U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Evaluation of Possible Alternatives to 

Lower the High Water Table of 

St. Charles Mesa, Pueblo County, 

Colorado

By Daniel L. Brendle

Denver, Colorado

2002

01–4190

Prepared in cooperation with 

PUEBLO COUNTY, COLORADO
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046
CONTENTS III

CONTENTS

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

Introduction............................................................................................................................................................................ 1

Purpose and Scope....................................................................................................................................................... 2

Description of Study Area ........................................................................................................................................... 3

Acknowledgments ....................................................................................................................................................... 3

Geohydrologic Setting of St. Charles Mesa........................................................................................................................... 3

Bedrock Surface........................................................................................................................................................... 6

Hydraulic Conductivity................................................................................................................................................ 8

Water-Table Surface..................................................................................................................................................... 8

Recharge ...................................................................................................................................................................... 10

Ditch Seepage .................................................................................................................................................... 10

Areal Recharge .................................................................................................................................................. 12

Discharge ..................................................................................................................................................................... 13

Spring and Seep Discharge................................................................................................................................ 13

Evapotranspiration ............................................................................................................................................. 14

Pumpage ............................................................................................................................................................ 14

Conceptual Model........................................................................................................................................................ 14

Simulation of Ground-Water Flow ........................................................................................................................................ 14

Model Description ....................................................................................................................................................... 15

Model Calibration........................................................................................................................................................ 15

Sensitivity Analysis ..................................................................................................................................................... 20

Evaluation of Possible Alternatives to Lower the High Water Table .................................................................................... 20

Reducing Areal Recharge ............................................................................................................................................ 21

Lining Various Portions of the Ditch ........................................................................................................................... 22

Installation of Drains and Dewatering Wells ............................................................................................................... 22

Model Limitations ................................................................................................................................................................. 31

Summary................................................................................................................................................................................ 34

References Cited.................................................................................................................................................................... 35

FIGURES

1–4. Maps showing:

1. Location of study area ....................................................................................................................................... 2

2. Topographic contours on and near St. Charles Mesa ........................................................................................ 4

3. Bedrock surface underlying St. Charles Mesa .................................................................................................. 5

4. Water table in May 1998, and the location of wells SCM–02 and SCM–28, St. Charles Mesa ....................... 7

5. Graph showing depth to water in wells SCM–02 and SCM–28, April 1997–September 1998................................ 8

6. Diagram showing components of the St. Charles Mesa hydrologic system ............................................................. 8

7–11. Maps showing:

7. Average monthly water table, June 1997–May 1998, and the simulated water table for the calibrated 

steady-state model ............................................................................................................................................. 9

8. Calibrated recharge rates for irrigated and nonirrigated areas, and the locations of irrigation wells 

and lateral headgates on the Mesa..................................................................................................................... 11

9. Model grid and simulated boundary conditions................................................................................................ 16

10. Hydraulic-conductivity zones used in the calibrated steady-state model.......................................................... 18

11. Difference between the measured and simulated water levels, and the locations of water-level 

measurement points........................................................................................................................................... 19

12. Root mean squared error between measured and simulated water levels as a result of varying model 

input-parameter values .............................................................................................................................................. 21
IV CONTENTS

13–16. Maps showing simulated water-table decline in the St. Charles Mesa terrace alluvial aquifer as a result 

of lining the Ditch from Aspen Street to: 

13. 21st Lane ........................................................................................................................................................... 23

14. 23rd Lane .......................................................................................................................................................... 24

15. 25th Lane........................................................................................................................................................... 25

16. Nicholson Road................................................................................................................................................. 26

17. Map showing simulated saturated thickness of the St. Charles Mesa terrace alluvial aquifer ................................. 27

18–20. Maps showing simulated saturated thickness remaining in the St. Charles Mesa terrace alluvial aquifer 

as a result of lining the Ditch from Aspen Street to: 

18. 21st Lane ........................................................................................................................................................... 28

19. 23rd Lane .......................................................................................................................................................... 29

20. 25th Lane........................................................................................................................................................... 30

21. Diagram showing relation between a pump intake and the position of the water table before and after 

implementation of an alternative to lower the water table ........................................................................................ 31

22–23. Maps showing:

22. Simulated water-table decline resulting from installation of two drains at 10 feet below land surface............ 32

23. Simulated water-table decline resulting from installation of 22 dewatering wells, each pumping at 

80 gallons per minute........................................................................................................................................ 33

TABLES

1. Average monthly flow and stage in Bessemer Ditch, average monthly seepage from Bessemer Ditch, and 

total diversions from Bessemer Ditch to lateral ditches for June 1997 through May 1998.................................... 12

2. Land-use areas and estimated recharge rates .......................................................................................................... 14

3. Description of Bessemer Ditch segments, the segment length, and the amount of simulated seepage for each 

Ditch segment.......................................................................................................................................................... 17

4. Water budget for the calibrated model .................................................................................................................... 20

CONVERSION FACTORS

Multiply By To obtain

inch 25.4 millimeter

foot (ft) 0.3048 meter

square foot (ft2) 0.09290 square meter

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

mile 1.69 kilometer

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

square mile (mi2) 2.590 square kilometer

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

square foot per second (ft2

/s) 929,664 square meter per day

inch per year (in/yr) 2.590 millimeter per year

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

gallon 7.48 cubic foot

cubic foot per second (ft3

/s) 378,432,000 inches per year per square foot

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived from a general 

adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
Abstract 1

Evaluation of Possible Alternatives to Lower the 

High Water Table of St. Charles Mesa,

Pueblo County, Colorado

By Daniel L. Brendle

Abstract

St. Charles Mesa, an upland terrace southeast of Pueblo, Colorado, has become increasingly urbanized as cultivated fields have been 

subdivided and converted to residential areas. In 

some areas, the water table in the terrace alluvial 

aquifer underlying St. Charles Mesa is very 

shallow. Bessemer Ditch, which delivers irrigation water to farms on the mesa and other areas 

of the lower Arkansas River Valley, traverses 

St. Charles Mesa along its southern side and is the 

principal source of recharge to the terrace alluvial 

aquifer. The ground-water flow system was 

assumed to be in a state of dynamic equilibrium 

(steady-state condition) for this study. A steadystate ground-water flow model of the terrace 

alluvial aquifer was constructed and calibrated. 

The model was run in transient state to evaluate 

possible alternatives of lowering the water table. 

The possible alternatives evaluated were 

(1) reducing areal recharge by reducing recharge 

to irrigated areas by 25 percent, (2) lining 

Bessemer Ditch from (a) Aspen Street to 21st 

Lane; (b) Aspen Street to 23rd Lane; (c) Aspen 

Street to 25th Lane; and (d) Aspen Street to 

Nicholson Road, (3) installing two drains at a 

depth of 10 feet below land surface upgradient 

from the high water table areas, and (4) installing 

22 dewatering wells within the high water table 

areas, each pumping at 80 gallons per minute. All 

alternatives evaluated were at least partly effective 

in lowering the water table. As the simulated 

extent of Bessemer Ditch lining was increased, 

the extent and magnitude of simulated water-table 

declines also increased. The maximum simulated 

declines in the water table were 3 feet when 

simulated areal recharge to irrigated areas was 

reduced by 25 percent, 29 feet when lining of 

Bessemer Ditch was simulated from Aspen Street 

to Nicholson Road, 6.8 feet when two drains were 

simulated at 10-foot depth, and 14.4 feet when 

22 dewatering wells, each pumping at 80 gallons 

per minute, were simulated. Lining Bessemer 

Ditch from Aspen Street to 25th Lane and from 

Aspen Street to Nicholson Road both resulted in 

water-table declines of at least 5 feet throughout 

most of the area. Except for reducing recharge to 

irrigated areas and installation of the two drains, 

all the alternatives evaluated probably would 

lower the water table enough to diminish the 

ground-water supply available for at least some 

existing wells.

INTRODUCTION

St. Charles Mesa (hereinafter, the Mesa), an 

upland terrace southeast of Pueblo, Colorado, has an 

area of about 10 mi2 (fig. 1). During the last 35 years, 

the Mesa has become increasingly urbanized as cultivated fields have been subdivided and converted to 

residential areas. The U.S. Geological Survey (USGS), 

in cooperation with Pueblo County, began a two-phase 

study in 1997 to define the extent of the area in which 

the water table is high and to evaluate possible alternatives to lower the high water table.

