aquifer conditions; diversions in operation have the potential to alter the local hydrologic regime. In all, conditions examined for the South Platte 

River alluvial aquifer call for a more frequent data collection 

program. It is suggested that all monitoring wells selected for 

trend monitoring be instrumented with continuous recorders 

that record water levels at least on a 4-hour frequency and 

that accompanying surveillance monitoring in the unstressed 

network be performed at least on a seasonal frequency. Some 

real-time monitoring would allow a proactive approach to 

water management. Monitoring of targeted subnetworks will 

be dependent on the intended application.

Monitoring Well Candidates

Wells considered for inclusion in the proposed waterlevel monitoring network in the South Platte River alluvial 

aquifer are dedicated for monitoring purposes according to 

State and Federal data records or were stated as such in personal communication with members of the Colorado House 

Bill 12-1278 working group. The goal is to use dedicated 

monitoring wells to reduce local influences on the water table 

such as pumping or artificial recharge. The total pool of monitoring well candidates consists of 396 wells (fig. 29). This 

candidate pool includes about 30 wells that were not analyzed 

in this study because of issues of data availability but were 

considered as potential candidates for the proposed monitoring 

network.

A breakdown of well ownership by agency is listed in 

table 3 (“Source Agency” column) along with available candidate monitoring wells from each source agency (“Available 

monitoring wells” column). The SPDSS network, managed 

by the Colorado Division of Water Resources, is a large, 

established water-level collection effort in the South Platte 

River alluvial aquifer that provides data and tools to support 

informed decisions on issues related to water resources. The 

SPDSS network contains 37 monitoring wells considered in 

this investigation. The arrangement of the SPDSS wells is 

not optimized for aquifer-scale evaluation because several 

wells are clustered in proximity to one another at the expense 

of large spatial gaps extending over large areas. The USGS 

National Water-Quality Assessment (NAWQA) network 

contains 23 monitoring wells dispersed along the South Platte 

River and complements SPDSS well locations in the majority 

of instances. Both the SPDSS and USGS NAWQA networks 

have a known construction history, generally produce reliable results, and have data collection programs in place. For 

these reasons, the two networks were considered for the trend 

monitoring network.

Other major contributions to the candidate pool of monitoring wells come from conservancy districts. The Central 

Colorado Water Conservancy District (CCWCD) has 15 monitoring wells considered for the network, and the Lower South 

Platte Water Conservancy District (LSPWCD) has 45 wells. 

Additional monitoring wells considered for the proposed network include 15 wells managed by the Colorado Department 

of Agriculture (CDA) with recent data records, 2 wells as part 

of the CSU network, 33 wells with miscellaneous ownership 

(other), and 7 tentative wells without a known data history. 

Many of the monitoring wells considered for the proposed 

monitoring network are used regularly by managing agencies, and the level of reliance is considered good. A portion 

of the identified USGS monitoring wells apparently have not 
Monitoring Well Network 47

been visited for several years. For those selected as part of 

the monitoring network, future field assessment is advised. 

Other wells with miscellaneous ownership and those denoted 

as tentative possess the greatest uncertainty of accessibility 

and suitability. The nearest suitable monitoring well from the 

remaining candidate pool is suggested if a replacement well is 

necessary. The suitability of the proposed monitoring network 

should be evaluated by direct field reconnaissance and future 

committee discussions as part of the implementation phase.

Monitoring-Well Network Optimization

The initial step in optimizing the monitoring network 

design was to approximate and detrend a depth to water 

surface of the South Platte River alluvial aquifer. Ordinary 

kriging assumes that nonstationary artifacts (trends) in the data 

have been removed in order for the underlying mathematical 

assumptions to remain valid (Isaaks and Srivastava, 1989). 

The complete groundwater-level dataset with records during 1953–2012 consisting of 1,669 wells (exhaustive dataset) 

was evaluated to approximate a representative depth to water 

surface across the South Platte River alluvial aquifer (fig. 5). 

Broad averaging was viewed as a practical option in this 

instance given that monitoring wells with different periods 

of records were considered in the analysis and hydrologic 

stresses changed over time. Time series of depths to groundwater level were smoothed using locally weighted scatterplot 

smoothing and averaged at each well location (Helsel and 

Hirsch, 2002). The best-fit exponential variogram model 

indicates a correlation scale (range) of about 6 mi (best-fit 

parameters: nugget of 357.4 ft2, sill of 3,460.6 ft2, and lag 

steps of 3,280.8 ft). The average separation distance between 

wells for the unstressed monitoring subnetwork should be less 

than about 10 mi to enable an overlap in the spatial correlation 

between well locations.

Cross-validation of kriging estimates was performed 

on the predictions of water-level depth at each well location. 

Cross-validation involved an iterative removal of one data 

point (well) from the complete dataset and use of the remaining data to compute an estimate at the location of the removed 

data point. Residuals between predicted and known values 

provided an assessment of error in terms of equivalent units 

of the data and are optimal near zero. Normalized residuals should have the property of unit variance (unit standard 

deviation), which provided an assessment of how closely the 

kriging variance represents the actual variability of prediction errors. Results indicate that mean residual and mean 

standardized errors were close to zero (–0.03 and –0.0015, 

respectively) and the root mean square standardized error 

was near unity (0.88), which were reasonable outcomes. The 

Figure 28. Required frequency of groundwater-level measurements based on system conditions. The 

South Platte River alluvial aquifer requires “more frequent” water level measurements in all circumstances. 

(Modified from Taylor and Alley, 2001)

Less frequent

Water level

measurements

Deep

confined

Aquifer

type and

position

Slow

Groundwater

flow and

recharge rate 

Less

development 

Aquifer

development

Less

variable 

Climatic

conditions 

More frequent

Shallow

unconfined Rapid

More

development

More

variable
48 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 29. Candidate monitoring wells in the South Platte River alluvial aquifer considered for analysis. (NAWQA, National WaterQuality Assessment Program)

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

Ownership of candidate monitoring wells

Central Colorado Water Conservation District

Colorado Department of Agriculture

Lower South Platte Water Conservation District

South Platte Decision Support System

U.S. Geological Survey (NAWQA)

U.S. Geological Survey (other)

Miscellaneous agencies

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Monitoring Well Network 49

average standard error and root mean-square errors are 11.05 

and 11.13 ft, respectively. Variation in predictions of depth to 

water of this magnitude were expected given the intensity of 

water use and water-level variability in the aquifer. The kriging interpolation revealed that the largest prediction errors (red 

dots) are along the fringes of the aquifer within the main stem 

paralleling the South Platte River and within major tributary 

channels south of the main stem (fig. 30). This showed that 

kriging predictions have greater accuracy, on average, along 

interior regions of the main stem of the South Platte River, 

whereas predictions along the aquifer periphery and lower 

tributaries were less accurate.

Given the spatial distribution of kriging error, optimization of the groundwater level monitoring networks was confined to areas along the main stem of the South Platte River. 

For monitoring wells located near the aquifer periphery along 

the main stem, the majority did not correlate to areas with 

greatest kriging errors. Monitoring wells outside of the optimization region or those with insufficient data history (denoted 

as tentative sites) were selected manually for the unstressed 

subnetwork to monitor ambient groundwater levels. 

Optimization of the unstressed (primary) monitoring 

subnetwork was assembled iteratively, building from a starting network. Initial ranking of proximity to administrative 

structures and average decree rate of structures within 10 mi 

of each candidate monitoring well were used to establish the 

backbone of the network. By excluding the top 10 monitoring 

wells per administrative structure category showing the largest 

potential influences on local water levels from consideration, 

all 23 USGS NAWQA wells and 29 of the 37 SPDSS wells 

were chosen as a starting well set. A limited number of SPDSS 

wells separated by relatively short distances were retained in 

the starting well set to allow for sampling multiple depths and 

examination of groundwater levels over a range of distances in 

a limited number of wells, which will be useful for geospatial 

analysis and characterizing small-scale variability in groundwater levels. Further design of the unstressed monitoring 

subnetwork was automated using the starting set of wells as 

an initial condition and adding wells to the network iteratively. 

