U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5107

Prepared in cooperation with Bureau of Reclamation

Trends in Recent Historical and Projected Climate Data 

for the Colorado River Basin and Potential Effects on 

Groundwater Availability

Colorado

River

Colorado

River

NEW MEXICO

COLORADO

WYOMING

UTAH

ARIZONA

NEVADA

CALIFORNIA

MEXICO

IDAHO

TEXAS

115°

40°

35°

30°

110° 105°
Cover. Map of the Colorado River Basin, western United States and northern Mexico
Trends in Recent Historical and Projected Climate Data 

for the Colorado River Basin and Potential Effects on 

Groundwater Availability

By Fred D Tillman, Subhrendu Gangopadhyay, and Tom Pruitt

Prepared in cooperation with Bureau of Reclamation

Scientific Investigations Report 2020–5107

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Tillman, F.D., Gangopadhyay, S., and Pruitt, T., 2020, Trends in recent historical and projected climate data for 

the Colorado River Basin and potential effects on groundwater availability: U.S. Geological Survey Scientific 

Investigations Report 2020–5107, 24 p., https://doi.org/10.3133/sir20205107.

Associated data for this publication:

Tillman, F.D., Gangopadhyay, S., and Pruitt, T., 2019, Soil-water balance groundwater infiltration model results for the 

Lower Colorado River Basin: U.S. Geological Survey data release, https://doi.org/10.5066/P9VLU0O6.

Tillman, F.D., 2017, Soil-water balance groundwater recharge model results for the upper Colorado River Basin 

[ver. 2.0, April 2017]: U.S. Geological Survey data release, https://doi.org/10.5066/F7ST7MX7.

ISSN 2328-0328 (online)
iii

Acknowledgments

This investigation of recent historical and projected climate data for the Colorado River Basin 

and potential effects on groundwater availability was supported by the Bureau of Reclamation’s 

Science and Technology and WaterSMART (Sustain and Manage America’s Resources for 

Tomorrow) Programs. 
iv

Contents

Acknowledgments........................................................................................................................................iii

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

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

Purpose and Scope ..............................................................................................................................3

Previous Studies ...................................................................................................................................3

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

Data and Methods .........................................................................................................................................5

Analyses of Recent Historical Climate Data for the Colorado River Basin..........................................8

Analyses of Projected Climate Data for the Colorado River Basin .....................................................10

Projected Groundwater Infiltration for the Colorado River Basin .......................................................15

Summary and Conclusions.........................................................................................................................18

References Cited..........................................................................................................................................19

Appendix 1. Computational Details and Limitations of the Soil-Water Balance Groundwater 

Infiltration Model............................................................................................................................23

References Cited..........................................................................................................................................24

Figures

1. Maps of the upper and lower Colorado River Basins within the western 

United States showing major tributaries and major aquifer systems..................................3

2. Maps of the Colorado River Basin within the United States showing: land surface 

elevation, mean annual precipitation, and mean annual temperature................................4

3. Graphs showing average monthly precipitation and average monthly temperature 

in the upper and lower Colorado River Basins during the 1896–2019 period.....................5

4. Graphs showing historical precipitation and temperature in the upper and lower 

Colorado River Basins during the 1896–2019 period...............................................................8

5. Graphs showing precipitation over three-month periods in the upper Colorado 

River Basin during the 1896–2019 period..................................................................................9

6. Graphs showing precipitation over three-month periods in the lower Colorado 

River Basin during the 1896–2019 period..................................................................................9

7. Graphs showing average temperature over three-month periods in the upper 

Colorado River Basin during the 1896–2019 period...............................................................10

8. Graphs showing average temperature over three-month periods in the lower 

Colorado River Basin during the 1896–2019 period...............................................................10

9. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of projected precipitation, proportional seasonal contribution to water-year 

precipitation, and change in seasonal precipitation relative to 1951–2015 precipitation 

for the upper Colorado River Basin .........................................................................................11

10. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of projected temperature, proportional seasonal contribution to 

water-year potential evapotranspiration (PET), and change in seasonal PET relative 

to 1951–2015 PET for the upper Colorado River Basin..........................................................12

11. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of projected precipitation, proportional seasonal contribution to 

water-year precipitation, and change in seasonal precipitation relative to 

1951–2015 precipitation for the lower Colorado River Basin...............................................13
v

12. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of projected temperature, proportional seasonal contribution to 

water-year potential evapotranspiration (PET), and change in seasonal PET relative

 to 1951–2015 PET for the lower Colorado River Basin.........................................................14

13. Boxplots comparing median decadal-average precipitation and temperature 

between recent historical and future periods for the upper and lower Colorado 

River Basins.................................................................................................................................15

14. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of simulated projected groundwater infiltration, proportional seasonal 

contribution to water-year infiltration, and change in seasonal infiltration relative 

to 1951–2015 infiltration for the upper Colorado River Basin ..............................................16

15. Diagrams and graphs showing median and interquartile range of 10-year moving 

averages of simulated projected groundwater infiltration, proportional seasonal 

contribution to water-year infiltration, and change in seasonal infiltration relative 

to 1951–2015 infiltration for the lower Colorado River Basin...............................................17

16. Boxplots comparing simulated groundwater infiltration between recent historical 

and future periods for the upper and lower Colorado River Basins.................................... 18

Table

1. Institutions providing climate model output used in the Colorado River Basin climate 

and groundwater infiltration investigation................................................................................7
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

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

 °F = (1.8 × °C) + 32.

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

 °C = (°F – 32) / 1.8.

Datum

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

(NAVD 88).
vii

Abbreviations

APWL accumulated potential water loss

AET actual evapotranspiration

BCSD Bias-Correction and Spatial Disaggregation method

CMIP Coupled Model Intercomparison Project

DEM digital elevation model

EPA U.S. Environmental Protection Agency

GCM global climate model

GHCN Global Historical Climatology Network

HUC hydrologic unit code

LOCA Localized Constructed Analog

NRCS National Resources Conservation Service

PET potential evapotranspiration

RCP Representative Concentration Pathway

SWB Soil-Water-Balance

USGS U.S. Geological Survey 

Trends in Recent Historical and Projected Climate Data 

for the Colorado River Basin and Potential Effects on 

Groundwater Availability

By Fred D Tillman,1 Subhrendu Gangopadhyay,2 and Tom Pruitt2

1

U.S. Geological Survey.

2

Bureau of Reclamation.

Abstract

Understanding recent historical and projected trends in 

precipitation and temperature in the Colorado River Basin, 

and estimating what the projected changes in these climate 

parameters may mean for groundwater resources in the 

region, is important for water managers and policymakers to 

sustainably manage water resources in the basin. Historical 

(1896–2019) precipitation and temperature data for the upper 

and lower Colorado River Basins were analyzed to better 

understand recent trends in climate data that may affect 

groundwater resources in the area. Historical data indicate 

multidecadal-scale cyclical patterns in precipitation in both the 

upper and lower basins. Although upper basin precipitation 

had no statistical trend over the recent historical period, 

the lower basin had a weak negative trend over this period. 

Multidecadal-scale cyclical patterns in temperature also are 

observed in historical climate data in both the upper and 

lower basins, at least until the early 1970s. Beginning at that 

time, both the upper and lower basins experienced strong, 

monotonic positive trends in temperature. Basic principles of 

hydrology indicate that periods of decreasing precipitation 

as well as increasing temperature would have a negative 

effect, that is, reduction in groundwater infiltration and hence, 

reduced recharge of aquifer systems.

Projected climate data from 97 Coupled Model 

Intercomparison Project phase 5 (CMIP5) ensemble members 

across the full range of Representative Concentration 

Pathway (RCPs) from water years 1951 through 2099 were 

evaluated to understand what current global climate models 

are projecting about future conditions in the Colorado River 

Basin, and what this might mean for groundwater systems 

in the region. Precipitation in the upper basin is projected to 

increase throughout the rest of the century, rising to 6 percent 

above the 1951–2015 historical period by mid-century and 

to 9 percent above the historical period by the end of the 

century. Temperature in the upper basin also is projected to 

be above the recent historical median throughout the rest 

of the century, with steady warming in decadal average 

temperatures expected until the last quarter of this century. 

In contrast to projected precipitation in the upper basin, 

precipitation in the lower basin is projected to be the same 

as, or slightly less than, the historical period throughout most 

of the rest of this century. Like projected temperature in the 

upper basin, temperature in the lower basin also is projected 

to be above the recent historical median throughout the rest 

of the century. Comparing median projections for all future 

decades with median results from all historical decades, future 

precipitation is expected to be greater than that of the past in 

the upper basin, though no significant difference is projected 

for precipitation in the lower basin. Significant increases 

(p-value<0.05) are expected in temperature in both the upper 

and lower basins.