During the first phase of the study, the extent of 

the high water table in November 1997 and water-table 
2 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

Map area

DENVER

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Pueblo Reservoir

Arkansas

River

Salt

Creek

St. Charles

River

Bessemer Ditch

PUEBLO

104 45' o 104 30' o

38 20' o

38 15' o

Fountain Creek

Study area

I-25

Bessemer

Ditch

Pueblo airport

weather station

Base from hydrology modified by Denver

Water Board, 1998, from U.S. Geological Survey

Digital line graphs, 1:100,000, 1981–1983. Roads and

cities from Colorado Department of Transportation,

1998. State plane projection. 1927 North American

datum, Colorado coordinate system, Southern zone

COLORADO

Hwy 50

Hwy 50

fluctuations from April 1997 to October 1998 were 

determined (Brendle, 1999) and a preliminary groundwater flow model was developed. The preliminary 

model was developed to determine which characteristics of the hydrologic system needed better definition 

to improve model accuracy. In the second phase of the 

study, the ground-water flow model was refined to 

evaluate possible alternatives to lower the high water 

table in problem areas. Possible alternatives evaluated 

to lower the water table are (1) reducing areal recharge 

by reducing recharge to irrigated areas by 25 percent, 

(2) lining Bessemer Ditch from (a) Aspen Street to 

21st Lane; (b) Aspen Street to 23rd Lane; (c) Aspen 

Street to 25th Lane; and (d) Aspen Street to Nicholson 

Road, (3) installing two drains at a depth of 10 feet 

below land surface upgradient from the high water 

table areas, and (4) installing 22 dewatering wells 

within the high water table areas, each pumping at 

80 gallons per minute.

Purpose and Scope

This report presents the results of simulations of 

ground-water flow in the terrace alluvial aquifer of the 

Mesa. The ground-water flow system of the Mesa, the 

development of the digital ground-water flow model, 

Figure 1. Location of study area.
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 3

and the use of the model to evaluate possible alternatives for lowering the water table are described. Areas 

of the Mesa having a high water table are defined in 

this report by November 1997 depth-to-water 

measurements.

Description of Study Area

The study area covers about 10 mi2 on the Mesa 

in southern Colorado (fig. 1). The Mesa is bounded by 

the Salt Creek Valley below its northwestern slope, the 

Arkansas River Valley below its northern slope, the 

St. Charles River Valley below its southeastern slope, 

and by shale-dominated hillocks southwest of the 

Bessemer Ditch (hereinafter, the Ditch). The Mesa 

was originally developed in the late 1800’s for use as 

an agricultural area with farm residences (Dumeyer, 

1975). Currently (2000), crops are grown on about 

4 mi2 of the Mesa. The Ditch was constructed during 

the late 1800’s on the southern side of the Mesa in part 

to deliver water to agricultural fields. Because the 

owners of the Ditch, the Bessemer Irrigation Ditch 

Company, have several relatively high priority water 

rights, water is conveyed through the Ditch every year. 

The Ditch is not lined where it traverses the Mesa, and 

the inflow of water from seepage from the Ditch and 

from irrigation has contributed to the water table 

becoming artificially high relative to the water table 

that probably existed before development of the Ditch 

and the start of irrigation on the Mesa. Twenty-two 

lateral ditches branch off the Ditch and are used to 

convey water to agricultural fields on the Mesa. Some 

lateral ditches are lined, others are unlined. The study 

area hereinafter will be referred to as “the modeled 

area.”

Acknowledgments

Data needed for the development of the groundwater flow model were provided by Dan Henrichs, 

Bessemer Irrigation Ditch Company; Dave Simpson, 

St. Charles Mesa Water District; Ina Bernard, Division 

of Water Resources; and 57 private well owners who 

allowed the USGS to measure water levels in their 

wells.

GEOHYDROLOGIC SETTING OF ST. 

CHARLES MESA

The Mesa is a remnant of a previously more 

extensive alluvial terrace that was deposited by the 

ancestral Arkansas River during the Pleistocene Epoch 

(Scott, 1969b). The alluvial deposits that compose the 

terrace alluvial aquifer on the Mesa include the 

Slocum Alluvium, generally south of Santa Fe Drive 

(fig. 2), and the Louviers Alluvium, generally north of 

Santa Fe Drive and in a narrow band along the southeastern side of the Mesa. These alluvial deposits 

generally consist of sand and gravel and range in 

thickness from about 7 to more than 38 ft (Scott, 

1969a). The alluvial deposits are overlain by approximately 5 to 32 ft of eolian sand of late Holocene age 

(Scott, 1969b).

Erosion of the alluvial terrace produced the 

current physiographic feature that resembles a mesa. 

The Mesa is bounded by fairly high-angle scarps on its 

northwestern and northern sides, a gradually 

descending slope on its southeastern side, and a topographic rise underlain by bedrock on its southwestern 

side. The land surface of the Mesa slopes downward 

to the north-northeast toward the Arkansas River 

Valley (fig. 2). The topographic contours east of Salt 

Creek, south of the Arkansas River, and west of the St. 

Charles River illustrate the relief in the vicinity of the 

Mesa (fig. 2).

The northwestern, northern, and southeastern 

sides of the Mesa are dissected by numerous washes 

that convey stormwater flow off the Mesa and 

commonly contain springs and seeps. These washes 

generally are less than one-fourth mile in length and 

have a central axis perpendicular to the trend of the 

scarp or slope. Due to the ground-water discharge 

from springs and seeps, these washes usually contain 

water-loving vegetation (phreatophytes) in their upper 

ends.

Rocks of Cretaceous age form the bedrock 

underlying the Mesa. The Pierre Shale directly underlies the terrace alluvial aquifer throughout most of the 

Mesa. The Niobrara Formation is exposed on the 

Mesa’s northwestern scarp and the southeastern slope 

(Scott, 1964; 1969b). Deformation of the Cretaceous 

rocks in the vicinity of the Mesa has caused the 

contact between the overlying Pierre Shale and the 

underlying Niobrara Formation to become exposed 

along the sides of the Mesa. The Pierre Shale that 

underlies the terrace alluvial aquifer acts as an 

impermeable lower boundary to the ground-water flow 
4 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado Modeled area EXPLANATION Topographic contours—Shows elevation of land surface. Contour interval is 10 feet. Datum is sea level 4750 Base from hypsography from U.S. Geological Survey Digital Raster Graphics, 1998. From U.S. Geological Survey 1:24,000 quadrangles: Devine, Northeast Pueblo, Southeast Pueblo, Vineland, 1974. Hydrology modified by Denver Water

Board, 1998, from U.S. Geological Survey Digital Line Graphs,1:100,000, 1981–1983. Roads and cities from Colorado Departmentof Transportation, 1998. State plane projection. 1927 NorthAmerican datum, Colorado coordinate system, Southern zone

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer D

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

4600

4600

4700

4750

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4700

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46

46

Bessemer Ditch

Figure 2. Topographic contours on and near St. Charles Mesa.
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 5

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4690

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4630

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Modeled–area boundaryBedrock contour—Shows elevation of bedrock surface. Contour interval is 10 feet. Datum is sea level

4720

EXPLANATION

104 34' o 104 30' o

38 16'

o

38 14'

o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer 

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

Nicholson Road

Figure 3. Bedrock surface underlying St. Charles Mesa.
6 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

system. Figure 3 shows the configuration and elevation 

of the bedrock surface. The contact between the 

terrace alluvial aquifer and the underlying bedrock on 

the steep sides of the Mesa generally lies between the 

top and the bottom of the scarp forming the sides of 

the Mesa. Because the contact between the bedrock 

and the overlying terrace alluvial aquifer is physiographically higher than the Arkansas River and St. 

Charles River alluvial aquifers, the shallow, unconfined ground-water flow system on the Mesa is 

perched and does not receive flow from adjacent aquifers.

Ground-water flow in the terrace alluvial aquifer 

is generally from the southwest to the northeast, with 

the highest water-table elevations in the vicinity of the 

Ditch near the southwestern corner of the study area. 

The water table for May 1998 (fig. 4) represents nearly 

average hydrologic conditions. The terrace alluvial 

aquifer of the Mesa is very permeable and receives 

recharge as seepage from the Ditch when water is 

being conveyed in the Ditch. Water levels in well 

SCM–28, which is near the Ditch (fig. 4), fluctuated 

about 14 ft during April 1997 through October 1998 

and increased relatively rapidly when water was 

diverted into the Ditch beginning in mid-February 

1998 (fig. 5). Water levels in well SCM–02 (fig. 5), 

which is farther away from the Ditch (fig. 4), fluctuated about 3 ft during this period. The Ditch conveys 

water beginning in mid-February to mid-March until 

mid-November. Recharge to the terrace alluvial 

aquifer occurs as seepage through the unlined bottom 

of the Ditch, as seepage from lateral ditches as water is 

delivered to irrigated fields, and by deep percolation of 

irrigation water, precipitation, and septic-system 

effluent.

Hydrographs of water levels measured in wells 

during April 1997 through October 1998 indicated that 

the ground-water levels in the terrace alluvial aquifer 

varied by less than 1 ft to more than 15 ft (Brendle, 

1999) (fig. 5). The ground-water flow system was 

assumed to be in a state of dynamic equilibrium 

(steady-state condition) for this study. Thus, average 

monthly water levels and pumpage, and Ditch diversions and precipitation for June 1, 1997, through 

May 31, 1998, were used in model construction and 

calibration.