The average kriging standard deviation was used to evaluate 

the optimal number of wells that should be adopted (fig. 31). 

As additional monitoring wells were added to the subnetwork, 

the average kriging error (uncertainty) in the interpolated 

water levels was reduced and observed to follow a power-law 

relation with a coefficient of determination (R2) fit of 0.91.

The optimal number of wells was chosen at the 32nd iteration where the benefit of adding new wells to the network was 

small. In total, 84 wells were selected from the optimization 

process in the optimization region near the main stem of the 

river (fig. 32).

Twelve additional wells were selected manually in tributaries south of the main stem of the river and adjacent areas 

within the optimization region where no monitoring wells 

were located in the evaluated dataset. The manual selection 

included all “tentative sites” (fig. 32) throughout the aquifer 

not evaluated during the optimization procedure and wells outside of the optimization region. The complete unstressed monitoring subnetwork was designed with 96 monitoring wells 

dispersed across the aquifer. In addition, two SPDSS wells 

were included as potential substitutes along the main stem 

of the South Platte River near Clear Creek north of Denver 

because of uncertainty of conditions nearby the selected wells. 

It was estimated that between 90 and 100 monitoring wells 

will be sufficient for most purposes of monitoring. An additional step will be needed to finalize the monitoring network, 

which allows flexibility in adjusting well selections during 

Table 3. All monitoring wells, trend monitoring wells, and monitoring subnetworks (unstressed, administrative structures, and 

hydrologic), by agency. “Other” sites indicate miscellaneous sources, and “tentative” sites have no known data history and require 

additional evaluation.

Source agency All monitoring 

wells

Trend

monitoring wells

Unstressed

subnetwork

Structure

subnetwork

Hydrologic

subnetwork

Central Colorado Water Conservancy 

District 15 0 3 5 4

Colorado Department of Agriculture 15 0 2 2 0

Colorado State University 2 0 0 1 0

Colorado Division of Water Resources 15 0 0 1 0

Lower South Platte Water Conservancy 

District 45 0 1 8 4

South Platte Decision Support System 37 29 31 7 29

U.S. Geological Survey 227 23 46 45 3

Other 33 0 6 11 0

Tentative 7 0 7 0 0

Total 396 52 96 80 40
50 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

the implementation phase of the network and using alternate 

wells. Detailed consideration of each well should include field 

reconnaissance to assess site conditions and review of site 

records held by respective agencies. The proposed network 

includes wells from USGS, SPDSS, CDA, CSU, DWR, 

LSPWCD, and other sources, demonstrating the need to gather 

community resources in order to characterize water levels 

across the South Platte River alluvial aquifer (table 2–1).

The hydrologic monitoring subnetwork includes wells to 

examine high water conditions or change in groundwater levels using data collected over the recent decade (2003–2012). 

During optimization, each candidate monitoring well was 

ranked as a function of location as determined by the kriging process and either the frequency of groundwater levels 

or trends in groundwater levels as the second metric. Results 

of the optimization are given in table 2–2 with well locations 

shown in figure 33. Twenty monitoring wells were identified 

to evaluate areas with high groundwater and another twenty 

to evaluate areas with appreciable water levels trends. It is 

recommended that a subset of the proposed well groups be 

adopted after direct field reconnaissance to assess additional 

factors evident from physical inspection of the site. Optimal 

monitoring wells for each component are dispersed from 

Greeley to Julesburg along the main stem of the South Platte 

River. There are primary groupings of wells identified east of 

Greeley and Gilcrest and west of Fort Morgan. Other selected 

wells are more isolated and located mainly between Sterling 

and Julesburg.

The structural target monitoring subnetwork is composed 

of wells to evaluate whether administrative structures could be 

affecting groundwater levels. Physical administrative structures considered in the analysis are reservoirs, engineered 

recharge areas, ditches, and wells and well fields. During 

optimization, each candidate monitoring well was ranked as 

a function of location as determined by the kriging process, 

proximity to administrative structures, and average decree 

rate within 10 mi of the well. Results of the optimization are 

given in table 2–3 with well locations shown in figure 34. 

Twenty monitoring well candidates were determined for each 

of four structures (ditch, recharge area, reservoir, and well 

or well field). As with the hydrologic target subnetwork, it is 

recommended that a subset of the proposed well groups for 

diversion targets be adopted after direct field reconnaissance 

to assess additional factors evident from physical inspection 

of the site. The optimal monitoring wells for each component 

are dispersed all along the main stem of the South Platte River. 

There are major groups of wells identified midway between 

Greeley and Fort Morgan at the intersection of Lost Creek 

with the South Platte River near Riverside Reservoir and midway between Fort Morgan and Sterling along the South Platte 

River. Most monitoring wells identified to examine the effects 

from wells or well fields are located in the eastern section 

of the study area, while most monitoring wells identified to 

examine the effects from reservoirs are located in the western 

section of the study area. Monitoring wells to examine the 

influence of ditches and engineered recharge areas are more 

dispersed along the South Platte River alluvial aquifer.
Monitoring Well Network 51

Figure 30. Kriging interpolation errors using depth to water in the 1,669 wells considered. Highest prediction errors (red dots) are 

located mainly along the aquifer periphery paralleling the South Platte River and in tributary channels to the south.

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1 Kriging standard error, in feet

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

6.757310–9.421185

9.421186–11.575860

11.575861–13.823460

13.823461–20.212428

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
52 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 31. Optimization results for the “unstressed” monitoring subnetwork. A reduction in standard error (uncertainty) 

is achieved by adding additional wells to the starting well subnetwork (backbone) composed of 52 wells (table 3). The 

dashed vertical line indicates the stopping point of optimization equivalent to 84 total wells near the main stem of the 

South Platte River in the optimization region (see fig. 32).

Added wells

All wells (backbone + added)

Power (standard error)

Standard error

EXPLANATION

Standard error = 24.9*(wells added)

-0.18

Coeffient of determination (R2) = 0.91

0

10

20

30

40

50

60

70

80

90

100

10

12

14

16

18

20

22

24

26

28

0 10 20 30 40

Total wells

Standard error, in feet

Number of wells added to network

Selected optimization
Monitoring Well Network 53

Figure 32. Proposed “unstressed” monitoring subnetwork composed of 96 monitoring wells. Eighty-four monitoring wells were 

selected along the main stem of the South Platte River using optimization analysis. Twelve additional wells were added manually along 

major tributaries. Two alternate SPDSS wells are also shown about 10 miles northeast of Denver, leading to 98 wells. (SPDSS, South 

Platte Decision Support System; NAWQA, National Water-Quality Assessment Program)

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

Monitoring well network (unstressed)

Central Colorado Water Conservation District

Colorado Department of Agriculture

Lower South Platte Water Conservation District

South Platte Decision Support System

U.S. Geological Survey (NAWQA)

U.S. Geological Survey (other)

Miscellaneous agencies

Tentative sites

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary

Optimization region
54 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Figure 33. Proposed target monitoring subnetwork to evaluate hydrologic stresses. Twenty monitoring well candidates were 

determined to examine each of two stresses (groundwater levels and groundwater-level change).

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

Target monitoring wells—hydrologic

High groundwater levels

Water-level change

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
Monitoring Well Network 55

Figure 34. Proposed target monitoring subnetwork to evaluate influences of administrative structures. Twenty monitoring well 

candidates were determined for each of four structures (ditch, recharge area, reservoir, and well or well field).