To estimate the effects of projected precipitation and 

temperature on groundwater systems in the region, results 

from the 97 member CMIP5 climate projection ensemble were 

used as input in a Soil-Water Balance (SWB) groundwater 

infiltration model for the Colorado River Basin. SWB 

simulation results indicate that the upper Colorado River 

Basin is expected to experience decades of above-historicalaverage groundwater infiltration through the end of the 

century. For the lower Colorado River Basin, simulated 

groundwater infiltration is projected to be consistently less 

than the recent (1951–2015) historical period for most of the 

remaining century. A comparison of the distribution of all 

median simulated groundwater infiltration results between 

recent historical and future periods indicates projected 

groundwater infiltration in the upper basin is significantly 

(p-value<0.05) greater over the combined 2020–2099 

future period than the recent (1951–2015) historical period. 

Moreover, in 41 of 71 (58 percent) possible future decades in 

this century, groundwater infiltration is projected to be greater 

than the 75th percentile of historical simulated groundwater 

infiltration. Projected groundwater infiltration in the lower 

Colorado River Basin across all future decades is significantly 

less than in the historical period. Of the 71 future decades 

in the century, projected groundwater infiltration in the 
2 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

lower basin is expected to be less than the 25th percentile of 

historical infiltration in 55 (77 percent) of the 10-year periods. 

Important differences in projected precipitation between 

the upper (increasing precipitation) and lower (decreasing 

precipitation) basins largely drive the different responses of 

simulated groundwater infiltration in the upper (increasing 

infiltration) and lower (decreasing infiltration) basins. It will 

be useful to revisit projections in groundwater infiltration in 

the Colorado River Basin when more up-to-date projections of 

precipitation become available from the next Coupled Model 

Intercomparison Project phases or by using climate input 

developments through Regional Climate Modeling efforts and 

stochastic weather generators.

Introduction

The Colorado River supplies water to more than 35 million 

people in the United States and 3 million people in Mexico 

(Bureau of Reclamation, 2013a; Colorado River Basin Salinity 

Control Forum, 2013). The Colorado River Basin drains parts 

of seven U.S. States and Mexico and is divided into upper 

and lower basins at the compact point of Lee Ferry, Arizona, 

located 1.6 kilometers (km) downstream of the mouth of the 

Paria River (fig. 1; Anderson, 2004). The flow of the Colorado 

River in the lower basin is mostly controlled by releases from 

Glen Canyon Dam. About 90 percent of the flow in the lower 

Colorado River at Lake Mead originates in the upper basin 

(U. S. Geological Survey, 2017). Groundwater discharge to 

streams makes up an estimated 21–58 percent of the flow in 

rivers and streams in the upper Colorado River Basin (Miller 

and others, 2014). Although the importance of lower-basin 

groundwater to flow in the Colorado River is substantially 

less than in the upper basin, large agricultural areas and 

fast-growing communities such as Phoenix and Tucson in 

the lower basin depend on groundwater to meet water needs. 

Additionally, important ecosystems in the semi-arid lower 

basin are sustained by groundwater discharge at spring sites. 

An understanding of recent trends in climate data, projected 

change in climate data, and projected change in groundwater 

infiltration in the Colorado River Basin is essential for 

the sustainable management of the rivers and aquifers in 

the region. The SECURE Water Act, passed by Congress 

in 2009, requires the Bureau of Reclamation to report to 

Congress every 5 years following the initial 2-year reporting 

requirement (that is, a first report in 2011) on effects and 

risks resulting from global climate change with respect to the 

quantity of water resources located in each major Reclamation 

River Basin (Bureau of Reclamation, 2011). To help address 

this reporting requirement, the USGS, in cooperation with the 

Bureau of Reclamation, completed an investigation analyzing 

trends in recent historical and current state-of-the-science 

projected climate data for the Colorado River Basin and 

potential effects on groundwater availability.

Purpose and Scope

The purpose of this report is to document the data, 

methods, and results from the investigation of recent 

historical and projected climate data, and simulated, projected 

groundwater infiltration in the Colorado River Basin. The 

study area for this investigation, as described below, comprises 

the upper Colorado River Basin as defined by USGS twodigit hydrologic unit code (HUC) number 14 and the lower 

Colorado River Basin as defined by USGS two-digit HUC 

number 15 within the United States (fig. 1). Climate data 

analyzed for this study include precipitation and temperature 

for water years (October–September) 1896 through 2019. 

Projected climate data include precipitation and temperature 

for water years 2020 through 2099. Groundwater infiltration 

is simulated using the Soil-Water Balance groundwater 

infiltration model (Westenbroek and others, 2018) for water 

years 2020 through 2099.

Previous Studies

Results from analyses of projected climate data and 

simulated groundwater infiltration in the upper Colorado River 

Basin described in this report was previously published by 

Tillman and others (2016). Several publications within the last 

5 years have reported on potential effects of projected climate 

change on water resources in the western or southwestern 

United States. Cook and others (2015) project increased 

drought severity with climate change in the southwest and 

central plains of western North America. In the Southwest, 

a reduction in cold season precipitation and an increase in 

evapotranspiration are expected to reduce soil moisture. 

Dettinger and others (2015) describe projections for continued 

and increased warming across the western United States, with 

continued changes in seasonality of snowmelt and streamflow, 

and a strong potential for increases in evaporative demands. 

A series of reports focuses on potential effects of projected 

climate change on agricultural, rangeland, and forest resources 

in the southwest, in particular the effect of declining snowpack 

on available irrigation water (Elias and others, 2016; Elias and 

others, 2018a,b; Havstad and others, 2018; Steele and others, 

2018; Thorne and others, 2018). The most recent National 

Climate Assessment by the U.S. Global Change Research 

Program (Gonzalez and others, 2018) describes climate model 

results that project increased temperatures in the southwestern 

United States, with subsequent increased evapotranspiration, 

lower soil moisture, reduced snow cover, and changes in the 

timing of snowmelt and runoff. Studies of the potential effects 

of projected climate change on groundwater recharge in the 

western United States (Meixner and others, 2016; Niraula and 

others, 2017) indicate potential declines in recharge across 

aquifers in the southern part of the study area. Prein and others 

(2016) use observation-based results to support projections of 

climate models that indicate a substantial increase in droughts 

and aridity in the Southwest.
Introduction 3

men20-2167_fig01

Basin and Range basin-fill aquifers

Basin and Range carbonate-rock aquifers

Colorado Plateau aquifers

Not a principal aquifer

Rio Grande aquifer system

Upper Tertiary aquifers

Surficial expression of major aquifer system

EXPLANATION

0 100 200 300 KILOMETERS

Geographic coordinate system 0 100 200 300 MILES

North American Datum of 1983

Upper Colorado 

River Basin

Lower Colorado 

River Basin

Virgin River

Little Colorado River

Dolores R iver

Gunnison River

White River Yampa River

Green Rive r

COLORADO

RIVER

Sa n Juan Riv er

Gulf of California

Gila River

Paria

River Lake

Powell

GLEN CANYON DAM

NEW MEXICO

COLORADO

WYOMING

UTAH

ARIZONA

NEVADA

CALIFORNIA

MEXICO

IDAHO

NEW MEXICO

COLORADO

WYOMING

UTAH

ARIZONA

NEVADA

CALIFORNIA

MEXICO

IDAHO

Tucson

Phoenix

Lee Ferry

Lake

Mead

Yuma

Coconino 

Plateau

115°

40°

35°

30°

110° 105° 115° 110° 105°

A B

Figure 1. Maps of the upper and lower Colorado River Basins within 

the western United States showing (A) major tributaries and (B) major 

aquifer systems (from U.S. Geological Survey [2003]).

Description of the Study Area

The Colorado River traverses 2,334 km from headwaters 

in the southern Rocky Mountains of Colorado to the Gulf 

of California in Mexico (fig. 1). The Colorado River and its 

tributaries drain an area of 640,000 square kilometers (km2) 

in parts of seven U.S. States (Wyoming, Colorado, Utah, New 

Mexico, Arizona, Nevada, and California) and two Mexican 

States. The Colorado River Compact of 1922 divided the 

Colorado River into upper and lower basins at the compact 

point of Lee Ferry, Arizona—roughly 0.6 km downstream 

from the mouth of the Paria River (fig. 1). For this study, the 

Colorado River Basin is defined by Region 14 HUC for the 

upper Colorado River Basin and Region 15 HUC for the lower 

Colorado River Basin (see https://water.usgs.gov/GIS/huc.

html). Furthermore, only the portion of the Colorado River 

Basin within the United States (91 percent) is considered 

in this study, owing to the unavailability of datasets in the 

Mexican portion of the basin. Major tributaries in the upper 

basin include the Yampa, Green, White, Gunnison, Dolores, 

and San Juan Rivers. Major lower basin tributaries include 

the Little Colorado, Virgin, and Gila Rivers. Land surface 

elevation in the basin ranges from mountain peaks more 

than 4,000 meters (m) above the North American Vertical 

Datum of 1988 (NAVD 88) in the Rocky Mountains to 20 m 

above NAVD 88 as the Colorado River leaves the United 
4 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

States near Yuma, Arizona (fig. 2A). The climate across 

the upper and lower basins is also diverse. Average annual 

precipitation (1981–2010) ranges from more than 1.8 m per 

year at the highest elevations in the upper basin to as little 

as 7 centimeters (cm) per year near Yuma, Arizona (fig. 2B). 