The hydrologic components that affect groundwater flow and storage in the terrace alluvial aquifer of 

the Mesa (fig. 6) are recharge by infiltration of water 

that seeps from the Ditch and lateral ditches as irrigation water is conveyed; recharge by infiltration of 

water from lawn and crop irrigation and septic-system 

effluent; recharge by infiltration of precipitation; 

discharge of ground water through springs and seeps 

on the northwestern, northern, and southeastern sides 

of the Mesa; discharge of water by evaporation or transpiration (evapotranspiration); and discharge of water 

by well pumpage, some of which results in discharge 

off the Mesa through lined drainage ditches. Seepage 

from the Ditch is the main source of inflow to the 

terrace alluvial aquifer (Dumeyer, 1975).

Infiltration of water from lawn and crop irrigation, septic-system effluent, and precipitation; 

discharge of ground water by evaporation or transpiration; and flow from springs and seeps on the northwestern, northern, and southeastern sides of the Mesa 

were not measured during this study.

Bedrock Surface

The map of the bedrock surface (fig. 3) was 

developed from well-construction reports obtained 

from the Colorado Division of Water Resources 

(DWR) in Denver; the bedrock/alluvium contact along 

the Mesa scarp, which is shown on the geologic maps 

of the Pueblo area (Scott, 1964; 1969b); and depths to 

bedrock from test holes shown on the geologic maps 

and in a previous USGS study (Major and others, 

1970). Most wells on the Mesa were drilled and 

installed to the top of the bedrock and fully penetrate 

the saturated thickness of the terrace alluvial aquifer. 

The depths to bedrock from lithologic logs for 

118 wells were used in constructing a contour map of 

the bedrock surface (fig. 3). Locations of the 118 wells 

obtained from DWR were plotted on a USGS topographic map using the reported distances from the 

north or south and the east or west section lines. The 

elevation of the land surface at each well was interpolated from the topographic contours. These points and 

15 data points from the geologic maps and from Major 

and others (1970) were used to calculate the elevation 

of the bedrock surface (bedrock elevation equals landsurface elevation minus depth to bedrock). Bedrocksurface elevations were machine contoured using the 

TOPOGRID and LATTICECONTOUR commands in 

the geographic information system (GIS) program 

ArcInfo (Environmental Systems Research Institute, 

1992). The resulting calculated contours of the 

bedrock surface were checked for validity and edited 

to more accurately represent the data points where 
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 7

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer 

0 1/2 1 2 

1-1/2

0 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

4745

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4700

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4695

4690

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4655

4730

Modeled–area boundaryWater-table contour—Shows elevation of the water table. Contour interval is 5 feet. Datum is sea levelWell location and identifier

4720

SCM-02

SCM-28

SCM-28

EXPLANATION

Figure 4. Water table in May 1998, and the location of wells SCM–02 and SCM–28, St. Charles Mesa.
8 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

EXPLANATION

Recharge

Discharge

precipitation

springs

and seeps

Arkansas

River

well

pumpage

septicsystem

effluent

lawn

and crop

irrigation Bessemer

Ditch

terrace alluvial aquifer

(sand and gravel)

bedrock (shale)

evaporation,

transpiration

alluvial

aquifer

necessary. Some calculated contours were smoothed to 

reduce what appeared to be unreasonable sinuosity.

Hydraulic Conductivity

Results from six specific-capacity tests were 

used to estimate the horizontal hydraulic conductivity 

of the terrace alluvial aquifer. One test completed by 

the USGS (Wilson, 1965) on the north side of the 

Mesa yielded a horizontal hydraulic conductivity of 

490 ft/d. Analysis of five specific-capacity tests 

completed by private well drillers in the vicinity of 

South Road and 25th Lane (fig. 4) yielded a range of 

estimated horizontal hydraulic-conductivity values of 

250 to 750 ft/d.

Water-Table Surface

Depth-to-water (water-level) measurements 

were obtained in 54 privately owned wells from April 

1997 through October 1998. Although measurements 

were obtained at varying intervals (weekly, biweekly, 

and monthly) depending on the well’s proximity to the 

Ditch and to areas where the high water table exists, 

only the monthly measurements were used in mapping 

the average monthly water table (fig. 7). The average 

monthly water-table surface was selected to represent 

the water table under the assumption of a steady-state 

condition.

Monthly depth-to-water measurements from 

June 1, 1997, through May 31, 1998, were averaged 

for the 54 wells. The average depth-to-water values 

were converted to water-table elevations above sea 

level and were machine contoured using the TOPOGRID and LATTICECONTOUR commands of 

Figure 5. Depth to water in wells SCM–02 and SCM–28, 

April 1997–September 1998.

Figure 6. Components of the St. Charles Mesa hydrologic system.

AMJ J AS S O O N D J FM A M J J A

1997 1998

28

0

4

8

12

16

20

DEPTH TO WATER BELOW LAND 

24

SURFACE, IN FEET

SCM-28

SCM-02
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 9

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104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditch

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

Modeled–area boundaryAverage water-table contour—Shows  elevation of the water table.  Contour interval is 10 feet.  Datum is sea levelSimulated water-table contour—Shows  elevation of the water table.  Contour interval is 10 feet.  Datum is sea level

47404740

EXPLANATION

Figure 7. Average monthly water table, June 1997–May 1998, and the simulated water table for the calibrated steady-state model
10 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

ArcInfo (Environmental Systems Research Institute, 

1992). The resulting machine contours of the watertable surface were checked for validity and edited to 

more accurately represent the data points where necessary, but the contours were not smoothed.

Recharge

Water that recharges the terrace alluvial aquifer 

of the Mesa comes from seepage from the Ditch and 

lateral ditches, precipitation, water used for lawn and 

crop irrigation (surface and ground water), and septic 

system effluent. Estimates of recharge to the terrace 

alluvial aquifer were made separately for Ditch 

seepage and for areal recharge (seepage from lateral 

ditches, precipitation, water used for lawn and crop 

irrigation, and septic-system effluent).

Areal recharge was estimated for agricultural 

(irrigated) areas and residential (nonirrigated) areas. 

The percentages of precipitation, applied lawn or crop 

irrigation water, and septic-system effluent on which 

the estimates were based were obtained from reports 

of previous studies in similar hydrologic or climatic 

settings (Watts and Lindner-Lunsford, 1992; Goodell, 

1988; Weist, 1965). The locations of irrigated parcels 

(fig. 8) were obtained from a digital coverage of the 

Mesa obtained from the DWR. Areas not indicated as 

irrigated were considered nonirrigated, even though 

the application of water in those areas was not necessarily uniform between discreet nonirrigated areas. 

Some nonirrigated parcels are residential lots, whereas 

others are vacant lots that receive no lawn watering.

Ditch Seepage

Although the Ditch through Pueblo is lined to 

limit seepage, the lining ends at Aspen Street on the 

Mesa. Where the Ditch is not lined, water seeps 

through the bottom and sides of the Ditch and 

recharges the terrace alluvial aquifer. Because the 

water table near the Ditch is approximately 4 ft below 

the bottom of the Ditch, the water table does not 

contribute flow back into the Ditch. The amount of 

seepage from the Ditch was estimated on the basis of 

measurements made during May 1999 and March and 

April 2000.

Because the Ditch had been flowing for about 

3 months in May 1999, steady seepage through the 

Ditch’s bottom and sides was assumed. Discharge 

measurements were made during one day at the Aspen 

Street bridge (upstream site), upstream from the 

25th Lane bridge over the Ditch, the overflow structure at Nicholson Road (downstream site), and at all 

the lateral ditches where a measurement was possible. 

Five measurements were made at the Aspen Street 

bridge and at the overflow structure at Nicholson 

Road, and three measurements were made upstream 

from the 25th Lane bridge over the Ditch. Flow into 

lateral ditches that were inaccessible for a measurement was estimated from the Bessemer Irrigation 

Ditch Company’s settings for the lateral headgates 

(Dan Henrichs, Bessemer Irrigation Ditch Company, 

oral commun., 1999).

The difference between the average of five flow 

measurements obtained at the upstream (258.4 ft3/s) 

and downstream (191.0 ft3/s) ends of the Ditch was 

67.4 ft3/s. A total seepage of 22.1 ft3/s from the Ditch 

to the terrace alluvial aquifer was calculated by 

subtracting lateral diversions from the difference 

between the upstream and downstream flows. The estimated seepage is greater than the presumed 5-percent 

error in the discharge measurements.

The relation between the rate of flow in the 

Ditch and the height of water in the Ditch for each 

flow rate (rating) was used to estimate the amount of 

seepage associated with each average monthly rate of 

flow. Additional flow measurements at the Aspen 

Street bridge were obtained in March 2000 (79.2 ft3/s) 

and in April 2000 (174 ft3/s) and were used to develop 

a rating curve for the Ditch. Because a plot of the logarithm of discharge (flow) against the logarithm of 

stage (height of water above the bottom of a stream or 

ditch) is approximately linear, it is possible to estimate 

the stage for a known (measured) flow rate (Kennedy, 

1984). Because the shape of the Ditch is approximately trapezoid, the stage is approximately the 

average height of water above the horizontal bottom of 

the Ditch. Monthly flow in the Ditch, which was 

provided by the Bessemer Irrigation Ditch Company 

(Dan Henrichs, Bessemer Irrigation Ditch Company, 

oral commun., 1999), was used to estimate average 

monthly stage. The average monthly stage in the Ditch 

at Aspen Street was assumed to approximate the 

average monthly stage in the reach of the Ditch 

traversing the Mesa.
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 11

10

10

10

5

5 5 5

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer D

0 1/2 1 2 MILES 1-1/2

0 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. Irrigated areas from the Colorado Division of Water Resources, 1998.State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

21 inches per year (irrigated)16 inches per year (irrigated)1.5 inches per year (nonirrigated)Irrigation wellLateral headgateDepth-to-water contour—Shows depth to water, in feet, November 1997 (Brendle 1999). 