Box Elder Creek

Kiowa Creek

South Platte River

South Platte River

Pawnee Creek

Wildcat Creek

Sand Creek

Lost Creek

Big Thompson Creek

Cache La Po udre River

Cedar Creek

Long Tree Creek

Saint Vrain Cr Big Dry Creek

Clear

Cr

Crow

Creek

Buck Creek

Beaver Creek

Badger Creek

Bijou Creek

DOUGLAS ELBERT

LINCOLN KIT CARSON

ARAPAHOE

JEFFERSON

ADAMS

BOULDER

WASHINGTON

YUMA

MORGAN

PHILLIPS

LARIMER

WELD

LOGAN

SEDGWICK

DENVER

BROOMFIELD

Denver

Fort Collins

Greeley

Fort

Morgan Fort

Morgan

Gilcrest

Julesburg

La Salle

Sterling

Base from U.S. Geological Survey digital data, 2009, 1:100,000

Lambert Conformal Conic projection (Colorado State Plane Central)

Standard parallels 38°27’N and 37°45’N, central meridian 105°00’W

0 10 20 MILES

0 10 20 KILOMETERS

105° 104° 103°

41°

40°

EXPLANATION

Water Division 1

Water District 1

Water District 2

Water District 64

South Platte River alluvial aquifer

Target monitoring wells—structures

Ditch

Recharge area

Reservoir

Well or well field

COLORADO

WYOMING NEBRASKA

COLORADO

Map area

Water Division 1

boundary
56 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Summary

The South Platte River Basin contains essential water 

resources for municipalities, agriculture, industry, and energy 

exploration in northeastern Colorado. The area of investigation is the South Platte River alluvial aquifer in Water District 

1, Water District 2, and Water District 64, which are areas 

defined by the Colorado Division of Water Resources. The 

districts compose a portion of the basin extending from the 

Front Range of the Rocky Mountains between Denver and 

Greeley to the eastern plains near Julesburg. The South Platte 

River system and underlying alluvial aquifer, together with 

transbasin water diversions, serve water to 70 percent of the 

State’s population. The South Platte River Basin will be facing 

recurring challenges to satisfy water needs of a population that 

is expected to increase by about 35 percent over three decades 

by 2030. 

Human and natural controls have particularly strong 

influence on groundwater conditions in the South Platte River 

alluvial aquifer. Coupled groundwater and surface-water 

interaction regulates the transfer of water between the river 

and the underlying aquifer. The flow of groundwater through 

the aquifer is controlled in part by geologic composition, 

particularly local deposits of prominent heterogeneity, and by 

variations in topography. Yearly precipitation controls water 

availability and varies substantially with periods of flooding 

and drought, affecting the magnitude and timing of inflow to 

the aquifer. Societal influences are imparted through water use 

and water management. The use of administrative structures 

to distribute water helps to achieve maximum beneficial use 

of the resource, but it also alters the natural cycling of the 

hydrologic system. Water that is drawn from the aquifer and 

replaced through augmentation plans occurs non-uniformly, 

leading to areas of altered recharge and depletion. 

Groundwater levels from 1,669 wells in the South Platte 

River alluvial aquifer were examined over six decades during 1953–2012. Curtailment of pumping from wells without 

authorized augmentation plans that began around 2002–2003 

caused groundwater levels to rise at some locations, leading to 

concern of “high” groundwater levels (depth to water of 10 ft 

or less). Average depths to groundwater evaluated per decade 

varied between 24 and 32 ft over the 60-yr record. Between 

1993–2002 and 2003–2012, groundwater levels declined about 

2 ft throughout the aquifer. For selected areas with shallow 

groundwater less than 20 ft below the land surface, groundwater levels had the reverse pattern and rose by about 0.6 ft 

during the same period, despite the overall decline.

High groundwater levels were observed in the South 

Platte River alluvial aquifer throughout the 60-yr record. Overall, more than one in four (about 29 percent) groundwaterlevel observations indicates conditions of high groundwater. 

High groundwater levels were identified in 17 to 33 percent of 

wells examined per decade with the largest percentages occurring over three decades from 1963 to 1992. Historically, 3 of 5 

decades in the first 50 years (1953–2002) of record showed a 

greater percentage of wells with high groundwater levels compared to the recent decade (2003–2012), implying that recent 

groundwater conditions are within the historical range of the 

previous half century in terms of well location. When accounting by observations, the recent decade had the highest percentage (30 percent) of high groundwater levels, although results 

were similar (26–29 percent) over three decades prior from 

1963 to 1992. Depending on the decade, areas with low to pervasive frequencies of high groundwater levels have occurred 

intermittently along the South Platte River, whereas areas to 

the south along tributaries typically had lower frequencies of 

high groundwater levels. Over six decades of examination, 

sections of the aquifer east of Sterling to Julesburg and from 

Greeley, La Salle, and Gilcrest to about 20 mi east, overall, 

were identified with the highest frequencies of high groundwater levels. There were also isolated areas of high groundwater levels in other locations, depending on the decade. The 

Local Moran’s I method was used to define four regions based 

on spatial autocorrelation of high groundwater levels using 

decades 1963–1973 and 2003–2012, for which data coverage was sufficient. Spatial groupings were similar between 

decades but showed some local distinction. Anomalies of high 

groundwater levels occurred mainly in the western and eastern 

sections of the aquifer.

Over the first five decades of record (1953–2002), trends 

of groundwater decline were identified intermittently along 

tributaries. Decadal trends were insignificant in areas near the 

South Platte River based on available data and selected statistical methods. During the recent decade, 2003–2012, following imposition of curtailment of pumping from wells without 

augmentation plans, there was a clear increase in groundwater 

levels. During this period, about 88 percent of wells with 

significant trends indicate rising groundwater levels, and the 

remaining 12 percent show declining groundwater levels. In 

comparison, about 81 percent of subwatersheds with significant trends show rising groundwater levels, and the remaining 

19 percent show declining groundwater levels. During this 

period, rates of change in groundwater levels varied between 

–1.2 and 2.8 ft/yr. Areas showing greatest groundwater rise 

in either wells or subwatersheds were centered west of Fort 

Morgan, near Gilcrest and La Salle, southeast along Box 

Elder Creek, and around Julesburg. The remaining areas with 

groundwater rise occurred intermittently along the South 

Platte River. Areas with groundwater decline were most extensive along tributaries inclusive to Lost Creek, Kiowa Creek, 

and Bijou Creek. Both Sterling and Julesburg showed groundwater level rise during the recent decade. 

Over the 60-yr record, about 65 percent of individual 

wells with sufficient records show significant trends. For 

these wells, 66 percent had declining groundwater levels and 

34 percent had rising groundwater levels. About 42 percent of 

subwatersheds with sufficient records show significant trends. 

For these wells, around 57 percent had declining groundwater 

levels and 43 percent had rising conditions. Areas near the 

South Platte River show trends that generally were less than 
Summary 57

0.125 ft/yr in magnitude. Areas with greatest groundwater 

decline were along tributaries in the central section of the 

aquifer between Kiowa Creek and Bijou Creek. The main 

areas of groundwater rise occurred between Fort Morgan and 

Julesburg and intermittently along tributaries between Kiowa 

Creek and La Salle.