Average annual temperatures (1981–2010) are below 0 °C 

in a few high-elevation locations in the Rocky Mountains 

and almost 25°C near Yuma, Arizona (fig. 2C). Monthly 

precipitation in the upper basin is fairly uniform throughout the 

year, with an average of 28–36 millimeters (mm) per month for 

all months except June, which receives about 20 mm (fig.3). 

Precipitation in the lower basin varies considerably during 

parts of the year, with 20–30 mm per month from September 

through March, declining to a low of 7.8 mm in June, then 

monsoonal storms in July and August result in 44 and 50 mm 

per month, respectively (fig. 3). Monthly temperature patterns 

in the upper and lower basins are identical, with lower basin 

temperatures 7–10 °C warmer than the upper basin (fig.3). For 

both the upper and lower basins, average monthly temperatures 

follow a sinusoidal pattern from highest average temperatures 

in July to lowest average temperatures in January (fig. 3).

The principal aquifers in the upper basin are the Colorado 

Plateau regional aquifers that are composed of permeable, 

moderately to well-consolidated sedimentary rocks ranging 

in age from Permian to Tertiary (fig. 1; Robson and Banta, 

1995), although groundwater in shallow alluvial deposits may 

be locally important in some locations in the southern Rocky 

Mountains (Apodaca and Bails, 2000). At least three groups 

of regional, productive water-yielding geologic units have 

been identified in the upper basin (Freethey and Cordy, 1991; 

Robson and Banta, 1995; Geldon 2003a,b). Aquifers contained 

in Tertiary-age formations of limited extent in the northern and 

southeastern parts of the basin overlie aquifers in Mesozoicage formations that also are present throughout most of the 

study area. Deeper aquifers in Paleozoic-age formations are 

present throughout much of the upper Colorado River Basin 

and may rise to land surface in uplifted areas. The Triassicage Chinle and Moenkopi Formations (confining units) limit 

MEXICO

71 1,872

Mean annual precipitation, 

in millimeters

EXPLANATION

B

MEXICO

20 4,362

Land surface elevation, in 

meters above NAVD 88

A

EXPLANATION

N

0 100 200 KILOMETERS

0 100 200 MILES

Figure 2 (pages 4–5). Maps of the Colorado River Basin (within the United States) showing: (A) land surface elevation above the North 

American Vertical Datum of 1988 (NAVD 88), (B) mean annual precipitation (1981–2010), and (C) mean annual temperature (1981–2010; 

PRISM Climate Group, 2019). Elevation data from the U.S. Geological Survey, National Geospatial Program.
Data and Methods 5

men20-2167_fig02

MEXICO

Mean annual temperature, 

in degrees Celsius

EXPLANATION

−5.57 24.6

C

N

0 100 200 KILOMETERS

0 100 200 MILES

Figure 2 (pages 4–5). —Continued

hydraulic connection between aquifers in Mesozoic- and 

Paleozoic-age formations throughout much of the upper 

Colorado River Basin (Freethey and Cordy, 1991; Geldon 

2003a,b). Areas with Mesozoic-age sandstone in contact with 

Paleozoic-age rocks and fractures associated with uplifted 

areas, however, may allow groundwater flow between the two 

systems (Geldon, 2003b). The principal aquifers in the lower 

basin are the basin-fill aquifers in the central and southern part 

of the basin (fig. 1; Robson and Banta, 1995). The basin-fill 

aquifers consist of unconsolidated gravel, sand, silt, and clay, 

or partially consolidated sedimentary or volcanic material 

(Robson and Banta, 1995). Structural deformations beginning 

in Tertiary time produced block faulting along steeply dipping 

normal faults. Downthrown parts of the blocks became basins 

and the upthrown parts became mountain ranges that mostly 

surround the basins. Basins were filled over time with erosional material from the mountains. Although not a principal 

aquifer system, the C and Redwall-Muav aquifers in northern 

Arizona provide important sources of water to communities on 

and near the Coconino Plateau (Bills and others, 2007). Permeable units of the C aquifer are contained in the Permian-age 

Kaibab Formation and Coconino Sandstone, and in the 

Esplanade Sandstone, which is the uppermost formation of the 

Permian- and Pennsylvanian-age Supai Group. The RedwallMuav aquifer is named for the Mississippian-age Redwall 

Limestone and the Cambrian-age Muav Formation that make 

up the top and bottom units of the aquifer system, respectively.

Month

Average monthly precipitation,

in millimeters

Average monthly temperature,

in degrees Celsius

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Upper Colorado River Basin

Lower Colorado River Basin

EXPLANATION

5

15

25

35

45

55

-10

-5

0

5

10

15

20

25

30

Upper Colorado River Basin

Lower Colorado River Basin

EXPLANATION

Figure 3. Graphs showing average monthly precipitation (top) 

and average monthly temperature (bottom) in the upper and 

lower Colorado River Basins during the 1896–2019 period (PRISM 

Climate Group, 2019).

Data and Methods

In order to understand the fundamental drivers of 

potential changes in groundwater recharge, first the climate 

parameters of precipitation and temperature are evaluated in 

both recent historical and projected future periods. Simulating 

groundwater infiltration using a numerical model is useful 

but adds additional uncertainty through questions about 

how one model might perform differently than another, how 

appropriate a particular model may be for a study area, or how 

model parameters are selected. By investigating changes in the 

basic climate parameters of precipitation and temperature that 

may increase or decrease the amount of available water for 

groundwater recharge, more direct information can be gained 

about the potential for changes in groundwater recharge that 

do not require numerical modeling. Recent historical climate 
6 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

data are investigated to determine if conditions that affect 

groundwater recharge have already begun to change. Projected 

climate data are investigated for current understanding of 

possible future conditions relevant to groundwater recharge. 

After analyzing historical and projected temperature changes 

in the study area, simulations using a numerical groundwater 

infiltration model provide additional evidence for outcomes 

suggested by the climate data.

Historical climate parameters of precipitation and 

mean monthly temperature for the Colorado River Basin 

study area were evaluated using the Gridded 5km Global 

Historical Climatology Network (GHCN)-Daily Temperature 

and Precipitation Dataset (nClimGrid; Vose and others 

2014a,b). The nClimGrid dataset uses station data from the 

GHCN with temperature bias correction and climatologically 

aided interpolation to address topographic and network 

variability, resulting in a dataset appropriate for evaluating 

regional climate trends (Vose and others, 2014a,b). Monthly 

gridded climate data, developed from the daily dataset, at 

approximately (~) 5-km spatial scale were available from 

water year (October–September) 1896 through 2019. Water 

year results (sum of precipitation, mean of temperature) 

are presented, but analyses of trends are performed on 

10-year moving averages of the water year results. A 10-year 

averaging period is used in this study to smooth out interannual variability and is the averaging time used in the 

Colorado River compact (that is, 10-year average deliveries).

Simulated future precipitation and temperature data for 

the study area were available through the year 2099 for 97 

climate projections from the Coupled Model Intercomparison 

Project phase 5 (CMIP5) multimodel archive (table 1). Each 

of the 97 ensemble members are results from a general climate 

model (GCM) run using a specified initial condition and one 

of four future emission scenarios known as a Representative 

Concentration Pathway (RCP). The four emission scenarios 

are for radiative forcing levels of 8.5, 6, 4.5, and 2.6 watts 

per square meter (W/m2) by the end of the century (Van 

Vuuren, 2011). RCP8.5 represents a high baseline emission 

scenario that presumes “business as usual” conditions prevail. 