RECHARGE RATES

10

EXPLANATION

Figure 8. Calibrated recharge rates for irrigated and nonirrigated areas, and the locations of irrigation wells and lateral headgate
12 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

Average monthly seepage from the Ditch (Q) 

was calculated using Darcy’s Law (eq. 1),

(1)

where

Q is the discharge (vertical flow) through the 

Ditch bottom (seepage), in cubic feet 

per second; 

K is the hydraulic conductivity of the Ditch 

bottom materials through which flow 

occurs, in feet per day;

A is the area through which flow occurs 

(area of Ditch bottom), in square feet;

dh is the change in head across the materials 

through which flow occurs (h2 – h1), in 

feet, where h1 is the initial height of water 

(average monthly stage in the Ditch) and 

h2 is the final height of water above the 

bottom of the Ditch, assumed to be 0 

(h1 = dh when h2 = 0); and

dx is the distance over which flow occurs 

perpendicular to the area through which 

flow occurs (x2 – x1), or the thickness of 

the Ditch bottom, in feet.

Because the hydraulic conductivity of the Ditch 

bottom (K) and the flow-path length (dx) are unknown 

and the area of the ditch bottom (A) is only approximate, the factor KA/dx was calculated. Equation 1 was 

rearranged to solve for the constant KA/dx by using the 

seepage from the Ditch (Q) estimated from the May 

1999 flow measurements and the stage (dh) at the time 

the seepage was estimated.

For Q of 22.1 ft3/s and dh of 4.17 ft (estimated 

stage for Q of 22.1 ft3/s), the factor KA/dx is 5.30 ft2/s 

(22.1 ft3/s ÷ 4.17 ft = 5.30 ft2/s). The average monthly 

seepage rates (Q) were estimated as the product of 

KA/dx and the estimated average monthly stage (h1). 

The average of the monthly seepage rates was 

13.2 ft3/s (table 1). 

Areal Recharge

The term “areal recharge” refers to recharge that 

is not focused at a point but that occurs over a larger 

area, such as agricultural fields or residential areas. 

Initially, two different areal recharge rates were estimated, one for irrigated areas and one for residential 

areas.

Seepage from lateral ditches was expected to 

contribute to recharge to the terrace alluvial aquifer. 

Lateral ditch seepage was not measured but was 

assumed to be accounted for in the overall areal 

recharge term.

Q K = – A dh/dx

Table 1. Average monthly flow and stage in Bessemer Ditch, average monthly seepage from Bessemer Ditch, and total 

diversions from Bessemer Ditch to lateral ditches for June 1997 through May 1998

Average monthly flow 

in Bessemer Ditch 

(cubic feet per second)

Average monthly 

stage (h1) in 

Bessemer Ditch

(feet)

Average monthly seepage 

from Bessemer Ditch 

(cubic feet per second)

Total of diversions to 

lateral ditches (cubic 

feet per second)

June 1997 263.54 4.22 22.36 55.66

July 276.63 4.34 23.00 56.65

August 263.55 4.22 22.37 52.96

September 167.67 3.27 17.33 38.82

October 115.66 2.73 14.47 35.69

November 52.37 2.02 10.71 11.72

December 0 0 0 0

January 1998 0 0 0 0

February 0 0 0 0

March 62.55 2.14 11.34 28.75

April 143.30 3.02 16.01 42.43

May 232.53 3.92 20.78 51.84

AVERAGE 131.48 2.49 13.20 31.21
GEOHYDROLOGIC SETTING OF ST. CHARLES MESA 13

There is no continuous-recording rain gage on 

the Mesa, but precipitation measurements obtained at 

the weather station at Pueblo Airport are considered 

representative of precipitation on the Mesa because 

neither the airport nor the Mesa is affected by 

orographic effects (Viessman and others, 1977). 

Precipitation measured at the weather station at Pueblo 

Airport (fig. 1), which lies about 5 miles north from 

the center of the Mesa, was 15.8 inches (4.18 ×

10–8 ft3

/s) from June 1, 1997, through May 31, 1998.

In agricultural areas, recharge to the terrace alluvial aquifer includes precipitation and ground water 

and surface water used for irrigation. Twenty-two 

percent of precipitation was assumed to recharge the 

terrace alluvial aquifer in the agricultural areas 

(Goodell, 1988). Goodell’s estimates are used for this 

study because the climatic conditions, precipitation, 

and soils on the Mesa are similar to areas of the Snake 

River Plain. Twenty-five percent of applied irrigation 

water was assumed to recharge the terrace alluvial 

aquifer (Weist, 1965). The average total diversion into 

lateral ditches was 31.2 ft3

/s (Dan Henrichs, Bessemer 

Irrigation Ditch Company, written commun., 1999), 

the average pumpage used for irrigation was 

985 gal/min (2.19 ft3

/s) (Colorado Division of Water 

Resources, Pueblo), and the precipitation was 

15.8 in/yr (4.18 × 10–8 ft/s). The recharge rate in irrigated areas was calculated using equation 2:

(2)

where

RA is the recharge rate in agricultural areas, in feet 

per second;

E is the average diversions into lateral ditches, in 

cubic feet per second;

F is the average pumpage, in cubic feet per 

second;

G is the total irrigated area, in square feet; and

H is the average annual precipitation, in feet per 

second.

In the residential area, 5 percent of precipitation 

was assumed to recharge the terrace alluvial aquifer 

based on a study by Watts and Lindner-Lunsford 

(1992) near La Junta, Colorado. Average household 

water use was 265 gal/d (4.10 × 10–4 ft3/s) for the 

2,100 households receiving municipal water from St. 

Charles Mesa Water District (David Simpson, St. 

Charles Mesa Water District, oral commun., February 

2000). Seventy percent of household water use was 

assumed to recharge the terrace alluvial aquifer, based 

on Goodell’s (1988) study in the Snake River Plain in 

Idaho. Household water use includes infiltration of 

water used for lawn irrigation and septic-system 

effluent. Septic systems were assumed to be uniformly 

distributed throughout the residential area. Recharge 

due to septic system effluent was accounted for by 

assuming a certain rate of infiltration of household 

water used. Equation 3 was used to estimate the 

recharge rate in residential areas:

(3)

where

RR is the recharge rate for residential area, 

in feet per second;

A is the number of households receiving 

municipal water;

B is the average water use per household, in 

cubic feet per second;

C is the total residential area, in square feet; and

D is the average annual precipitation, in feet per 

second.

The estimates of recharge rates for residential 

and irrigated areas are listed in table 2. The distribution of areal recharge is shown in figure 8.

Discharge

Water that discharges from the terrace alluvial 

aquifer of the Mesa occurs as spring and seep 

discharge, evapotranspiration, and ground-water 

pumpage. Ground water that is pumped from the 

terrace alluvial aquifer can recharge the ground-water 

system, can flow off the Mesa through drainage 

ditches, or can be lost through evaporation. 

Spring and Seep Discharge

Previously, spring and seep discharge from the 

Mesa was estimated by Dumeyer (1975) to be 13 ft3

/s. 

Cain and others (1980) made several discharge 

measurements in the Arkansas River in the reach north 

of the Mesa. No measurable tributary inflows were 

observed in this reach, and spring and seep discharge 

from the Mesa of 21 ft3/s was calculated. The current 

(2000) spring and seep discharge from the Mesa was 

assumed to be between 13 and 21 ft3

/s for this study.

RA ( ) E F + × 0.25

G = ----------------------------------- + H × 0.22

RR A B × × 0.7

C = ----------------------------- + D × 0.05
14 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

Evapotranspiration

An evapotranspiration (ET) rate of 47 in/yr and 

an extinction depth of 5 ft was estimated for the Mesa 

area. This estimate was based on a curve of ET relative 

to depth to water developed for a study in the San Luis 

Valley in Colorado (Emery, 1970, fig. 3), and is lower 

than the pan evaporation rate for this area of 60 in/yr. 

The extinction depth is the depth below which the ET 

rate is zero. Above the extinction depth, the ET rate 

increases linearly to the maximum rate at the ground 

surface. The area of the Mesa within which ET is 

expected to occur occupies about one-fifth of a square 

mile. This area corresponds with the areas defined in 

November 1997 as having a depth to water less than 

5 ft (fig. 8).