Net differences in average groundwater levels and standard deviation of groundwater levels in subwatersheds with 

sufficient data coverage between 1963–1972 and 2003–2012 

(40-year span) were used to support and extend the trend 

analyses. Overall, about 58 percent of subwatersheds with 

sufficient records show a net decrease in average groundwater 

levels. Net differences in average groundwater levels between 

1963–1972 and 2003–2012 (40-year span) were generally 

consistent with trend directions over 1953–2012 (60 years 

of record). Disparities were present mainly in areas between 

Fort Morgan and Sterling. More subwatersheds had declining 

water levels west of Fort Morgan, whereas more subwatersheds had rising water levels east of Fort Morgan. Deviations 

in this regional pattern are more substantial west of Fort 

Morgan. Reconciling an overall decline in groundwater levels 

with groundwater rise in some areas implies that groundwater has been redistributed, regionally from upgradient to 

downgradient sections of the aquifer. In areas near and east of 

Fort Morgan along the main stem of South Platte River, the 

standard deviation of detrended groundwater levels amplified 

between 1963–1972 and 2003–2012. Upon initial inspection, 

the increase is not explained by changes in precipitation or 

discharge of the South Platte River, which saw decreases in 

variability between periods. Other factors might have affected 

groundwater level fluctuations, possibly development in the 

region or greater complexity of water use and water management over 40 years. Mechanisms influencing groundwater 

fluctuations could be evaluated through field studies by comparing direct measurements in identified areas against carefully selected baselines. 

Precipitation and transbasin diversions were the primary source of water that entered the South Platte River and 

underlying alluvial aquifer. Changes in groundwater levels 

were responses to the magnitude and timing of water inflow 

and physical characteristics and stresses on the aquifer. Correlations between precipitation, river discharge, and percent 

frequency of high groundwater levels were examined to 

determine whether climate and hydrologic inputs may cause 

groundwater to be shallow in some areas. Annual and decadal 

analyses indicate that high groundwater levels were moderately correlated to river discharge. The strongest relation was 

between climate (precipitation) and surface-water conditions 

(river discharge). From 1993–2002 to 2003–2012, the pattern 

of frequency of high groundwater levels and river discharge 

changed, which was caused in part from the curtailment of 

pumping in wells. During this period, frequency of high 

groundwater levels increased, precipitation remained fairly 

constant, and river discharge decreased from greater reliance 

on surface water.

Comparisons between seasonal high groundwater levels 

and discharge of the South Platte River also support that 

administrative structures can influence high groundwater 

levels in some cases. The analyses showed that peak river 

discharge from April to June did not correlate to the period 

with greatest frequency of high groundwater levels, which 

was typically between July and September after spring runoff. In 5 of the 6 decades examined, excluding 1973–1982, 

peaks of high groundwater levels occurred from July to 

September, coincident to when administrative structures 

were used extensively for irrigation. Recent (2003–2012) 

patterns of quarterly (seasonal) high groundwater levels 

following the curtailment of pumping were generally in line 

with the previous historical record (1953–2002). Frequencies 

of high groundwater levels ranged between approximately 

10 to 40 percent of observations per quarter-year duration, 

which signified that strong subannual variability was typical. 

There were greater differences in the percentages of high 

groundwater levels between April–June and July–September 

prior to the early 1970s and before the Water Right Determination and Administration Act of 1969, which administered 

augmentation plans, than during following decades. Inferences drawn from previous work did not clearly support the 

paradigm of bank storage as a control on seasonal peaks in 

high groundwater levels, but more study is needed on temporal lags of groundwater and surface water exchange. Administrative structures are known to be active and have been 

correlated to areas with high groundwater levels. Engineered 

recharge areas, such as recharge ponds, are used to replenish water to the aquifer and unlined ditches can lose water to 

the aquifer during transport, both of which may contribute to 

local high groundwater levels.

The strongest correlations between geographic attributes 

and frequency of high groundwater levels were found in 

characteristics of land-surface elevation at a well and proximity of a well to streams and rivers. Wells located higher in 

elevation tended to exhibit lower frequencies of high groundwater levels. Depressions (low spots) have tended to promote 

focused recharge, leading to elevated local groundwater levels. 

Wells located closer to the South Platte River often had greater 

frequency of high groundwater levels, whereas those along 

tributaries tended to have lower frequencies of high groundwater levels.

Analysis was used to test whether administrative structures were related to high groundwater levels. The results 

showed that a few characteristics were correlated to high 

groundwater levels at moderate to high magnitudes; remaining characteristics showed insignificant to low correlations. 

Measures of separation distance from a well to an administrative structure, number of decreed structures in a defined 

search area, and average decree rate assigned to structures 

within a defined search area were evaluated for correlations 

to frequency of high groundwater levels. Results showed 

that areas with more impacted river reaches or engineered 

recharge areas or those at greater distances from well fields 
58 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

tended to have higher frequencies of high groundwater levels. 

Engineered recharge areas, such as recharge ponds, are the 

primary structures that create high groundwater levels in 

recharging water to the aquifer. Impacted reaches are areas 

that may receive diverted water along the South Platte River, 

thereby having locally high groundwater levels, but they do 

not directly produce high groundwater levels. Pumping wells 

helped to remediate high groundwater levels by removing 

water from the aquifer and could be the preferred administrative tool to lower groundwater levels in desired areas in the 

future. Areas with more wells or well fields or areas farther 

from engineered recharge areas tended to have lower frequencies of high groundwater levels. Overall, influences on 

groundwater levels were in some cases direct (local) over 1 mi 

or less but could extend to several miles, often manifesting as 

diffuse effects from multiple surrounding structures.

Regardless of the cause(s) of observed long-term change 

in groundwater levels, greater knowledge and control of 

aquifer conditions would allow for improved water management and more efficient use of water resources. One solution 

would be to update existing groundwater-level monitoring by 

using real-time recordings and to develop a set of contingency 

procedures of water administration and allocation based on 

present aquifer conditions. Additional large water storage 

facilities could be built to better regulate groundwater levels 

and allow increased water use during times of need. Diversions could be constructed at the downstream end of the aquifer near Julesburg to return excess water back upstream for 

recirculation or to transport water downstream across the State 

line to Nebraska if the resources were dispensable, avoiding 

excess storage of groundwater in lower reaches of the aquifer. 

Implementing these tools would enable more precise water 

management, provide real-time management decisions using 

real-time monitoring, and offer additional administrative 

structures at key locations for greater flexibility in distributing 

and storing water.

Management decisions require high-quality and easily 

accessible data. Providing a quality, continuous data record at 

key well locations would improve characterization of groundwater levels and inform decisions that could allow more 

efficient use of Colorado’s water. To enable this opportunity 

in the future, a three-component monitoring network consisting of a primary unstressed subnetwork and two targeted 

subnetworks was developed using a procedure that weighs the 

benefit of each well location in characterizing groundwater 

levels against potential influences of nearby administrative 

structures. The network was designed to characterize ambient groundwater levels (1—unstressed subnetwork) and target 

areas of structure influences (2—targeted structural subnetwork) and areas experiencing frequent high groundwater levels or those showing substantial change in groundwater levels 

(3—targeted hydrologic subnetwork). Performing ground 

truths and verifications of site records were beyond the scope 

of this study, but such would be required as a next step. Finalization of the network will require confirmation of long-term 

site accessibility, adequate field conditions, and further consideration of miscellaneous details of each well. New monitoring 

wells might be needed in some areas. Once established, the 

finalized monitoring network will generate consistent time 

series of groundwater levels useful for both administering and 

quantifying water resources. Groundwater levels should be 

periodically examined to determine whether recent patterns of 

groundwater rise persist in the future.

Acknowledgments

The author is indebted to the Colorado Water Conservation Board and the Colorado Water Institute for seeking technical assistance from the U.S. Geological Survey to evaluate 

groundwater levels in the South Platte River alluvial aquifer as 

part of the scientific study developed in Colorado House Bill 

12-1278. The Open Water Foundation is thanked for providing 

several important updates to TSTool, which was used to query 

State records. The Colorado Division of Water Resources provided numerous benefits to the study including periodic record 

updates, discussions on data reliability, and clarifications of 

procedures used for water administration. The author is appreciative of comments provided by the expert panel assigned 

to the study and of collaborations with the following team 

members: Reagan Waskom, Mary-Lou Smith, Dick Stenzel, 

Steve Malers, and Takis Oikonomou. In addition, reviews by 

Edward Banta, Peter Barkman, and Peter McMahon helped to 

significantly improve earlier versions of the report. 