RCP2.6 represents a rapid decrease in carbon emissions in 

the early part of this century with a subsequent stabilization 

of atmospheric CO2 concentrations by mid-century. Two 

medium stabilization scenarios (RCP4.5 and RCP6) represent 

future emissions between these end members (Van Vuuren, 

2011). In this investigation, the range of future emission 

scenarios are all considered equally likely. GCMs are typically 

run at coarse spatial resolutions of ~100–200 km so climate 

output must be downscaled to finer spatial scales for climate 

effect assessments. There is a continuum of downscaling 

methods ranging from statistical approaches to physically 

based modeling. This study uses the statistical downscaling 

Bias-Correction and Spatial Disaggregation method (BCSD; 

Wood and others, 2004) to develop monthly precipitation 

and temperature fields at 1/8° × 1/8° (latitude × longitude) 

spatial resolution from the GCM native scales to be consistent 

with earlier analyses conducted for the upper Colorado River 

Basin (Tillman and others, 2016), though other statistically 

downscaled CMIP5 projections (for example, using the 

Localized Constructed Analog [LOCA]; Pierce and others, 

2014, 2015) are also currently available. Little difference is 

observed in the study area between downscaled climate data 

using the BCSD and LOCA methods (Bracken, 2016).

A historical resampling and scaling technique (Wood 

and others, 2002) was subsequently used to disaggregate the 

monthly precipitation and temperature data to daily values. 

Projected daily precipitation and temperature data are available from the downscaled climate and hydrology projections 

archive hosted by Lawrence Livermore National Laboratory 

(Bureau of Reclamation, 2013b; https://gdo-dcp.ucllnl.org/

downscaled_cmip_projections/dcpInterface.html). Summarized 

projected climate results for the study area discussed in this 

manuscript are available on the USGS ScienceBase web site 

(Tillman, 2017; Tillman and others, 2019).

To investigate the integrated effects of projected 

temperature and precipitation on groundwater resources in 

the study area, projected climate data from the 97 CMIP5 

ensemble members were used in the Soil-Water Balance 

(SWB) groundwater infiltration model (Westenbroek 

and others, 2018). The SWB model estimates potential 

groundwater infiltration by calculating water-balance 

components using a modified version of the ThornthwaiteMather (Thornthwaite, 1948; Thornthwaite and Mather, 1957) 

soil-water-balance approach (see Appendix 1 for model details 

and limitations). Groundwater infiltration is calculated on a 

daily time step as the difference between sources and sinks of 

water and change in soil moisture:

INFILTRATION = water sources – water sinks– Δ soil moisture(1)

where

water sources = rainfall + snowmelt + inflow; and

water sinks = interception + outflow + AET.

Sources of water in the SWB model include rainfall, 

snowmelt, and inflow from other model cells and sinks of 

water include interception, outflow to other model cells, and 

actual evapotranspiration (AET). The SWB groundwater 

infiltration model has been used in several regional 

groundwater studies in the United States including the High 

Plains aquifer (Stanton and others, 2011), the Lake Michigan 

Basin (Feinstein and others, 2010), basins in Wisconsin 

(Dripps and Bradbury, 2009) and Minnesota (Smith and 

Westenbroek, 2015), the Northern Atlantic Coastal Plain 

aquifer system (Masterson and others, 2013), and the upper 

Colorado River Basin (Tillman and others, 2016). Summarized 

SWB model results discussed in this manuscript are available 

on the USGS ScienceBase web site (Tillman, 2017; Tillman 

and others, 2019).

Results of both projected climate data and simulated 

groundwater infiltration were aggregated into water years 

and then subsequently averaged over 10-year periods, 

moving every year. The 10-year moving average smooths 

out inter-annual variability and makes trends more apparent. 

Median values for the 97 climate projections and simulated 
Data and Methods 7

Table 1. Institutions providing climate model output used in the Colorado River Basin climate and groundwater infiltration investigation.

[Abbreviations: BCC, Beijing Climate Center, China Meteorological Administration; CCCMA, Canadian Centre for Climate Modelling and Analysis; 

CMCC, Centro Euro-Mediterraneo per I Cambiamenti Climatici; CNRM-CERFACS, Centre National de Recherches Météorologiques /Centre Européen de 

Recherche et Formation Avancée en Calcul Scientifique; CSIRO-BOM, Commonwealth Scientific and Industrial Research Organization (CSIRO) and Bureau 

of Meteorology (BOM), Australia; CSIRO-QCCCE, Commonwealth Scientific and Industrial Research Organization in collaboration with Queensland Climate 

Change Centre of Excellence; FIO, The First Institute of Oceanography, State Oceanic Administration, China; INM, Institute for Numerical Mathematics; IPSL, 

Institut Pierre-Simon Laplace; LASG-CESS, LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences and CESS,Tsinghua University; MIROC, 

Atmosphere and Ocean Research Institute (The University of Tokyo), National Institute for Environmental Studies, and Japan Agency for Marine-Earth Science 

and Technology; MIROC(2), Japan Agency for Marine-Earth Science and Technology, Atmosphere and Ocean Research Institute (The University of Tokyo), 

and National Institute for Environmental Studies; MOHC, Met Office Hadley Centre (additional HadGEM2-ES realizations contributed by Instituto Nacional 

de Pesquisas Espaciais); MPI-M, Max-Planck-Institut für Meteorologie (Max Planck Institute for Meteorology); MRI, Meteorological Research Institute; 

NASA GISS, National Aeronautics and Space Administration Goddard Institute for Space Studies; NCAR, National Center for Atmospheric Research; NCC, 

Norwegian Climate Centre; NOAA GFDL, National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory; NSF-DOE-NCAR, 

National Science Foundation - Department of Energy - National Center for Atmospheric Research Community Earth System Model Contributors]

Modeling Center or Group Model Name Representative Concentration Pathway 

2.6 4.5 6.0 8.5

BCC BCC-CSM 1.1    

BCC BCC-CSM 1.1(m)  

CCCMA CanESM2   

CMCC CMCC-CM  

CNRM-CERFACS CNRM-CM5  

CSIRO-BOM Access 1.0  

CSIRO-QCCCE CSIRO-mk3.6.0    

FIO FIO-ESM    

INM INM-CM4  

IPSL IPSL-CM5A-MR    

IPSL IPSL-CM5B-LR  

LASG-CESS FGOALS-g2   

MIROC MIROC5    

MIROC(2) MIROC-ESM    

MIROC(2) MIROC-ESM-CHEM    

MOHC HadGEM2-AO    

MOHC HadGEM2-CC  

MOHC HadGEM2-ES    

MPI-M MPI-ESM-LR   

MPI-M MPI-ESM-MR   

MRI MRI-CGCM3   

NASA GISS GISS-E2-H-CC 

NASA GISS GISS-E2-R    

NASA GISS GISS-E2-R-CC 

NCAR CCSM4(RSMAS)    

NCC NorESM1-M    

NOAA GFDL GFDL-CM3    

NOAA GFDL GFDL-ESM2G    

NOAA GFDL GFDL-ESM2M    

NSF-DOE-NCAR CESM1(BGC)  

NSF-DOE-NCAR CESM1(CAM5)    
8 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

groundwater infiltration are used to describe the central 

tendency (50th percentile) of the projected climate and 

infiltration results, with the interquartile range (25th and 75th 

percentiles) of results presented to illustrate variability in 

the projected data and model results. Results for projected 

climate data and simulated groundwater infiltration are 

presented as a percentage of the recent historical (water years 

1951–2015) average. For projected temperature results, both 

mean temperature and potential evapotranspiration (PET) are 

discussed. The Hargreaves-Samani (Hargreaves and Samani, 

1985) estimation of PET, which is used in this study in the 

estimation of actual evapotranspiration (AET) in equation 1, 

is directly related to temperature and is more easily compared 

on a seasonal scale (for example, percent contribution from 

a season to a year) than temperature. Projected climate and 

SWB simulation analyses for the upper Colorado River 

Basin in this study are based on previously published results 

(Tillman and others, 2016).

Descriptive, correlative, and hypothesis-testing statistics 

were performed using the R statistical platform, version 3.6.0 

(R Core Team, 2019). A significant statistical test result was 

defined as having a p-value<0.05. 

Analyses of Recent Historical Climate 

Data for the Colorado River Basin

Historical precipitation and temperature data for the 

study area were available from water year 1896 through 

2019. Patterns are difficult to observe in individual wateryear values, but 10-year moving averages illustrate 10-to 

20-year cycles in precipitation in both the upper and lower 

basins (fig. 4). Ten-year moving averages of precipitation 

show more variability in the lower basin (variance is 778 

square millimeters [mm2]) than the upper basin (variance is 

315 mm2). Relative to the long-term (1896–2019) average, 

EXPLANATION

Upper Colorado River Basin Lower Colorado River Basin

200

300

400

500

600

Water year precipitation, in millimeters

150

250

350

450

550

4

6

8

10

Water-year mean temperature, in degrees Celsius

Water year precipitation, in millimeters

Water-year mean temperature, in degrees Celsius

Water Year

1896 1916 1936 1956 1976 1996 2016

12

14

16

18

1896 1916 1936 1956 1976 1996 2016

Water Year

Water-year sum of precipitation 1896–2019 water-year average

Water-year average temperature Ten-year moving average

Figure 4. Graphs showing historical precipitation (top plots) and temperature (bottom plots) in the upper and lower Colorado 

River Basins during the 1896–2019 period (Vose and others, 2014a,b). Note different vertical scales for the two basins.
Analyses of Recent Historical Climate Data for the Colorado River Basin 9

the maximum wet period for the upper basin is the decade 

1978–1987 (13 percent above the mean) and the decade 1979–

1988 (21 percent above the mean) for the lower basin. The 

maximum dry periods during the recent historical record are 

1896–1905 for the upper basin (9 percent below average) and 

1946–1955 for the lower basin (15 percent below average). 