Pumpage

Monthly pumpage from 86 irrigation wells 

(rates ranged from 11.41 to 531 gal/min (2.54 × 10–2

to 1.18 ft3/s)) obtained from DWR was used in the 

model. Figure 8 shows the locations of the irrigation 

wells. Pumpage from household wells was not 

included in the pumpage estimates. Most water 

for domestic use is provided by the St. Charles Mesa 

Water District. Pumpage from household wells is 

used mostly to supplement water provided by the 

St. Charles Mesa Water District.

Conceptual Model

The terrace alluvial aquifer of the Mesa is a 

shallow, unconfined, ground-water system perched on 

impermeable shale. The terrace alluvial aquifer does 

not receive water discharged from neighboring 

ground-water systems because it is perched above both 

the Arkansas and St. Charles alluvial aquifers. Watertable elevations are highest near the southwest corner 

of the Mesa. Ground water generally flows toward the 

northeast. Recharge to the aquifer occurs primarily as 

seepage from the Ditch and lateral ditches, and infiltration of lawn and crop irrigation water, septic-system 

effluent, and precipitation. Discharge from the aquifer 

occurs primarily as outflow from springs and seeps, 

evapotranspiration, and pumpage. Springs and seeps 

are intermittently dispersed along the northwestern, 

northern, and southeastern sides of the Mesa.

SIMULATION OF GROUND-WATER FLOW

The numerical model of ground-water flow, 

MODFLOW–96 (Harbaugh and McDonald, 1996), 

which uses a finite-difference approximation method, 

was used to simulate ground-water flow in the terrace 

alluvial aquifer of the Mesa. The terrace alluvial 

aquifer of the Mesa was simulated as a one-layer 

system with an underlying impermeable boundary 

Table 2. Land-use areas and estimated recharge rates

[Ditch, Bessemer Ditch in fig. 2]

Area

classifications

Area

(square feet)

Percentage of precipitation, household 

wastewater, or irrigation water recharging 

the terrace alluvial aquifer

Estimated 

recharge rate 

(cubic feet per 

second per 

square foot)

Rate used in 

calibrated 

model

(cubic feet per 

second per 

square foot)

Residential (nonirrigated) area 158,748,125 5a precipitation; 70b household wastewater 5.81 × 10–9 4.00 × 10–9

Irrigated parcels less than 0.5 

mile from the Ditch between 

Aspen Street and South Road

18,438,410 22c precipitation; 25c irrigation water 8.59 × 10–8 5.54 × 10–8

Irrigated parcels greater than 0.5 

mile from the Ditch, or parcels 

within 0.5 mile of the Ditch 

from South Road to Nicholson 

Road

90,326,016 22c

 precipitation; 25c irrigation water 8.59 × 10–8 4.23 × 10–8

a

Watts and Lindner-Lunsford, 1992.

b

Goodell, 1988.

c

Weist, 1965.
SIMULATION OF GROUND-WATER FLOW 15

(bedrock). A grid, consisting of 98 columns by 

58 rows, was used to divide the modeled area into 

300-ft by 300-ft cells (fig. 9). Each cell represents an 

area of 90,000 ft2

 within which characteristics of the 

flow system are assumed to be uniform.

Hydrologic characteristics of the flow system 

were measured (seepage from the Ditch at relatively 

high flow and water levels in wells) or estimated 

(hydraulic conductivity of the aquifer, ET rate and 

extinction depth, recharge rates for irrigated and 

nonirrigated areas of the Mesa, and the extent of areas 

through which spring and seep discharge occurs) for 

input into the model.

Data that define the geometry and hydraulic 

properties of the terrace alluvial aquifer, and the locations of hydrologic flows (such as spring and seep 

discharge) and stresses (such as ground-water 

pumpage) were developed as digital coverages in 

ArcInfo (Environmental Systems Research Institute, 

1992) for input to a graphical user interface (GUI), 

developed by the USGS and Argus Interware, which 

runs on the GIS program ArgusONE (Argus) (Shapiro 

and others, 1997). Argus creates the model grid and 

populates each model cell with the flow-system 

parameters (land-surface elevation, bedrock elevation, 

water-table elevation, recharge rate, and so forth) that 

are later used as input files for MODFLOW–96.

Model Description

The terrace alluvial aquifer has real physical 

boundaries, such as the impermeable bedrock, which 

acts as a lower boundary to limit flow in a downward 

direction. The model boundaries simulate hydrologic 

boundaries that control the amount and rate of water 

entering and leaving the simulated terrace alluvial 

aquifer of the Mesa. Boundaries of the model are the 

horizontal-flow boundaries, a lower no-flow boundary 

at the bottom of the model’s active cells, and an upper, 

specified-flow boundary.

The horizontal-flow boundaries are no-flow 

cells surrounding most of the lateral extent of active 

cells, cells containing injection wells to simulate Ditch 

seepage, and drain cells used to simulate spring and 

seep discharge (fig. 9). The lateral extent of the model 

corresponds to the Ditch on the southwestern side and 

the bedrock-alluvium contact on the northwestern, 

northern, and southeastern sides of the Mesa (Scott, 

1964; 1969b). The drain cells on the sides of the Mesa 

are used to simulate ground-water discharge from 

springs and seeps. The drain cells generally are located 

at the head of the washes that dissect the steep sides of 

the Mesa. No-flow cells were simulated between these 

washes because there is negligible ground-water 

discharge along the scarp between the washes. A 

no-flow boundary was used upgradient from the Ditch 

because the small area of alluvium southwest of the 

Ditch does not contribute significantly to flow in the 

system. 

The bottom of the active model cells is a noflow boundary that represents the bedrock surface, 

which directly underlies the alluvial deposits that 

compose the terrace alluvial aquifer of the Mesa.

The water table is the upper boundary of the 

model. This boundary is a specified-flow boundary, 

whose level may move vertically in response to 

recharge and discharge conditions.

The preliminary model developed during the 

first phase of this study indicated that better definitions 

were needed on the amount of Ditch seepage and the 

hydraulic conductivity of the terrace alluvial aquifer. 

Based on three flow measurements made in the Ditch, 

an average Ditch seepage of 13.2 ft3/s was calculated 

and used in the model. A value of hydraulic conductivity of 500 ft/d (5.78 × 10–3 ft/s) for the terrace alluvial aquifer was used initially in the model. Estimates 

of initial recharge to residential areas was 2.2 in/yr 

(5.81 × 10–9 ft/s), and in irrigated areas initial recharge 

was 32.5 in/yr (8.59 × 10–8 ft/s). Monthly pumpage 

from 86 wells (rates ranged from 11.41 to 531 gal/min 

[2.54 × 10–2 to 1.18 ft3

/s]) obtained from DWR was 

used in the model. The pumpage was simulated both 

as discharge from the ground-water flow system and 

as a component in estimating the amount of simulated 

recharge to irrigated fields.

Model Calibration

The model input layers were entered into the 

Argus GUI (Shapiro and others, 1997) and the model 

was calibrated to the average monthly water table for 

June 1, 1997, through May 31, 1998. Figure 7 shows 

the average monthly water table for June 1, 1997, 

through May 31, 1998, and the simulated water table 

for the calibrated steady-state model. The spring and 

seep discharge of 13 ft3

/s estimated by Dumeyer 

(1975) and 21 ft3/s calculated from measurements 

made by Cain and others (1980) were used to define a 
16 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado 104 34' o 104 30' o 38 16' o 38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditc

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

Injection–well cellsDrain cell(conductance = 0.0120 ft /s)

Drain cell(conductance = 0.2025 ft /s)Drain cell(conductance = 0.0975 ft /s)

Other active cells

Inactive model cells 2 2

2

EXPLANATION

Figure 9. Model grid and simulated boundary conditions.
SIMULATION OF GROUND-WATER FLOW 17

calibration range. The values and spatial distributions 

of hydraulic conductivity, recharge, drain conductance, and Ditch seepage were adjusted during calibration to minimize the difference between simulated and 

average water-table elevations and to match the estimated spring and seep discharge. Adjustments to the 

values were made within limits considered likely for 

the hydrologic system. For example, hydraulic 

conductivities for valley-fill deposits in the Arkansas 

River Valley determined by Wilson (1965) ranged 

from 32 to 2,045 ft/d (3.70 × 10–4 to 2.37 × 10–2 ft/s). 

Thus, the calibrated values for hydraulic conductivity 

were kept within these limits.

During model calibration, the distribution of 

hydraulic conductivity (fig. 10) was changed from a 

single value for the entire aquifer to one that was based 

on the distribution of the Slocum Alluvium and 

Louviers Alluvium that form the terrace alluvial 

aquifer. The revised distribution of hydraulic conductivity resulted in a better match between average 

measured and simulated water levels. Hydraulic 

conductivity was also adjusted, alternately with drain 

conductance, to better match the estimated discharge 

through seeps and springs. Simulated Ditch seepage 

was adjusted during calibration by redistributing injection wells from cells in areas where simulated heads 

along the Ditch were too high to cells in areas along 

the Ditch where the simulated heads were too low. 

Table 3 lists the distribution of simulated Ditch 

seepage and the length of the Ditch through which the 

simulation occurs in the calibrated model.