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Appendixes 61

Appendix 1

The general premise in kriging interpolation is to predict the spatial field as linear combinations of data observations using 

spatial covariance to compute weights that minimize the error variance of the estimate. Kriging is an optimal interpolator in the 

sense that it is unbiased with respect to the mean and it minimizes the error variance of the estimates, which has resulted in the 

widespread use of kriging for interpolating earth science data (de Marsily, 1986; Isaaks and Srivastava, 1989). 

Ordinary kriging was used in this study for spatial interpolation of water-level data. Ordinary kriging requires the kriging 

weights sum to unity leading to the relations

Z Z

n

*

( ) u u = ( ) ( ) u =

∑α

λα α

1

(1)

subject to

α

λα

=

∑ ( ) = 1

1

n

u (2)

where Z*(u) is the realized value at location u, uα are the n data locations, and λα are the kriging weights. By minimizing a 

Lagrange parameter (μ) used in the minimization of the kriging error, a set of equations is derived for the kriging weights as 

follows

( ) ( ) ( ) ( )

1

, 1, ..

n

CCn β βα α

β

λ μα

=

∑ u u u u uu − + = − =... (3)

subject to

β

λβ

=

∑ ( ) = 1

1

n

u (4)

where C(uβ – uα ) is the covariance function equivalent to C(h) with h a lag distance parameter between data points. Covariance 

is then derived from the variogram analysis using the relation

C C ( ) h h = ( ) 0 −γ ( ) (5)

where the covariance at lagged distances between data pairs C(h) is equal to the variance C(0) minus the variogram model γ (h). 

Spatial relations are specified with a variogram model rather than a direct measure of covariance, which is defined for the current 

investigation as an exponential model written in the form

( ) 1 exp , 0

0, 0

C C o γ φ

      −   +− >     =           =

h h

h

h

(6)

where Co is the nugget, C is the sill, and φ is the range giving an estimate of the correlation length of the dataset. Once the kriging weights are determined, kriging variance σ 2

(u) can then be determined as

σ λ μ α

α α

2

1

( ) u u = ( ) 0 − ( ) ( ) u u − − ( ) u =

C C ∑

n

(7)

The kriging variance provided a measure of the uncertainty (error) of the spatial field and an indication of areas where additional 

monitoring wells would provide the most benefit to characterize groundwater levels in the aquifer. Kriging was performed on a 

raster grid in ArcGIS. The nearest cell in the raster grid was used to estimate the kriging variance at each monitoring well location. Assuming kriging errors are normally distributed, there is a 96 percent probability that the actual value at the monitoring 

well location is near the predicted value, plus or minus two times the square root of the kriging standard deviation.
62 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Appendix 2

Table 2–1. Candidate wells for the “unstressed” monitoring subnetwork. Proposed candidates to be evaluated during the 

implementation phase.

[ID, identifier; UTM, Universal Transverse Mercator; LSPWCD, Lower South Platte Water Conservancy District; CCWCD, Central Colorado Water 

Conservancy District; DWR, Colorado Division of Water Resources; GPS, Global Positioning System; NWIS, National Water Information System; 

USGS, U.S. Geological Survey; Rech., recharge; N/A, not applicable; SPDSS, South Plate Decision Support System; CDA, Colorado Department of 

Agriculture; Resrv., reservoir; CSU, Colorado State University; MISC., miscellaneous]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Ditch LSPWCD LSPWCD 41275 LLWU 9 MAP Manual 674831 4519976

Ditch Hydrobase CCWCD SB00406434CD SB00406434CD MAP Manual 539347 4457229

Ditch Hydrobase CCWCD 402359104375200 SB00506514BDD MAP Manual 531260 4472187

Ditch Hydrobase DWR SB00805230BA PRN-4 GPS Daily 649263 4500256

Ditch NWIS USGS OTHER 401858104224400 SB00406313AAC MAP Manual 552729 4463026

Ditch NWIS USGS OTHER 402237104290101 SB00506330BBA1 MAP Manual 543792 4469721

Ditch NWIS USGS OTHER 401855104202000 SB00406217ADA MAP Manual 556129 4462958

Ditch NWIS USGS OTHER 405655102222300 SB01104509BDA MAP Manual 721074 4536374

Ditch NWIS USGS OTHER 402119103322600 SB00505532DAA MAP Manual 623889 4468210

Ditch NWIS USGS OTHER 402623103251300 SB00605433CAC MAP Manual 633936 4477759

Ditch NWIS USGS OTHER 401834104144000 SB00406118DAD MAP Manual 564159 4462375

Ditch NWIS USGS OTHER 401633103520200 SB00405833AAA MAP Manual 596261 4458986

Ditch NWIS USGS OTHER 405824102150700 SB01204433DAC MAP Manual 731182 4539432

Ditch NWIS USGS OTHER 400823104494600 SB00206713ADD2 MAP Manual 514484 4443277

Ditch NWIS USGS OTHER 405133102421900 SB01004810BCD MAP Manual 693368 4525656

Ditch NWIS USGS OTHER 402110104004900 SB00505931DDA MAP Manual 583721 4467378

Ditch NWIS USGS OTHER 402258103291800 SB00505523DCD MAP Manual 628272 4471337

Ditch NWIS USGS OTHER 402753103195300 SB00605330AAA MAP Manual 641423 4480673

Ditch NWIS USGS OTHER 402105104451200 SB00506635CCB MAP Manual 520903 4466786

Ditch NWIS USGS OTHER 402446104334200 SB00506409CBA MAP Manual 537146 4473663

Rech. Hydrobase INDIVIDUAL 52W METRO MAP Manual 515206 4436903

Rech. Hydrobase SPDSS DSS18HND SC00106736 GPS Daily 513914 4419040

Rech. Hydrobase INDIVIDUAL S Adams MW N/A MAP Manual 517080 4417626

Rech. Hydrobase LSPWCD South LLWU 4 MAP Manual 671486 4520069

Rech. Hydrobase INDIVIDUAL WT-03 N/A MAP Manual 687766 4523095

Rech. Hydrobase MISC. S Adams B MW11 N/A MAP Manual 510674 4416615

Rech. Hydrobase LSPWCD MW-9 N/A MAP Manual 728103 4541891

Rech. Hydrobase SPDSS DSS09STR SB00605320AD MAP Daily 643032 4481645

Rech. Hydrobase INDIVIDUAL 124W N/A MAP Manual 511323 4419284

Rech. Hydrobase LSPWCD Manuello 3 N/A MAP Manual 654488 4507211

Rech. CDA CDA WLM009 N/A GPS Daily 525478 4464094

Rech. Hydrobase SPDSS DSS16HND SC00206712 GPS Daily 513441 4416629

Rech. NWIS USGS OTHER 401724103414800 SB00405725AAA MAP Manual 610739 4460757

Rech. NWIS USGS OTHER 402334103271600 SB00505419ABB MAP Manual 631130 4472497
Appendix 63

Table 2–1. Candidate wells for the “unstressed” monitoring subnetwork. Proposed candidates to be evaluated during the 

implementation phase.—Continued

[ID, identifier; UTM, Universal Transverse Mercator; LSPWCD, Lower South Platte Water Conservancy District; CCWCD, Central Colorado Water 

Conservancy District; DWR, Colorado Division of Water Resources; GPS, Global Positioning System; NWIS, National Water Information System; 