More recently, 10-year moving averages of precipitation 

have been mostly below the long-term (1896–2019) average. 

Beginning with the 1993–2002 10-year period, 78 percent of 

upper basin and 100 percent of the lower basin decades have 

experienced precipitation less than the 1896–2019 average 

(fig. 4). Although upper basin precipitation data indicate no 

statistical trend (p-value>0.05) over the 1896–2019 historical 

period, the lower basin has a weak negative trend over this 

period (Kendall’s τ =−0.2, p-value=0.001).

Multidecadal-scale cyclical patterns in temperature also 

are observed in historical climate data in both the upper and 

lower basins, at least until the early 1970s (fig. 4). The coolest 

period for both the upper and lower basins was the 1912–1921 

decade, when temperatures were 0.86 °C (12 percent) and 

0.70 °C (5 percent) below historical means, respectively. Prior 

to the early 1970s, the warmest period for both the upper and 

lower basins was the 1934–1943 decade, when temperatures 

were 0.3 °C (4 percent) and 0.1 °C (0.6 percent) above their 

respective long-term means. Beginning in the early 1970s, 

both the upper basin (Kendall’s τ=0.83, p-value<2.5×10−15) 

Year

Ten-year moving average of three-month precipitation, in millimeters

October–December

January–March

April–June

July–September

70

90

110

130

70

90

110

130

70

90

110

130

70

90

110

130

1900 1920 1940 1960 1980 2000 2020

Figure 5. Graphs showing precipitation over three-month 

periods in the upper Colorado River Basin during the 1896–2019 

period (Vose and others, 2014a,b). Dashed line indicates 

significant slope of linear regression and significant correlation 

with time (p-value<0.05).

and lower basin (Kendall’s τ=0.91, p-value<2.2×10−16) 

experienced strong, monotonic positive trends in temperature, 

with a maximum rise above long-term means to date of 1.1 °C 

for both basins.

Seasonal precipitation in the upper basin over the recent 

historical period (fig. 5) is characterized by significantly 

increasing precipitation in the October–December months 

Year

Ten-year moving average of three-month precipitation, in millimeters

October–December

January–March

April–June

July–September

100

120

140

1900 1920 1940 1960 1980 2000 2020

15

25

35

45

45

65

85

105

125

40

60

80

100

Figure 6. Graphs showing precipitation over three-month 

periods in the lower Colorado River Basin during the 1896–2019 

period (Vose and others, 2014a,b). Dashed line indicates 

significant slope of linear regression and significant correlation 

with time (p-value<0.05).

(Kendall’s τ=0.25, p-value=7.9×10−5) and significantly 

decreasing precipitation in the January–March months 

(Kendall’s τ=−0.36, p-value=9.7×10−9). Lower basin seasonal 

precipitation (fig. 6) decreased in the historical period in 

April–June (Kendall’s τ=−0.36, p-value=8.2×10−9) and July–

September (Kendall’s τ =−0.22, p-value =4.1×10−4) months. 

Significant positive trends in temperature are observed in 

the recent historical period across all seasons for both the 

upper (fig. 7) and lower (fig. 8) basins, with greatest increases 

in July–September for the upper basin (Kendall’s τ=0.61, 

p-value<2.2×10−16) and in October–December for the lower 

basin (Kendall’s τ=0.54, p-value<2.2×10−16).

Possible effects of observed trends in climate data on 

groundwater resources can be discussed conceptually by 

evaluating a simple water balance equation for groundwater 

infiltration, such as equation 1. No trend in decadal 
10 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

men20-2167_fig07

Year

Ten-year moving average of three-month average temperature, in degrees Celsius

October–December

January–March

April–June

July–September

16.5

17.5

18.5

1900 1920 1940 1960 1980 2000 2020

10.0

10.8

11.6

12.4

-3.0

-2.0

-1.0

0.0

1.0

2.0

Figure 7. Graphs showing average temperature over threemonth periods in the upper Colorado River Basin during the 1896–

2019 period (Vose and others, 2014a,b). Dashed line indicates 

significant slope of linear regression and significant correlation 

with time (p-value<0.05).

men20-2167_fig08

Year

Ten-year moving average of three-month average temperature, in degrees Celsius

October–December

January–March

April–June

July–September

23.0

24.0

25.0

26.0

1900 1920 1940 1960 1980 2000 2020

17.0

18.0

19.0

20.0

6.0

7.0

8.0

9.0

9.2

10.2

11.2

Figure 8. Graphs showing average temperature over threemonth periods in the lower Colorado River Basin during the 1896–

2019 period (Vose and others, 2014a,b). Dashed line indicates 

significant slope of linear regression and significant correlation 

with time (p-value<0.05).

precipitation in the upper basin means little change in the 

longer term “water sources” part of equation 1. The negative 

trend in decadal precipitation in the lower basin would result 

in less groundwater infiltration over time, all other conditions 

being the same in equation 1. However, other terms in 

equation 1 are not the same over the recent historical period. 

Temperature increases in both the upper and lower basins 

since the early 1970s would increase evapotranspiration rates, 

increasing the “water sinks” part of equation 1, resulting in a 

decrease in water available for groundwater infiltration. This 

increase in water removed from the system permanently would 

compound the possible reduction in groundwater infiltration 

caused solely by the decrease in precipitation noted in lower 

basin climate records (Cook and others, 2015). Significantly 

decreasing precipitation in the January–March months 

observed in the upper basin may further decrease groundwater 

infiltration above estimates using annual precipitation 

values, as spring snowmelt is a principal source of water 

for groundwater infiltration in the upper basin (Earman and 

others, 2006; Whitehead, 1996). 

Analyses of Projected Climate Data for 

the Colorado River Basin

GCM precipitation and temperature data for the study 

area were available from water years 1951 through 2099. 

Median and interquartile ranges of 10-year moving averages 

of 97 CMIP5 climate model projections are presented relative 

to results from the recent historical period (1951–2015). In 

the upper basin (fig. 9A), precipitation is expected to increase 

throughout the projected future period. By mid-century, 

median projected decadal precipitation is expected to be about 

6 percent greater than the recent historical period (1951–2015). 

By the end of the century, projected precipitation rises to 

9 percent above the 1951–2015 period. Most of the 97 climate 

projections concur with the projected increase in precipitation, 

with even the 25th percentile of results barely below, and 

during a few periods above, the historical median. The relative 

seasonal contribution of precipitation in a water year (fig. 9B) 

is not projected to change over time, with about one-quarter of 
Analyses of Projected Climate Data for the Colorado River Basin 11

men20-2167_fig09 Percent of 1951–2015 seasonal contribution

104%

118%

102% 102% 106%

113%

103% 101% 104%

110%

101% 102%

75

100

125

50

EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 10-year 

moving 

averages

Median of 10-year moving 

averages 

of all ensembles

Greater than the historical 

median

Less than the historical median

25% 25%

23% 27%

25% 25%

23% 27%

25% 25%

22% 28%

Seasonal proportion of 

water year

75

100

125

2020 2030 2040 2050 2060 2070 2080 2090 2100

Projected precipitation, as a percent of the median 

of precipitation for the 1951–2015 climate period

Upper Colorado River Basin—Precipitation

B

C

Year

2020–2050 2050–2075 2075–2099

Time period

A

Figure 9. Diagrams and graphs showing median and interquartile range of 10-year moving averages of 

projected precipitation (A), proportional seasonal contribution to water-year precipitation (B), and change in 

seasonal precipitation relative to 1951–2015 precipitation (C) for the upper Colorado River Basin. Pie diagrams 

and bar graphs summarize results over decades 2020–29 to 2045–54, 2046–55 to 2070–79, and 2071–80 to 

2090–99. Line graph in A adapted from Tillman and others (2016) , used with permission. %, percent.

precipitation projected in each 3-month period throughout the 

century. Seasonal contributions relative to the historical period 

(fig. 9C), however, indicate that most of the projected increase 

in precipitation is in January–March, during which cooler 

temperatures may provide favorable conditions for increased 

groundwater infiltration. The projected substantial increase 

in January–March precipitation, however, is not supported by 

trends in recent historical data (fig. 5).