The distribution of recharge from irrigated fields 

was adjusted to account for the expected seepage 

losses from lateral ditches delivering water to fields by 

assigning higher recharge rates to irrigated fields near 

the Ditch (table 2). Because water delivered to irrigated fields farther from the Ditch must flow through 

lateral ditches adjacent to fields near the Ditch, 

seepage losses from laterals nearer the Ditch likely 

would be greater.

The model was considered calibrated when the 

root mean squared error (RMSE) and the mean of the 

differences were minimized between the average 

water-table elevations and the simulated water-table 

elevations at points corresponding to the 54 watertable-measurement locations. The RMSE is a measure 

of the magnitude of differences (errors) between 

average measured and simulated water levels over the 

entire modeled area (Lucey and others, 1995). The 

mean of the differences is a measure of the systematic 

error; it approaches zero when the sum of the differences between average measured and simulated water 

levels that are greater than zero equals the sum of the 

differences that are less than zero (Lucey and others, 

1995). The RMSE is calculated as shown in equation 4 

(Anderson and Woessner, 1992):

(4)

where

M is the average measured water level, in feet;

S is the simulated water level, in feet; and

N is the number of comparisons.

The RMSE and the mean of the differences for 

the calibrated model were, respectively, 3.85 ft and 

–0.44 ft. The –0.44 ft mean of the differences indicates 

that the simulated heads are on average 0.44 ft higher 

than the mean of the average measured water levels in 

the wells used for comparison.

The differences between the average water-table 

elevations and the simulated water-table elevations at 

points are shown in figure 11. Differences greater than 

zero indicate simulated water levels are lower than the 

average water-table elevations, and differences less 

than zero indicate simulated water levels are higher 

than the average water-table elevations.

The water budget for the model (table 4) is 

useful in evaluating whether the calibrated model 

adequately represents the hydrologic system of the 

Mesa. Discharge from springs and seeps was caliRMSE

M – S )2 ∑(

N = -----------------------------

.

Table 3. Description of Bessemer Ditch segments, the 

segment length, and the amount of simulated seepage for 

each Ditch segment

Description

(see fig. 10)

Length of 

ditch 

segment 

(miles)

Amount of 

simulated 

seepage

(cubic 

feet/second)

From Aspen Street to Nicholson Road 3.31 13.2

From 21st Lane to Nicholson Road 2.28 7.4

From 23rd Lane to Nicholson Road 1.74 6.3

From 25th Lane to Nicholson Road 1.19 3.7

From Aspen Street to 21st Lane 1.03 5.8

From 21st Lane to 23rd Lane 0.54 1.1

From 23rd Lane to 25th Lane 0.55 2.6
18 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado 104 34' o 104 30' o 38 16' o 38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditc

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

518 feet per day

HYDRAULIC CONDUCTIVITY

432 feet per day

Modeled–area boundary

EXPLANATION

Figure 10. Hydraulic-conductivity zones used in the calibrated steady-state model.
SIMULATION OF GROUND-WATER FLOW 19 DIFFERENCE (measured minus simulated) 5 to 10 feet 0 to 5 feet –5 to 0 feet –10 to –5 feet –15 to –10 feet –20 to –15 feet Water-level measurement point Modeled–area boundary EXPLANATION

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS RIVER

Salt Creek

Bessemer Ditch

St.

Charles

R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer

D

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

Figure 11. Difference between the measured and simulated water levels, and the locations of water-level measurement points.
20 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

brated to be between the discharge estimated from 

previous studies (13 to 21 ft3/s). The primary inflow 

simulated by the model is Ditch seepage (70.4 percent 

of total inflow), and the primary outflow is spring and 

seep discharge (84.7 percent of total outflow). The 

water budget indicates the significant effect of seepage 

from the Ditch on the terrace alluvial aquifer. The 

discrepancy of 0.01 ft3/s between the inflow to the 

model and the outflow from the model is due to 

rounding errors when calculating the budget terms.

Sensitivity Analysis

Sensitivity testing of a model is performed to 

determine uncertainties in the calibrated model which 

are caused by uncertainties in parameter values 

(Anderson and Woessner, 1992). The model is considered to be more sensitive to a particular parameter if a 

change in the parameter results in a relatively large 

increase in the RMSE. Parameters to which a model is 

very sensitive need to be defined as well as possible to 

improve model accuracy.

An analysis of model sensitivity was performed 

by changing parameter values incrementally to determine the effect of those changes on the RMSE. Drain 

conductance, hydraulic conductivity, areal recharge 

rate, and Ditch seepage were individually changed to 

50, 75, 95, 110, 125, and 150 percent of their calibrated values to determine the associated change in 

RMSE for the model (fig. 12). The model did not 

converge on a solution when Ditch seepage was 

reduced below 95 percent of the calibrated value. 

When the seepage was reduced to below 95 percent of 

its calibrated value, cells near the Ditch were 

completely dewatered. The model also failed to 

converge when hydraulic conductivity was increased 

to greater than 110 percent of the calibrated value. 

Large increases in hydraulic conductivity allowed too 

much discharge of water from the model, thus simulating complete dewatering of local portions of the 

terrace alluvial aquifer. The order of decreasing sensitivities for the remaining parameters was as follows: 

decreased hydraulic conductivity, increased ditch 

seepage, decreased drain conductance, increased areal 

recharge rate, increased drain conductance, and 

decreased areal recharge rate.

The model was relatively insensitive to 

decreases in the areal recharge rate. The model yielded 

lower RMSE values for decreases in recharge rate to 

75 and 95 percent of the calibrated value. These lower 

values of recharge were not used in the calibrated 

model because they were lower than values considered 

reasonable.

EVALUATION OF POSSIBLE 

ALTERNATIVES TO LOWER THE 

HIGH WATER TABLE

Possible alternatives to lower the high water 

table of the Mesa were tested by running transientstate simulations using the hydraulic parameters from 

the calibrated steady-state model. The alternatives 

were simulated in transient-state mode because the 

model would not converge in steady-state mode when 

the Ditch seepage was entirely cut off. In steady-state 

mode, model cells were simulated to become prematurely dry when the Ditch seepage was decreased to 

simulate each of the Ditch-lining alternatives. In transient-state mode, the model approaches a solution in 

smaller increments, and fewer model cells were simulated to become dry. Steady-state results can be 

approximated by running a steady-state model in tranTable 4. Water budget for the calibrated model

[Ditch, Bessemer Ditch in fig. 2]

Inflow

Rate of

simulated 

inflow

(cubic feet per 

second)

Percentage of 

total inflow Outflow

Rate of

simulated 

outflow

(cubic feet 

per second)

Percentage of 

total outflow

Injection wells to simulate 

Ditch seepage

13.18 70.4 Withdrawal wells 2.20 11.8

Areal recharge 5.53 29.6 Drains to simulate springs and seeps 15.86 84.7

Evapotranspiration 0.66 3.5

TOTAL INFLOW 18.71 100 TOTAL OUTFLOW 18.72 100
EVALUATION OF POSSIBLE ALTERNATIVES TO LOWER THE HIGH WATER TABLE 21

50

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

60 70 80 90 100 110 120 130 140 150

PERCENT CHANGE IN CALIBRATED PARAMETER VALUE

ROOT MEAN SQUARED ERROR, IN FEET

DRAIN CONDUCTANCE

HYDRAULIC CONDUCTIVITY

AREAL RECHARGE RATE

DITCH SEEPAGE RATE

sient mode until subsequent model runs induce only 

relatively small changes in storage in the aquifer.

Several model simulations were run to evaluate 

various possible alternatives for lowering the water 

table of the Mesa. The possible alternatives simulated 

were (1) reducing areal recharge by reducing recharge 

to irrigated areas by 25 percent, (2) lining Bessemer 

Ditch from (a) Aspen Street to 21st Lane; (b) Aspen 

Street to 23rd Lane; (c) Aspen Street to 25th Lane; and 

(d) Aspen Street to Nicholson Road, (3) installing two 

drains at a depth of 10 feet below land surface upgradient from the high water table areas, and (4) installing 

22 dewatering wells within the high water table areas, 

each pumping at 80 gallons per minute.

The calibrated steady-state water levels were 

used as the starting water-table elevations in the transient simulations to calculate the effectiveness of the 

alternatives in lowering the water table. The effectiveness of the alternatives for lowering the high water 

table was based on the amount and extent of simulated 

water-level decline associated with each alternative. 

The model calculates the amount of decline for each 

alternative relative to the starting heads in the system. 

The saturated thickness simulated to be remaining in 

the terrace alluvial aquifer for each of the alternatives 

was calculated by subtracting the simulated watertable declines and the bedrock-surface elevations from 

the simulated initial water-table surface elevations.

 Maps showing the amount and extent of watertable declines were generated for each of the six alternatives that were at least partially effective in lowering 

the water table in the areas defined in November 1997 

as having a high water table. The November 1997 

water-level measurements were selected to represent 

the high water table because the highest water table in 

most wells occurs in autumn. November 1997 depthto-water contours (Brendle, 1999) were superimposed 

on the maps for comparison with the mapped, simulated water-level declines for each alternative. A simulation was considered effective in alleviating the high 

water table if water-table declines were at least 

5 ft in the areas where a high water table existed in 

November 1997.