USGS, U.S. Geological Survey; Rech., recharge; N/A, not applicable; SPDSS, South Plate Decision Support System; CDA, Colorado Department of 

Agriculture; Resrv., reservoir; MISC., miscellaneous]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Rech. NWIS USGS OTHER 400908104494500 SB00206607CBB MAP Manual 514505 4444665

Rech. NWIS USGS OTHER 403540103114100 SB00705209BAC MAP Manual 652716 4495302

Rech. NWIS USGS OTHER 404928102522700 SB01004919DBC MAP Manual 679226 4521442

Rech. NWIS USGS OTHER 401925104232700 SB00406312CBD MAP Manual 551709 4463852

Rech. NWIS USGS OTHER 401855104084000 SB00406018BDB MAP Manual 572650 4463100

Rech. NWIS USGS OTHER 404450103022800 SB00905115DCD MAP Manual 665338 4512541

Resrv. NWIS USGS OTHER 401736104172900 SB00406223DCB MAP Manual 560184 4460554

Resrv. LSPWCD LSPWCD ANDERS 1 LWU 14 MAP Manual 652122 4505621

Resrv. Hydrobase INDIVIDUAL WT-05 N/A MAP Manual 687979 4523374

Resrv. Hydrobase SPDSS DSS03OCH SB00406120DD MAP Daily 565829 4460261

Resrv. Hydrobase CCWCD SB00406535BB N/A MAP Manual 530726 4458443

Resrv. Hydrobase MISC. S Adams MW11 N/A MAP Manual 510674 4416615

Resrv. Hydrobase SPDSS DSS38BLZ SB00505533 GPS Daily 624931 4468483

Resrv. Hydrobase INDIVIDUAL SB00306107AC State Well MAP Manual 563651 4454657

Resrv. Hydrobase INDIVIDUAL South Adams MW N/A MAP Manual 517080 4417626

Resrv. Hydrobase INDIVIDUAL 52W METRO MAP Manual 515206 4436903

Resrv. NWIS USGS OTHER 401858104224400 SB00406313AAC MAP Manual 552729 4463026

Resrv. NWIS USGS OTHER 401821104110300 SB00406114CCD MAP Manual 569285 4462019

Resrv. NWIS USGS OTHER 401815104190000 SB00406222BBA MAP Manual 558027 4461739

Resrv. NWIS USGS OTHER 402032104033900 SB00406002ACD MAP Manual 579723 4466162

Resrv. NWIS USGS OTHER 402001104235800 SB00406311AAB MAP Manual 550970 4464956

Resrv. NWIS USGS OTHER 401855104202000 SB00406217ADA MAP Manual 556129 4462958

Resrv. NWIS USGS OTHER 404301103090900 SB00905227DDD MAP Manual 656004 4508975

Resrv. NWIS USGS OTHER 394855104504801 2052 MAP Manual 513078 4407265

Resrv. NWIS USGS OTHER 401902104213000 SB00406218AAA MAP Manual 554475 4463162

Resrv. NWIS USGS OTHER 404857102573600 SB01005029AAD MAP Manual 672009 4520314

Well Hydrobase CCWCD SB00406533AA N/A MAP Manual 528985 4458375

Well Hydrobase LSPWCD JULS #1 N/A MAP Manual 729597 4541741

Well Hydrobase SPDSS DSS09STR SB00605320AD MAP Daily 643032 4481645

Well Hydrobase CCWCD SB00406434DD SB00406434DD MAP Manual 540011 4456930

Well Hydrobase CSU 405804102200400 SB01104502ABB MAP Manual 724259 4538600

Well Hydrobase MISC. WH-06 N/A MAP Manual 690686 4523912

Well LSPWCD LSPWCD P Ranch 3 41297 LWU 13 MAP Manual 635325 4473363

Well LSPWCD LSPWCD Well #2 41276 LLWU 7 MAP Manual 674863 4519994
64 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Table 2–1. Candidate wells for the “unstressed” monitoring subnetwork. Proposed candidates to be evaluated during the 

implementation phase.—Continued

[ID, identifier; UTM, Universal Transverse Mercator; LSPWCD, Lower South Platte Water Conservancy District; CCWCD, Central Colorado Water 

Conservancy District; DWR, Colorado Division of Water Resources; GPS, Global Positioning System; NWIS, National Water Information System; 

USGS, U.S. Geological Survey; Rech., recharge; N/A, not applicable; SPDSS, South Plate Decision Support System; CDA, Colorado Department of 

Agriculture; Resrv., reservoir; MISC., miscellaneous]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Well CDA CDA WLM009 N/A GPS Daily 525478 4464094

Well Hydrobase SPDSS DSS03OCH SB00406120DD MAP Daily 565829 4460261

Well NWIS USGS OTHER 402334103271600 SB00505419ABB MAP Manual 631130 4472497

Well NWIS USGS OTHER 402701103224000 SB00605426DCC MAP Manual 637520 4478996

Well NWIS USGS OTHER 402008103292100 SB00405511AAA MAP Manual 628291 4466094

Well NWIS USGS OTHER 405247102390500 SB01004706BBB MAP Manual 697849 4528058

Well NWIS USGS OTHER 402311103295000 SB00505523BDC MAP Manual 627511 4471725

Well NWIS USGS OTHER 401633103520200 SB00405833AAA MAP Manual 596261 4458986

Well NWIS USGS OTHER 403331103184900 SB00705321BCC MAP Manual 642732 4491124

Well NWIS USGS OTHER 404450103022800 SB00905115DCD MAP Manual 665338 4512541

Well NWIS USGS OTHER 401843104065700 SB00406017DAB MAP Manual 575085 4462753

Well NWIS USGS OTHER 405608102273300 SB01104615ADA MAP Manual 713867 4534710
Appendix 65

Table 2–2. Candidate wells for the hydrologic target subnetwork to examine trends and high groundwater levels. Proposed candidates 

to be evaluated during the implementation phase.

[ID, identifier; UTM, Universal Transverse Mercator; CCWCD, Central Colorado Water Conservancy District; GPS, Global Positioning System; LSPWCD, 

Lower South Platte Water Conservancy District; SPDSS, South Plate Decision Support System; NWIS, National Water Information System; USGS, 

U.S. Geological Survey; NAWQA, National Water-Quality Assessment Program]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Trend Hydrobase CCWCD SB00206503BB SB00206503BB MAP Manual 529050 4447152