Temperature in the upper basin also is projected to be 

above the recent historical median throughout the rest of the 

century, with steady warming in decadal average temperatures 

expected until the last quarter of this century, when projected 

temperature levels out (fig. 10A). The climate models are in 

even greater agreement about projected warming temperatures 

than projected increases in precipitation, as evidenced by 

a narrower interquartile range and the 25th percentile of 
12 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

men20-2167_fig10

110% 111% 108% 107%

115% 116% 112% 110%

119% 121%

115% 111%

42%

11%

10%

37%

42%

11%

10%

37%

41%

11%

10%

37%

Percent of 1951–2015 

seasonal PET contribution

Seasonal proportion of 

water year PET

75

100

125

2020 2030 2040 2050 2060 2070 2080 2090 2100

Projected temperature, as a percent of the median 

temperature for the 1951–2015 climate period

75

100

125

50

A EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 

10-year moving 

averages

Median of 10-year moving 

averages of all 

ensembles

Greater than the historical 

median

Less than the historical 

median 

Upper Colorado River Basin—Temperature

B

C

Year

2020–2050 2050–2075 2075–2099

Time period

Figure 10. Diagrams and graphs showing median and interquartile range of 10-year moving averages of 

projected temperature (A), proportional seasonal contribution to water-year potential evapotranspiration (PET)

(B), and change in seasonal PET relative to 1951–2015 PET (C) for the upper Colorado River Basin. Pie diagrams 

and bar graphs summarize results over decades 2020–29 to 2045–54, 2046–55 to 2070–79, and 2071–80 to 2090–99. 

Line graph in A modified from Tillman and others (2016), used with permission. %, percent. 

ensembles warmer than the historical median throughout all 

decades of the century. Relative seasonal contributions of 

projected increased temperature, as demonstrated by analyses 

of potential evapotranspiration (PET) in fig. 10B, remain 

constant throughout the century. Lowest seasonal PET is 

projected to remain during the cooler months of January–

March, although this season is projected to have the greatest 

increase in temperature (fig. 10C). The increase in temperature 

during January–March may negate the effect on increased 

water available for groundwater infiltration that would be 

expected from the projected increase in precipitation during 

the same months (fig. 9). Projected warming throughout all 

seasons is supported by positive trends in temperature in 

recent historical data (fig. 7).

In contrast to projected precipitation in the upper basin 

(fig. 9), precipitation in the lower basin is projected to be 

the same as, or slightly (<3 percent) less than, the historical 

period throughout most of the rest of this century (fig. 11A). 
Analyses of Projected Climate Data for the Colorado River Basin 13

men20-2167_fig11

99% 98%

85%

102%

95% 95%

84%

103%

97% 97%

80%

105%

40%

24%

9%

28%

41%

23%

8%

27%

41%

24%

8%

27%

Lower Colorado River Basin—Precipitation

Percent of 1951–2015 

seasonal contribution

Seasonal proportion of 

water year

75

100

125

2020 2030 2040 2050 2060 2070 2080 2090 2100

Projected precipitation, as a percent of the median 

of precipitation for the 1951–2015 climate period

75

100

125

50

A EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 10-year 

moving averages

Median of 10-year moving 

averages of all ensembles

Greater than the historical 

median

Less than the historical 

median

Year

2020–2050 2050–2075 2075–2099

Time period

B

C

Figure 11. Diagrams and graphs showing median and interquartile range of 10-year moving averages of projected 

precipitation (A), proportional seasonal contribution to water-year precipitation (B), and change in seasonal precipitation 

relative to 1951–2015 precipitation (C) for the lower Colorado River Basin. Pie diagrams and bar graphs summarize results 

over decades 2020–29 to 2045–54, 2046–55 to 2070–79, and 2071–80 to 2090–99.%, percent.

This conclusion is based on the median of the 97 climate 

ensembles, but variability in climate projections demonstrated 

by the interquartile range of results indicates precipitation 

could be slightly above or below the historical period. As 

with the upper basin, the interquartile range of projected 

precipitation in the lower basin is substantially wider than 

for projected temperature, indicating a lack of certainty 

among climate model-projected precipitation that has been 

documented in other studies (Schaller and others, 2011). The 

relative seasonal contribution of precipitation in a water year 

(fig. 11B), is projected to remain mostly the same throughout 

the century, with a slight decrease in the water-year proportion 

of precipitation during April–June and a slight increase in 

relative July–September precipitation over time. For the 

seasonal periods with the greatest contribution to wateryear precipitation, October–December and January–March 

are projected to be slightly (<3 percent) less than during 

the historical period (fig. 11C) , whereas July–September 
14 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

precipitation is projected to be slightly greater (maximum 

of 5 percent) than during the historical period. Projected 

increase in July–September precipitation is unlikely to benefit 

groundwater systems in much of the lower basin, however, as 

high temperatures and resulting PET rates during this season 

would likely limit additional available water for groundwater 

infiltration. Projected decreases in April–June months are 

supported by negative trends in historical precipitation data 

(fig. 6). Projected increase in July–September precipitation 

contrasts with a negative trend in these months during the 

recent historical period (fig. 6).

Like projected temperature in the upper basin, 

temperature in the lower basin also is projected to be above 

the recent historical median throughout the rest of the century 

(fig. 12A). A narrow interquartile range, and 25th percentile 

of ensembles for all remaining decades of the century 

warmer than the historical median, demonstrate consensus 

on this projected warming among climate models. Relative 

seasonal contributions of projected increased temperature, 

as demonstrated by analyses of potential evapotranspiration 

(PET) (fig. 12B), remain mostly constant throughout the 

century, with highest PET during the warmer April–June and 

107% 107% 106% 105%

111% 110% 109% 107% 113% 113% 111% 108%

36%

14%

35%

14% 36%

14%

35%

14% 36%

14%

36%

14%

Percent of 1951–2015 

seasonal PET contribution

75

100

125

50

Seasonal proportion of 

water year PET

75

100

125

2020 2030 2040 2050 2060 2070 2080 2090 2100

Projected temperature, as a percent of the median

temperature for the 1951–2015 climate period

Lower Colorado River Basin—Temperature

A EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 10-year 

moving averages

Median of 10-year moving 

averages of all ensembles

Greater than the historical 

median

Less than the historical median

Year

2020–2050 2050–2075 2075–2099

Time period

B

C

Figure 12. Diagrams and graphs showing median and interquartile range of 10-year moving averages of projected 

temperature (A), proportional seasonal contribution to water-year potential evapotranspiration (PET) (B), and change 

in seasonal PET relative to 1951–2015 PET (C) for the lower Colorado River Basin. Pie diagrams and bar graphs 

summarize results over decades 2020–29 to 2045–54, 2046–55 to 2070–79, and 2071–80 to 2090–99.%, percent.
Projected Groundwater Infiltration for the Colorado River Basin 15

July–September months and the lowest during the cooler 

October–December and January–March months. All seasonal 

PET is projected to increase, relative to the historical period, 

throughout the rest of the century (fig. 12C), with the greatest 

increase (13 percent) in both the October–December and 

January–March months. Increases in temperature during 

the cooler October–December and January–March months 

(fig.12C) combined with projected decreases in precipitation 

during these same periods (fig. 11C) are expected to decrease 

the amount of available water for groundwater infiltration. 

Projected warming across all seasons is supported by positive 

trends in temperature in recent historical data (fig. 8).

Comparing median projections for all future decades 

with median results from all historical decades (fig. 13), future 

precipitation is expected to be greater than that of the past 

in the upper basin (Wilcoxon rank sum, p-value<2.2×10−16, 

one-tailed), though no significant difference is projected for 

precipitation in the lower basin (Wilcoxon rank sum, p-value 

5

10

15

20

1951 to 2015 2020 to 2099 1951 to 2015 2020 to 2099

EXPLANATION

300

320

340

360

380

400

420

90th percentile

75th percentile

25th percentile

10th percentile

50th percentile (median)

Mean

Time Period

Precipitation, in millimeters

Upper Colorado River Basin Lower Colorado River Basin

Median of average parameter values during all ten-year periods

Temperature, in degrees Celsius

Figure 13. Boxplots comparing median decadal-average 

precipitation and temperature between recent historical and 

future periods for the upper and lower Colorado River Basins.

= 0.999, one-tailed). Moreover, the 10th percentile of average 

annual precipitation in all future decades exceeds precipitation in the 90th percentile of historical decades in the upper 

basin. Comparing the distribution of projected and historical 

temperature in the Colorado River Basin, significant increases 

in temperature are expected in both the upper (Wilcoxon rank 

sum, p-value<2.2×10−16, one tailed) and lower (Wilcoxon rank 

sum, p-value<2.2×10−16, one tailed) basins (fig. 13).