Reducing Areal Recharge

The alternative of reducing areal recharge in 

irrigated areas by 25 percent resulted in a maximum 

decline in the water table of about 3 ft near the northeast corner of the Mesa. The water-table declines 

resulting from this alternative were only about 1 ft in 

the areas defined as having a high water table in 

Figure 12. Root mean squared error between measured and simulated water levels as a 

result of varying model input-parameter values.
22 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

November 1997 and were, thus, considered ineffective 

in lowering the high water table in those areas. For 

these reasons, no map is shown of the resulting watertable declines or the saturated thickness in the terrace 

alluvial aquifer for this alternative. Larger reductions 

in the rate of areal recharge to irrigated areas were not 

evaluated because they were considered to be too large 

to be reasonably implemented.

Lining Various Portions of the Ditch

The effects of lining various portions of the 

Ditch along its reach across the Mesa were simulated 

by removing injection wells used to simulate seepage 

of Ditch water into the ground-water flow system. 

Four alternatives simulated lining the Ditch: from 

Aspen Street to 21st Lane; from Aspen Street to 

23rd Lane; from Aspen Street to 25th Lane; and from 

Aspen St. to Nicholson Road (the entire Ditch across 

the Mesa).

The three alternatives that simulated partial 

lining of the Ditch resulted in partial dewatering of the 

terrace alluvial aquifer. All the Ditch-lining alternatives resulted in simulated maximum water-table 

declines of about 27 to 29 ft (figs. 13–16).

Based on the November 1997 depth-to-water 

contours, as the length of the simulated Ditch lining 

increased, the extent of the area in which the water 

table was simulated as being lowered also increased. 

Simulation of lining the Ditch from Aspen Street to 

21st Lane resulted in water-table declines of slightly 

less than 5 ft to 27 ft in the area west of 25th Lane and 

south of Everett Road (fig. 13). Increases in the extent 

of Ditch lining caused increasingly greater simulated 

areas with at least 5 ft of decline in the water table 

(figs. 14–16). Lining the entire Ditch resulted in simulated declines in the water table of at least 5 ft in most 

of the modeled area (fig. 16). However, none of the 

Ditch-lining alternatives appear to have been effective 

in lowering the water table at least 5 ft throughout the 

entire area having a high water table north of Everett 

Road.

The saturated thickness of the terrace alluvial 

aquifer resulting from each of the Ditch-lining alternatives was compared with the initial saturated thickness 

of the aquifer (fig. 17). As the extent of simulated 

Ditch lining increased, the simulated saturated thickness in the aquifer decreased (figs. 18–20). A simulated saturated thickness map is not shown for lining 

the entire Ditch because model results indicate almost 

complete dewatering of the aquifer.

All the Ditch lining alternatives likely would 

result in water-table declines that could cause at least 

some private wells to become unusable because of 

aquifer dewatering, with the more extensive Ditch 

lining resulting in the largest number of wells 

becoming unusable. Wells could stop producing water 

if implementation of an alternative lowered the water 

table to a position lower than the pump intake (fig. 21).

Installation of Drains and Dewatering 

Wells

Alternatives were simulated to evaluate the 

potential effects of subsurface drains and dewatering 

wells on water levels in the area of the high water table 

on the Mesa. One alternative simulated two drains that 

were 10 ft below land surface (fig. 22) and the other 

simulated 22 dewatering wells, each pumping at 80 

gal/min (0.18 ft3

/s) (fig. 23). Maximum simulated 

water-table declines for these two alternatives were 

much smaller than for the alternatives involving lining 

the Ditch: 6.8 ft for the drain alternative and 14.4 ft for 

the dewatering-well alternative.

Simulated water-table declines for the drain 

alternative were greatest in the vicinity of Santa Fe 

Drive and 25th and 27th Lanes and north of Everett 

Road, between 23rd and 25th Lanes (fig. 22). Existing 

wells in these areas would stop producing water if 

water-table declines were sufficient to lower the water 

table below the pump intakes (fig. 21).

Simulated water-table declines for the dewatering-well alternative were greater than 5 ft between 

23rd and 27th Lanes and north of County Farm Road 

(fig. 23). This alternative for lowering the water table 

might cause some wells to stop producing water in the 

areas where the greatest simulated declines occurred: 

northeast of Everett Road and 23rd Lane and southeast 

of Santa Fe Drive and 23rd Lane.

The drain and dewatering-well alternatives were 

more effective in lowering the water table in the high 

water table areas than lining the Ditch from Aspen 

Street to either 21st Lane or 23rd Lane. The drain and 

dewatering-well alternatives were less effective in 

lowering the water table in the high water table areas 

than lining the Ditch from Aspen Street to either 25th 

Lane or to Nicholson Road.
EVALUATION OF POSSIBLE ALTERNATIVES TO LOWER THE HIGH WATER TABLE 23

10

10

10 5

5 5 5

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditc

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

SIMULATED WATER-TABLE DECLINE

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feet20 to 27 feetDepth-to-water contour—Shows depth to water in feet, November 1997 (Brendle, 1999)

Modeled–area boundary

10

EXPLANATION

Figure 13. Simulated water-table decline in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch from Aspen 
24 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

10

10

10 5

5 5 5

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Dit

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feet20 to 29 feet

Depth-to-water contour—Shows depth to  water, in feet, November 1997 (Brendle, 1999)

10

Modeled–area boundary

EXPLANATION

SIMULATED WATER-TABLE DECLINE

Figure 14. Simulated water-table decline in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch from Aspen 
EVALUATION OF POSSIBLE ALTERNATIVES TO LOWER THE HIGH WATER TABLE 25

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Di

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feet20 to 29 feet

Depth-to-water contour—Shows depth to  water, in feet, November 1997 (Brendle, 1999)

SIMULATED WATER-TABLE DECLINE

10

Modeled–area boundary

EXPLANATION

10

10

10

5

5 5 5

Figure 15. Simulated water-table decline in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch from Aspe
26 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

10

10

10

5

5

5 5

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditch

0 1/2 1 2 MILES 1-1/2

0 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feet20 to 29 feet

Depth-to-water contour—Shows depth to  water, in feet, November 1997 (Brendle, 1999)

SIMULATED WATER-TABLE DECLINE

10

Modeled–area boundary

EXPLANATION

Figure 16. Simulated water-table decline in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch from Aspen 

Nicholson Road.
EVALUATION OF POSSIBLE ALTERNATIVES TO LOWER THE HIGH WATER TABLE 27

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer D

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feetGreater than 20 feet

SIMULATED SATURATED THICKNESS

Modeled–area boundary

EXPLANATION

Figure 17. Simulated saturated thickness of the St. Charles Mesa terrace alluvial aquifer.
28 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado 104 34' o 104 30' o 38 16' o 38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditch

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feetGreater than 20 feet

SIMULATED SATURATED THICKNESS

Modeled–area boundary

EXPLANATION

Figure 18. Simulated saturated thickness remaining in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch f21st Lane.
EVALUATION OF POSSIBLE ALTERNATIVES TO LOWER THE HIGH WATER TABLE 29

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer D

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feetGreater than 20 feet

SIMULATED SATURATED THICKNESS

Modeled–area boundary

EXPLANATION

Figure 19. Simulated saturated thickness remaining in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch to 23rd Lane.
30 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado 104 34' o 104 30' o 38 16' o 38 14' o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditch

0 1/2 1 2 MILES 1-1/2

0 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 10 feet10 to 15 feet15 to 20 feetGreater than 20 feet

SIMULATED SATURATED THICKNESS

Modeled–area boundary

EXPLANATION

Figure 20. Simulated saturated thickness remaining in the St. Charles Mesa terrace alluvial aquifer as a result of lining the Ditch from A

to 25th Lane.
MODEL LIMITATIONS 31

LAND SURFACE

PUMP

MOTOR

WATER TABLE BEFORE

IMPLEMENTATION

OF ALTERNATIVE

PUMP

INTAKE

POSSIBLE WATER

TABLE AFTER

IMPLEMENTATION OF

ALTERNATIVE

BEDROCK

MODEL LIMITATIONS

The ground-water flow model developed for this 

study is useful for evaluating the ground-water flow 

system and potential effects of alternatives for 

lowering the high water table of the Mesa. The model 

is a simplified mathematical representation of a 

complex and dynamic physical system. Model cells, 

which represent areas approximately 300 ft by 300 ft, 

cannot adequately represent changes in the flow 

system that occur over distances of less than 300 ft, 

such as the cones of depression near pumping wells.

The ground-water flow system of the Mesa was 

modeled as a steady-state system, assuming no change 

in ground-water storage within a year’s seasonal cycle 

of ground-water-level fluctuations. Calibration of the 

model to transient conditions would have entailed 

specifying the temporal variation in rates of inflow and 

outflow to simulate seasonal water-level fluctuations 

observed in the terrace alluvial aquifer. The level of 

detail needed to define temporal variation in inflow 

and outflow rates is not supported by the available data 

and is beyond the scope of this study.