Trend Hydrobase LSPWCD MW-3 N/A MAP Manual 724162 4540486

Trend Hydrobase LSPWCD JULS #3 N/A MAP Manual 729410 4541614

Trend Hydrobase LSPWCD MW-9 N/A MAP Manual 728103 4541891

Trend Hydrobase LSPWCD MW-7 N/A MAP Manual 725954 4540390

Trend Hydrobase SPDSS DSS05WLD SB00406025DC MAP Daily 581459 4458872

Trend Hydrobase SPDSS DSS31WLD SB00405909 GPS Daily 586677 4463746

Trend Hydrobase SPDSS DSS06WLD SB00406012BD MAP Daily 581079 4464562

Trend Hydrobase SPDSS DSS16HND SC00206712 GPS Daily 513441 4416629

Trend Hydrobase SPDSS DSS04WIG SB00306004AA MAP Daily 577084 4456773

Trend Hydrobase SPDSS DSS32WLD SB00505819 GPS Daily 591716 4462149

Trend Hydrobase SPDSS DSS39BLZ SB00405501 GPS Daily 628323 4466118

Trend Hydrobase SPDSS DSS10STR SB00805325BD MAP Daily 648052 4499728

Trend Hydrobase SPDSS DSS09STR SB00605320AD MAP Daily 643032 4481645

Trend Hydrobase SPDSS DSS20BBD SB00406511 GPS Daily 531972 4463306

Trend Hydrobase SPDSS DSS08BLZ SB00505430AB MAP Daily 631135 4470905

Trend Hydrobase SPDSS DSS12JBG SB01104502AA MAP Daily 725059 4538386

Trend Hydrobase SPDSS DSS24KRY SB00506426 GPS Daily 540380 4469775

Trend Hydrobase SPDSS DSS02MLK SB00406606CC MAP Daily 514881 4464737

Trend Hydrobase SPDSS DSS03OCH SB00406120DD MAP Daily 565829 4460261

High Hydrobase SPDSS DSS22KRY SB00506409 GPS Daily 537065 4473634

High Hydrobase CCWCD 402359104375200 SB00506514BDD MAP Manual 531260 4472187

High Hydrobase CCWCD SB00406534AB SB00406534AB MAP Manual 529809 4458349

High Hydrobase CCWCD SB00206511AA SB00206511AA MAP Manual 532243 4445363

High Hydrobase SPDSS DSS41JBG SB01204432 GPS Daily 729908 4538955

High Hydrobase SPDSS DSS21BBD SB00406514 GPS Daily 530623 4461948

High Hydrobase SPDSS DSS25KRY SB00506422AB GPS Daily 539391 4471365

High Hydrobase SPDSS DSS42JBG SB01204433 GPS Daily 731284 4539462

High Hydrobase SPDSS DSS19BBD SB00406510 GPS Daily 530176 4463299

High Hydrobase SPDSS DSS38BLZ SB00505533 GPS Daily 624931 4468483

High Hydrobase SPDSS DSS40JBG SB01104502 GPS Daily 725056 4537008

High Hydrobase SPDSS DSS20BBD SB00406511 GPS Daily 531972 4463306

High Hydrobase SPDSS DSS30BXE SB00406401DD GPS Daily 543009 4464965

High Hydrobase SPDSS DSS11ILF SB00905109DA MAP Daily 664088 4514246

High Hydrobase SPDSS DSS39BLZ SB00405501 GPS Daily 628323 4466118

High Hydrobase SPDSS DSS29BXE SB00406401BB GPS Daily 541776 4466398

High Hydrobase SPDSS DSS26KRY SB00506414 GPS Daily 540926 4472708

High NWIS USGS NAWQA 402213104015501 SB00505930CBA GPS Daily 582198 4469302

High NWIS USGS NAWQA 402104104404501 SB00506532DDB GPS Daily 527363 4466513

High NWIS USGS NAWQA 405159102444201 SB01004805CCC GPS Daily 689980 4526367
66 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Table 2–3. Candidate wells for the administrative structures target subnetwork to examine influences of structures on wells. Proposed 

candidates to be evaluated during the implementation phase.

[ID, identifier; UTM, Universal Transverse Mercator; CCWCD, Central Colorado Water Conservancy District; CDA, Colorado Department of Agriculture; N/A, 

not applicable; GPS, Global Positioning System; LSPWCD, Lower South Platte Water Conservancy District; MISC., miscellaneous; SPDSS, South Platte Decision Support System; NWIS, National Water Information System; USGS, U.S. Geological Survey; NAWQA, National Water-Quality Assessment Program]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Monitor Hydrobase CCWCD SB00206511AA SB00206511AA MAP Manual 532243 4445363

Monitor Hydrobase CCWCD SB00406434DD SB00406434DD MAP Manual 540011 4456930

Monitor Hydrobase CCWCD 402359104375200 SB00506514BDD MAP Manual 531260 4472187

Monitor CDA CDA WLM040 N/A GPS Daily 522076 4459957

Monitor CDA CDA WLM001 N/A GPS Daily 515808 4459581

Monitor LSPWCD LSPWCD ANDERS 1 LWU 14 MAP Manual 652122 4505621

Monitor Hydrobase MISC. SAC-MW-09 N/A MAP Manual 508370 4404222

Monitor Hydrobase MISC. S Adams MW N/A MAP Manual 517080 4417626

Monitor Hydrobase MISC. S Adams MW2 N/A MAP Manual 510030 4416494

Monitor Hydrobase MISC. SB00306107AC State Well MAP Manual 563651 4454657

Monitor Hydrobase MISC. SAC-MW-14 N/A MAP Manual 508832 4411817

Monitor Hydrobase MISC. WH-10 N/A MAP Manual 694417 4525302

Monitor Hydrobase SPDSS DSS01KRY SB00406304AB GPS Daily 547482 4466531

Monitor Hydrobase SPDSS DSS04WIG SB00306004AA MAP Daily 577084 4456773

Monitor Hydrobase SPDSS DSS05WLD SB00406025DC MAP Daily 581459 4458872

Monitor Hydrobase SPDSS DSS06WLD SB00406012BD MAP Daily 581079 4464562

Monitor Hydrobase SPDSS DSS22KRY SB00506409 GPS Daily 537065 4473634

Monitor Hydrobase SPDSS DSS23KRY SB00506422BC GPS Daily 538590 4470625

Monitor Hydrobase SPDSS DSS24KRY SB00506426 GPS Daily 540380 4469775

Monitor Hydrobase SPDSS DSS25KRY SB00506422AB GPS Daily 539391 4471365

Monitor Hydrobase SPDSS DSS26KRY SB00506414 GPS Daily 540926 4472708

Monitor Hydrobase SPDSS DSS27KRY SB00506411 GPS Daily 541726 4473431

Monitor Hydrobase SPDSS DSS28KRY SB00506413 GPS Daily 542268 4472190

Monitor Hydrobase SPDSS DSS29BXE SB00406401BB GPS Daily 541776 4466398

Monitor Hydrobase SPDSS DSS30BXE SB00406401DD GPS Daily 543009 4464965

Monitor Hydrobase SPDSS DSS31WLD SB00405909 GPS Daily 586677 4463746

Monitor Hydrobase SPDSS DSS32WLD SB00505819 GPS Daily 591716 4462149

Monitor Hydrobase SPDSS DSS33WLD SB00305807 GPS Daily 591511 4455666

Monitor Hydrobase SPDSS DSS33WLD SB00305807 GPS Daily 591511 4455666

Monitor Hydrobase SPDSS DSS39BLZ SB00405501 GPS Daily 628323 4466118

Monitor Hydrobase SPDSS DSS19BBD SB00406510 GPS Daily 530176 4463299

Monitor Hydrobase SPDSS DSS20BBD SB00406511 GPS Daily 531972 4463306

Monitor Hydrobase SPDSS DSS21BBD SB00406514 GPS Daily 530623 4461948

Monitor Hydrobase SPDSS DSS10STR SB00805325BD MAP Daily 648052 4499728

Monitor Hydrobase SPDSS DSS11ILF SB00905109DA MAP Daily 664088 4514246

Monitor Hydrobase SPDSS DSS12JBG SB01104502AA MAP Daily 725059 4538386

Monitor Hydrobase SPDSS DSS13JBG SB01204429AC MAP Daily 729153 4541142

Monitor Hydrobase SPDSS DSS40JBG SB01104502 GPS Daily 725056 4537008

Monitor Hydrobase SPDSS DSS41JBG SB01204432 GPS Daily 729908 4538955

Monitor Hydrobase SPDSS DSS42JBG SB01204433 GPS Daily 731284 4539462
Appendix 67

Table 2–3. Candidate wells for the administrative structures target subnetwork to examine influences of structures on wells. Proposed 

candidates to be evaluated during the implementation phase.—Continued

[ID, identifier; UTM, Universal Transverse Mercator; CCWCD, Central Colorado Water Conservancy District; CDA, Colorado Department of Agriculture; N/A, 

not applicable; GPS, Global Positioning System; LSPWCD, Lower South Platte Water Conservancy District; MISC., miscellaneous; SPDSS, South Platte Decision Support System; NWIS, National Water Information System; USGS, U.S. Geological Survey; NAWQA, National Water-Quality Assessment Program]