Projected Groundwater Infiltration for 

the Colorado River Basin

Projected precipitation and temperature results from the 

97 CMIP5 climate projections were used to drive changes 

in simulated groundwater infiltration using the Soil-Water 

Balance model. As previously described by Tillman and 

others (2016), central tendency of SWB simulation results 

indicate that the upper Colorado River Basin is expected 

to experience mostly decades of above-historical-average 

groundwater infiltration through the end of the century 

(fig. 14A). Owing to the compounded variability of both 

precipitation and temperature inputs to the model, the 

interquartile range of simulated groundwater infiltration 

is relatively large. The largest proportion of water-year 

simulated groundwater infiltration is during the April–June 

months, when 65–74 percent of infiltration occurs (fig. 14B). 

The increase in January–March contribution to water-year 

simulated infiltration through the end of the century comes at 

the expense of a decrease in April–June contribution over the 

same period. Relative to the historical period, a large increase 

is projected in January–March groundwater infiltration and a 

decrease is projected in April–June infiltration (fig. 14C), the 

two seasons that account for about 95 percent of water-year 

infiltration (fig. 14B). Projected January–March precipitation 

increases over historical decades by 18 percent by the end of 

the century (fig. 9C), but large increases in PET (21 percent) 

also are expected during these months (fig. 10C). Relatively 

cool temperatures during the January–March months (only 

10 percent of water-year PET, fig. 10B), however, result in 

much of the increase in precipitation being available for 

groundwater infiltration. The projected 18 percent decrease 

in simulated April–June groundwater infiltration relative to 

historical decades (fig. 14C) can be explained by fairly flat 

projected precipitation (fig. 9C) combined with projected 

increase in PET (fig. 10C) resulting in less water available 

for groundwater systems. In addition to these direct effects, 

precipitation that was historically stored as snow during 

January–March and released for infiltration during the 

April–June snowmelt season may be reduced by warming 

temperatures, resulting in increased January–March and 

decreased April–June simulated infiltration. Simulated 

infiltration during the July–September months decreases by 

35 percent by the end of the century, owing to increasing 

temperature (fig. 10), and simulated October–December 
16 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

men20-2167_fig14

120%

175%

93%

70%

130%

201%

85% 66%

142%

254%

82% 65%

65%

1%

29%

5%

70%

1%

24%

5%

74%

1%

20%

4%

Percent of 1951–2015 

seasonal contribution

Seasonal proportion of 

water year

Projected groundwater infiltration, as a percent of the 

median infiltration for the 1951–2015 climate period

75

100

125

2020 2030 2040 2050 2060 2070 2080 2090 2100

Upper Colorado River Basin—Groundwater infiltration

100

200

150

50

250

A EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 10-year 

moving averages

Median of 10-year moving 

averages of all ensembles

Greater than the historical 

median

Less than the historical 

median

Year

2020–2050 2050–2075 2075–2099

Time period

B

C

Figure 14. Diagrams and graphs showing median and interquartile range of 10-year moving averages of 

simulated projected groundwater infiltration (A), proportional seasonal contribution to water-year infiltration (B), 

and change in seasonal infiltration relative to 1951–2015 infiltration (C) for the upper Colorado River Basin. Pie 

diagrams and bar graphs summarize results decades 2020–29 to 2045–54, 2046–55 to 2070–79, and 2071–80 to 

2090–99. Line graph in A modified from Tillman and others (2016), used with permission. %, percent.

infiltration increases by 42 percent, owing to increasing 

precipitation (fig. 9), although these seasons contribute only 

1 percent and 5 percent to water-year infiltration, respectively 

(fig. 14B). Results from simulated groundwater infiltration 

indicate that projected increases in precipitation during winter 

months provides additional water for groundwater infiltration 

despite projected increases in temperature during these months. 

Increases in projected precipitation during warmer months of 

the year, however, are not enough to compensate for projected 

increase in PET in these water-limited months, resulting in 

declines in projected infiltration during these seasons.

In the lower Colorado River Basin, simulated 

groundwater infiltration is projected to be consistently less 

than the recent historical period for most of the remaining 

century (fig.15). Decadal average groundwater infiltration 

is expected to decrease by as much as 16 percent below 

the recent historical period by 2059–2068. In contrast to 

upper basin simulated groundwater infiltration (fig. 14), 
Projected Groundwater Infiltration for the Colorado River Basin 17

men20-2167_fig15

100% 101%

62%

110%

91% 86%

53%

117%

96%

87%

43%

115%

60

100

80

40

120

63%

3%

27%

7%

61%

3%

28%

8%

61%

2%

29%

8%

60

80

100

120

140

2020 2030 2040 2050 2060 2070 2080 2090 2100

Percent of 1951–2015 

seasonal contribution

Seasonal proportion of 

water year

Projected groundwater infiltration, as a percent of the 

median infiltration for the 1951–2015 climate period

Lower Colorado River Basin—Groundwater infiltration

A EXPLANATION

Time period

October to December

January to March

April to June

July to September

Interquartile range of 10-year 

moving averages

Median of 10-year moving 

averages of all ensembles

Greater than the historical 

median

Less than the historical 

median

Year

2020–2050 2050–2075 2075–2099

Time period

B

C

Figure 15. Diagrams and graphs showing median and interquartile range of 10-year moving averages 

of simulated projected groundwater infiltration (A), proportional seasonal contribution to water-year 

infiltration (B), and change in seasonal infiltration relative to 1951–2015 infiltration (C) for the lower Colorado 

River Basin. Pie diagrams and bar graphs summarize results over decades 2020–29 to 2045–54, 2046–55 to 

2070–79, and 2071–80 to 2090–99.%, percent.

most water-year lower basin infiltration occurs during the 

January–March period (fig. 15B), although some decrease 

in this proportion is projected by the end of the century. A 

small decrease in projected precipitation during January–

March (fig. 11C) combined with a substantial increase in PET 

(7–13 percent; fig. 12C) during this time is responsible for 

this projected decline in groundwater infiltration. Although 

the water-year proportion of October–December simulated 

groundwater infiltration is projected to increase over time 

(fig. 15B), the total amount of infiltration is projected to 

decrease during this season (fig. 15C). Similar to projected 

infiltration during the January–March months, the reduction 

in October–December infiltration is a response to a small 

decrease in projected precipitation (fig. 11C) combined with 

a substantial projected increase in PET (7–13 percent; fig. 

12C) during these months. The only season projected to 

experience an increase in groundwater infiltration is during 

the July–September months (by as much as 17 percent during 

mid-century; fig. 15C). However, the July–September months 

only account for about 8 percent of the simulated recharge in a 
18 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

water year (fig. 15B), even though almost 40 percent of wateryear precipitation falls during these warm months (fig. 11B). 

A comparison of the distribution of all median simulated 

groundwater infiltration results between recent historical 

and future periods (fig. 16) reveals projected groundwater 

infiltration in the upper basin to be significantly greater over 

the combined 2020–2099 future period than the historical 

period (Wilcoxon rank sum, p-value=2×10−6, one tailed). In 

41 of 71 (58 percent) possible future decades in this century, 

groundwater infiltration is expected to be greater than the 75th 

percentile of historical simulated groundwater infiltration. 

Projected groundwater infiltration in the lower Colorado River 

Basin across all future decades is significantly less than in 

the historical period (Wilcoxon rank sum, p-value=8×10−11, 

one tailed). Of the 71 future decades in the century, projected 

groundwater infiltration in the lower basin is expected to be 

less than the 25th percentile of historical infiltration in 55 

(77 percent) of the 10-year periods. 

2.5

3.0

3.5

1951 to 2015 2020 to 2099

10.0

10.5

11.0

EXPLANATION

90th percentile

75th percentile

25th percentile

10th percentile

50th percentile (median)

Mean

Time Period

Median of average simulated water-year groundwater infiltration during all 

10-year periods, in units of x109 cubic meters

Upper Colorado River Basin

Lower Colorado River Basin

Figure 16. Boxplots comparing simulated 

groundwater infiltration between recent historical 

and future periods for the upper and lower 

Colorado River Basins. Upper Colorado River Basin 

boxplots modified from Tillman and others (2016), 

used with permission.

Summary and Conclusions

Groundwater discharge to streams is an important source 

of flow in rivers and streams in the upper Colorado River 

Basin. Although the importance of lower basin groundwater 

to flow in the Colorado River is substantially less than in 

the upper basin, large agricultural areas and fast-growing 

communities such as Phoenix and Tucson in the lower basin 

depend on groundwater to meet water needs. Additionally, 

many ecosystems in the semi-arid lower basin are sustained 

by groundwater discharge at spring sites. An understanding 

of recent trends in climate data, projected change in climate 

data, and projected change in groundwater infiltration in 

the Colorado River Basin is necessary for the sustainable 

management of the rivers and aquifers in the region. 