The final water-table elevations output by the 

model reflect the approximate water-table elevation 

for each model cell that results from a particular alternative of lowering the water table and may not represent the exact water-table elevation that could occur in 

a small area upon implementation of a particular alternative. Model results near the steep sides of the Mesa 

may not accurately represent the actual ground-water 

flow system because water-level measurements were 

not available in those areas and the accuracy of the 

calibrated model in those areas is unknown. Because 

the water-table surface slopes more steeply near the 

northwestern, northern, and southeastern sides of the 

Mesa, where ground-water discharge occurs, it is difficult to accurately simulate ground-water flow and 

water levels in these areas.

If the assumptions used in development of this 

model are inaccurate—for example, if the distribution 

and values of hydraulic conductivity are not accurately 

represented in the model—then the values of other 

parameters modified during the calibration procedure 

also may be inaccurate and the model results of the 

alternatives for lowering the water table could overestimate or underestimate the actual effects of implementing the alternatives.

Figure 21. Relation between a pump 

intake and the position of the water table 

before and after implementation of an 

alternative to lower the water table.
32 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

10

10

10

5

5

5 5

104 34' o 104 30' o

38 16'

o

38 14'

o

ARKANSAS

RIVER

Salt Creek

Bessemer Ditch

St. Charles R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer Ditch

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

0 to 5 feet5 to 6.8 feet

SIMULATED WATER-TABLE DECLINE

Modeled–area boundaryDrain

Depth-to-water contour—Shows depth to  water, in feet, November 1997 (Brendle, 1999)

10

EXPLANATION

Figure 22. Simulated water-table decline resulting from installation of two drains at 10 feet below land surface.
MODEL LIMITATIONS 33

104 34' o 104 30' o

38 16' o

38 14' o

ARKANSAS RIVER

Salt Creek

Bessemer Ditch

St.

Charles

R iver

Nicholson Road

South Road

County Farm Road

Santa Fe Drive

Everett Road

29th Lane

Baxter Road

23rd Lane

Aspen Street

21st Lane

25th Lane

27th Lane

30th Lane

Bessemer

D

0 1/2 1 2 MILES 1-1/20 0.5 1 2 KILOMETERS 1.5

Base from hydrology modified by Denver Water Board, 1998, from U.S. Geological SurveyDigital line graphs, 1:100,000, 1981–1983. Roads and cities from Colorado Department of Transportation, 1998. State plane projection. 1927 North American datum, Colorado coordinate system, Southern zone

Depth-to-water contour—Shows depth to  water, in feet, November 1997 (Brendle, 1999)

10

0 to 5 feet5 to 10 feet10 to 14.4 feet

SIMULATED WATER-TABLE DECLINE

Modeled–area boundaryDewatering well

EXPLANATION

10

10

10

5

5

5

5

Figure 23. Simulated water-table decline resulting from installation of 22 dewatering wells, each pumping at 80 gallons per min
34 Evaluation of Possible Alternatives to Lower the High Water Table of St. Charles Mesa, Pueblo County, Colorado

SUMMARY

St. Charles Mesa is an upland terrace southeast 

of Pueblo, Colorado, with an area of about 10 mi2. The 

water table is near the land surface in parts of the 

Mesa. The U.S. Geological Survey, in cooperation 

with Pueblo County, began a two-phase study in 1997 

to define the extent of the area in which a high water 

table has occurred (phase 1) and to evaluate possible 

alternatives to lower the high water table (phase 2).

Alluvial deposits that compose the terrace alluvial aquifer on the Mesa generally are composed of 

sand and gravel and range in thickness from about 7 to 

more than 38 ft. The shale bedrock underlying the 

terrace alluvial aquifer has very low permeability and 

acts as an impermeable lower boundary to groundwater flow in the terrace alluvial aquifer.

The lateral boundaries of the Mesa are highangle scarps on its northwestern and northern sides, a 

gradual slope on its southeastern side, and a topographic rise, underlain by bedrock, on its southwestern 

side. The northwestern, northern, and southeastern 

sides of the Mesa are dissected by numerous washes 

that commonly contain springs and seeps that 

discharge ground water and convey stormwater flow 

off the Mesa.

The hydrologic components that affect groundwater flow and storage in the terrace alluvial aquifer of 

the Mesa are recharge due to seepage from the Ditch 

and lateral ditches; recharge by infiltration of water 

from lawn and crop irrigation and septic-system 

effluent; recharge by infiltration of precipitation; 

discharge of ground water through springs and seeps 

on the northwestern, northern, and southeastern sides 

of the Mesa; discharge of water by evaporation or transpiration; and discharge of water by well pumpage, 

some of which results in discharge off the Mesa 

through lined drainage ditches. Seepage from the 

Ditch is the main source of inflow and discharge from 

seeps and springs is the main source of outflow from 

the terrace alluvial aquifer.

 The ground-water flow system of the Mesa was 

assumed to be in a state of dynamic equilibrium 

(steady-state condition). Average monthly water 

levels, precipitation, pumpage, and Ditch diversions 

for June 1, 1997, through May 31, 1998, were used in 

model construction and calibration. The model was 

calibrated to the average monthly water table for June 

1, 1997, through May 31, 1998, and to estimated 

spring and seep discharge from previous studies.

Alternatives of lowering the high water table 

were evaluated using the calibrated steady-state model 

of the ground-water flow system of the Mesa in transient mode. The modeled area was subdivided into a 

98-column by 58-row grid of 300- by 300-ft cells. 

Several possible alternatives of lowering the water 

table were evaluated and included: (1) reducing areal 

recharge by reducing recharge to irrigated areas by 

25 percent, (2) lining Bessemer Ditch from (a) Aspen 

Street to 21st Lane; (b) Aspen Street to 23rd Lane; 

(c) Aspen Street to 25th Lane; and (d) Aspen Street to 

Nicholson Road, (3) installing two drains at a depth of 

10 feet below land surface upgradient from the high 

water table areas, and (4) installing 22 dewatering 

wells within the high water table areas, each pumping 

at 80 gal/min.

The alternative of reducing areal recharge in 

irrigated areas by 25 percent resulted in a maximum 

decline in the water table of about 3 ft near the northeast corner of the Mesa. The water-table declines 

resulting from this alternative were only about 1 ft in 

the areas defined as having a high water table in 

November 1997, and were thus considered ineffective 

in lowering the high water table.

Based on the November 1997 depth-to-water 

contours, as the simulated extent of Ditch lining 

increased, so did the extent of the area throughout 

which the water table was simulated as being lowered 

at least 5 ft. All the Ditch-lining alternatives resulted in 

simulated maximum water-table declines of about 27 

to 29 ft. Simulation of lining the Ditch from Aspen 

Street to 21st Lane resulted in water-table declines of 

slightly less than 5 ft to 27 ft in the area west of 

25th Lane and south of Everett Road. None of the 

Ditch-lining alternatives were effective in lowering the 

water table throughout the entire high water table area 

north of Everett Road. The alternative of lining the 

Ditch completely resulted in an almost complete 

dewatering of the aquifer.

All the Ditch-lining alternatives probably would 

result in water table declines sufficient to cause at least 

some private wells to become unusable because of 

aquifer dewatering, with the more extensive Ditch 

lining resulting in the most wells becoming unusable. 

Wells could stop producing water if the position of the 

water table after implementation of an alternative was 

lower than the pump intake.

Two additional possible alternatives to lower the 

high water table were evaluated: the installation of two 

drains at a depth of 10 ft below land surface and instal-
REFERENCES CITED 35

lation of 22 dewatering wells each pumping at 80 

gal/min (0.18 ft3/s). Maximum simulated water-table 

declines for these two alternatives were much smaller 

than for the alternatives involving lining the Ditch: 

6.8 ft for the drain alternative and 14.4 ft for the 

dewatering-well alternative.

Simulated water-table declines for the drain 

alternative were greatest in the vicinity of Santa Fe 

Drive and 25th and 27th Lanes and north of Everett 

Road, between 23rd and 25th Lanes. Wells in these 

areas would stop producing water if water-table 

declines were sufficient to lower the water table below 

the pump intake.

Simulated water-table declines for the 

dewatering-well alternative were greater than 5 ft 

between 23rd and 27th Lanes and north of County 

Line Road. This alternative for lowering the water 

table might cause some wells to stop producing water 

in the areas northeast of Everett Road and 23rd Lane 

and southeast of Santa Fe Drive and 23rd Lane, where 

the greatest simulated declines occurred.

The drain and dewatering-well alternatives were 

more effective in lowering the water table in the high 

water table areas than lining the Ditch from Aspen 

Street to either 21st or 23rd Lanes. The drain and 

dewatering-well alternatives were less effective in 

lowering the water table in the high water table areas 

than lining the Ditch from Aspen Street to either 

25th Lane or to Nicholson Road.

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