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Monitor Hydrobase SPDSS DSS02MLK SB00406606CC MAP Daily 514881 4464737

Monitor Hydrobase SPDSS DSS14DIA SC00306411AB GPS Daily 541351 4406855

Monitor Hydrobase SPDSS DSS15DIA SC00206310CD GPS Daily 549072 4414951

Monitor Hydrobase SPDSS DSS16HND SC00206712 GPS Daily 513441 4416629

Monitor Hydrobase SPDSS DSS18HND SC00106736 GPS Daily 513914 4419040

Monitor Permit data TENTATIVE Permit_43985-MH N/A MAP Manual 548565 4429404

Monitor Permit data TENTATIVE Permit_41236-MH N/A MAP Manual 569396 4442414

Monitor Permit data TENTATIVE Permit_37246-MH N/A MAP Manual 620413 4440721

Monitor Permit data TENTATIVE Permit_270967 Receipt_3608968 MAP Manual 558541 4419491

Monitor Permit data TENTATIVE Permit_283850 Receipt_3647042A MAP Manual 555910 4452566

Monitor Permit data TENTATIVE Permit_244198 Receipt_0497162A MAP Manual 551124 4442399

Monitor NWIS USGS NAWQA 400237104500301 SB00106619BAB GPS Daily 515050 4432612

Monitor NWIS USGS NAWQA 400711104481801 SB00206620CCA GPS Daily 516651 4441090

Monitor NWIS USGS NAWQA 401440103373201 SB00305610ACD GPS Manual 616888 4455792

Monitor NWIS USGS NAWQA 401449104064801 SB00306009BBB GPS Manual 575460 4455525

Monitor NWIS USGS NAWQA 401544103443101 SB00305703BBA GPS Daily 606954 4457614

Monitor NWIS USGS NAWQA 401702103483901 SB00405825ADD GPS Daily 601099 4459918

Monitor NWIS USGS NAWQA 401750104143101 SB00406120CAB GPS Manual 564428 4461002

Monitor NWIS USGS NAWQA 402018103571801 SB00405902CBC GPS Manual 582198 4469302

Monitor NWIS USGS NAWQA 402034103301001 SB00405502BCD GPS Daily 627072 4466874

Monitor NWIS USGS NAWQA 402104104404501 SB00506532DDB GPS Daily 527363 4466513

Monitor NWIS USGS NAWQA 402150103322801 SB00505528CCC GPS Daily 623901 4469174

Monitor NWIS USGS NAWQA 402213104015501 SB00505930CBA GPS Daily 582198 4469302

Monitor NWIS USGS NAWQA 402538103242001 SB00505404ADD GPS Daily 635090 4476591

Monitor NWIS USGS NAWQA 402658104400001 SB00606533ABD1 GPS Daily 528484 4477745

Monitor NWIS USGS NAWQA 402955103163501 SB00605311CCC GPS Daily 646174 4484515

Monitor NWIS USGS NAWQA 403426103200401 SB00705317BCB1 GPS Daily 640972 4492841

Monitor NWIS USGS NAWQA 404106103082201 SB00805201CCD GPS Daily 657216 4505455

Monitor NWIS USGS NAWQA 404320103053801 SB00905130DAA GPS Daily 660923 4509662

Monitor NWIS USGS NAWQA 404450103013501 SB00905123BDD GPS Daily 666519 4512477

Monitor NWIS USGS NAWQA 405039102485601 SB01004915BCC GPS Daily 684126 4523748

Monitor NWIS USGS NAWQA 405159102444201 SB01004805CCC GPS Daily 689980 4526367

Monitor NWIS USGS NAWQA 405658102231201 SB01104509CCC GPS Daily 719968 4536460

Monitor NWIS USGS NAWQA 405801102284501 SB01104603BBC GPS Daily 712183 4538092

Monitor NWIS USGS OTHER 400938104053901 SB00206004DDD MAP Manual 577098 4445968

Monitor NWIS USGS OTHER 405252102364500 SB01104733CCC MAP Manual 701122 4528301

Monitor NWIS USGS OTHER 401217104515100 SB00306723CCD2 MAP Manual 511515 4450487

Monitor NWIS USGS OTHER 394755104481601 8027 MAP Manual 516696 4405422

Monitor NWIS USGS OTHER 404938102555300 SB01005022BDD MAP Manual 674392 4521634

Monitor NWIS USGS OTHER 403409103123400 SB00705217CDA MAP Manual 651527 4492470
68 Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks

Publishing support provided by: 

Denver and Rolla Publishing Service Centers

For more information concerning this publication, contact: 

Director, USGS Colorado Water Science Center 

Box 25046, Mail Stop 415 

Denver, CO 80225 

(303) 236-4882

Or visit the Colorado Water Science Center Web site at: 

http://co.water.usgs.gov/

This publication is available online at: 

http://dx.doi.org/10.3133/sir20155015

Purpose Source Agency Primary ID Secondary ID Locator Data type UTM X UTM Y

Monitor NWIS USGS OTHER 401356104462201 SB00306616AAA MAP Manual 519285 4453555

Monitor NWIS USGS OTHER 401855104202000 SB00406217ADA MAP Manual 556129 4462958

Monitor NWIS USGS OTHER 401855104084000 SB00406018BDB MAP Manual 572650 4463100

Monitor NWIS USGS OTHER 401320104364801 SB00306616DAC MAP Manual 532855 4452492

Monitor NWIS USGS OTHER 402753103195300 SB00605330AAA MAP Manual 641423 4480673

Monitor NWIS USGS OTHER 401724103414800 SB00405725AAA MAP Manual 610739 4460757

Monitor NWIS USGS OTHER 402349103290000 SB00505513CCB MAP Manual 628670 4472917

Monitor NWIS USGS OTHER 405340102423300 SB01104834BBB MAP Manual 692938 4529564

Monitor NWIS USGS OTHER 402105104451200 SB00506635CCB MAP Manual 520903 4466786

Monitor NWIS USGS OTHER 403826103095200 SB00805227AAA MAP Manual 655172 4500474

Monitor NWIS USGS OTHER 400935104503100 SB00206712BAB MAP Manual 513415 4445495

Monitor NWIS USGS OTHER 402120104233400 SB00506336CBB MAP Manual 551519 4467396

Monitor NWIS USGS OTHER 405434102354700 SB01104728AAB MAP Manual 702393 4531484

Monitor NWIS USGS OTHER 402027103555300 SB00405901CAA MAP Manual 590719 4466133

Monitor NWIS USGS OTHER 400213104032101 SB00106023DAA MAP Manual 580508 4432282

Monitor NWIS USGS OTHER 400635103463001 SB00205729ADC MAP Manual 604356 4440652

Table 2–3. Candidate wells for the administrative structures target subnetwork to examine influences of structures on wells. Proposed 

candidates to be evaluated during the implementation phase.—Continued

[ID, identifier; UTM, Universal Transverse Mercator; CCWCD, Central Colorado Water Conservancy District; CDA, Colorado Department of Agriculture; N/A, 

not applicable; GPS, Global Positioning System; LSPWCD, Lower South Platte Water Conservancy District; MISC., miscellaneous; SPDSS, South Platte Decision Support System; NWIS, National Water Information System; USGS, U.S. Geological Survey; NAWQA, National Water-Quality Assessment Program]
Wellman—Evaluation of Groundwater Levels, South Platte River Alluvial Aquifer, 1953–2012, and Design of Initial Well Networks—SIR 2015–5015

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155015