Historical (1896–2019) precipitation and temperature 

data for the upper and lower Colorado River Basins were 

analyzed to better understand recent trends in climate data that 

may affect groundwater resources in the area. Historical data 

indicate multidecadal-scale cyclical patterns in precipitation 

in both the upper and lower basins. Although upper basin 

precipitation had no statistical trend over the recent historical 

period, the lower basin had a weak negative trend in 

precipitation over this period. Seasonal precipitation in the 

upper basin over the recent historical period was characterized 

by a significant increase in precipitation in the October–

December months and a significant decrease in precipitation 

in the January–March months. Lower basin seasonal 

precipitation significantly decreased in the historical period 

in April–June and July–September months. Multidecadalscale cyclical patterns in temperature also are observed in 

historical climate data in both the upper and lower basins, 

at least until the early 1970s. Beginning at that time, both 

the upper and lower basins experienced strong, monotonic 

positive trends in temperature. Significant positive trends in 

temperature are observed in the historical period (1896–2019) 

across all seasons for both the upper and lower basins, with 

greatest increases in July–September for the upper basin and 

in October–December for the lower basin.

Projected climate data from 97 Coupled Model 

Intercomparison Project phase 5 (CMIP5) climate projections 

across the full range of Representative Concentration 

Pathways (RCPs) from water years 1951 through 2099 were 

evaluated to understand what current global climate models 

are projecting about future conditions in the Colorado River 

Basin, and what this might mean for groundwater systems 

in the region. Precipitation in the upper basin is projected to 

increase throughout the rest of the century, rising to 6 percent 

above the 1951–2015 historical period by mid-21st century 

and to 9 percent above the historical period by the end of the 

century. Although not supported by recent trends in historical 

climate data, most of the increase in precipitation is projected 

to occur in January–March, during which cooler temperatures 

will likely provide favorable conditions for increased 

groundwater infiltration. Temperature in the upper basin also is 

projected to be above the recent historical median throughout 
References Cited 19

the rest of the century, with steady warming in decadal 

average temperatures expected until the last quarter of this 

century. Warming is projected during all seasons, a projection 

supported by positive temperature trends in the historical data.

In contrast to the upper basin, precipitation in the lower 

basin is projected to be the same as, or slightly less than, the 

1951–2015 recent historical period throughout most of the 

rest of this century. For the seasonal periods with the greatest 

contribution to water-year precipitation, October–December 

and January–March are projected to be less than during the 

historical period whereas July–September precipitation is 

projected to be slightly greater than during the historical 

period. Projected increase in July–September precipitation 

in the lower basin is unlikely to benefit groundwater systems 

as higher temperatures and resulting higher PET rates during 

this season would likely limit additional available water for 

groundwater infiltration. Like projected temperature in the 

upper basin, temperature in the lower basin also is projected 

to be above the recent historical median throughout the rest 

of the century. Temperature (through analysis of PET) is 

projected to increase across all seasons in the lower basin, 

with the greatest increase in both the October–December 

and January–March months. Increases in temperature during 

the cooler October–December and January–March months 

combined with projected decreases in precipitation during 

these same periods would be expected to result in a decrease 

in the amount of available water for groundwater infiltration. 

Comparing median projections for all future decades with 

median results from all historical decades, future precipitation 

is expected to be greater than that of the past in the upper 

basin, whereas no significant difference is projected for 

precipitation in the lower basin. Significant increases are 

expected in temperature in both the upper and lower basins.

Projected precipitation and temperature results from the 

97 CMIP5 climate projections were used as input in a SoilWater Balance (SWB) groundwater infiltration model for the 

Colorado River Basin. SWB simulation results indicate that 

the upper Colorado River Basin is expected to experience 

mostly decades of above-historical-average groundwater 

infiltration through the end of the 21st century. An increase 

is projected in January–March groundwater infiltration and a 

decrease is projected in April–June infiltration, the two seasons 

that account for about 95 percent of water-year infiltration. 

For the lower Colorado River Basin, simulated groundwater 

infiltration is projected to be consistently less than the recent 

historical period for most of the remaining century. Decadal 

average groundwater infiltration is expected to decrease by 

as much as 16 percent below the recent historical period by 

2059–2068. A comparison of the distribution of all median 

simulated groundwater infiltration results between recent 

historical and future periods indicates projected groundwater 

infiltration in the upper basin will be significantly greater 

over the combined 2020–2099 future period compared to 

the 1951–2015 recent historical period. Moreover, in 41 

of 71 (58 percent) possible future decades in this century, 

groundwater infiltration is projected to be greater than the 75th 

percentile of historical simulated groundwater infiltration. 

Projected groundwater infiltration in the lower Colorado 

River Basin across all future decades is significantly less 

than in the historical period. Of the 71 future decades in the 

century, projected groundwater infiltration in the lower basin 

is expected to be less than the 25th percentile of historical 

infiltration in 55 (77 percent) of the 10-year periods.

Among the general climate model (GCM)-projected 

climate data, there is substantially more uncertainty (as 

shown by wider interquartile range of results) in projected 

precipitation than projected temperature. GCM projections 

generally agree on increasing temperatures in coming years 

in both the upper and lower basins. Important differences in 

projected precipitation between the upper basin (increasing 

precipitation) and lower basin (decreasing precipitation) 

largely drive the different responses of simulated groundwater 

infiltration in the upper (increasing infiltration) and lower 

(decreasing infiltration) basins. It may be useful to revisit 

projections in groundwater infiltration in the Colorado River 

Basin when more up-to-date projections of precipitation 

become available from the next CMIP exercise to see if 

current projections are still valid. Additionally, previous work 

in the upper Colorado River Basin (Tillman and others, 2017) 

indicates that projected climate and groundwater infiltration 

results summarized over a large basin might not be shared 

equally among subregions within the basin. Additional studies 

on important groundwater-dependent subregions within the 

lower Colorado River Basin may be useful.

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Nation: U.S. Geological Survey National Water Information 

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time series for U.S. climate divisions: Journal of Applied 

Meteorology and Climatology, v. 53, p. 1,232–1,251, 

https://doi.org/10.1175/JAMC-D-13-0248.1.

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R.J., 2018, SWB Version 2.0—A soil-water-balance code 

for estimating net infiltration and other water-budget 

components: U.S. Geological Survey Techniques and 

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tm6A59.

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States, segment 8, Montana, North Dakota, South 

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P., 2002, Long-range experimental hydrologic forecasting 

for the Eastern United States: Journal of Geophysical 

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B:CLIM.0000013685.99609.9e.
Appendix 1. Computational Details and Limitations of the Soil-Water Balance 

Groundwater Infiltration Model

The Soil-Water-Balance (SWB) groundwater infiltration 

model (Westenbroek and others, 2018) estimates spatial and 

temporal variations in groundwater infiltration by calculating 

water balance components at daily time steps. SWB follows a 

modified Thornthwaite-Mather soil-water-balance accounting 

approach (Thornthwaite, 1948; Thornthwaite and Mather, 

1957). Sources and sinks of water within each grid cell in 

the model domain are estimated based on climate data and 

landscape characteristics, and groundwater infiltration is then 

estimated as the difference between the change in soil moisture 

and these sources and sinks (equation 1 in the main text).

SWB infiltration simulations require spatially 

gridded datasets for land cover, hydrologic soil group, 

available soil-water capacity, overland flow direction, daily 

maximum temperature, daily minimum temperature, and 

daily precipitation. SWB simulations also require tabular 

information including runoff curve numbers, interception 

values, vegetation rooting depths, and maximum daily 

recharge values for each combination of hydrologic soil 

group and land-cover type. In SWB simulations, inflow to 

a model cell is surface flow from adjacent cells, which is 

calculated using the National Resources Conservation Service 

(NRCS) curve number rainfall-runoff relation. Runoff flow 

direction is determined using a flow-direction grid derived 

from a digital elevation model. Interception is a user-specified 

amount of precipitation that is trapped and used by vegetation 

and outflow from a cell is calculated in the same manner as 

inflow. Several methods are available for estimating potential 

evapotranspiration (PET) in the SWB model, from which 

actual evapotranspiration (AET) is calculated. For Colorado 

River Basin simulations, the Hargreaves-Samani (Hargreaves 

and Samani, 1985) method is used as it produces spatially 

variable estimates of PET from spatially varying minimum 

and maximum air temperature data for each daily time step:

PET=0.0135×RS×(T+17.8) with RS=KRS×RA×TD0.5

where 

PET is potential evapotranspiration, in units of 

equivalent water evaporation;

RS is incoming solar radiation, in units of 

equivalent water evaporation;

T is mean air temperature, in degrees Celsius;

KRS is a calibration coefficient;

RA is extraterrestrial radiation, in units of 

equivalent water evaporation; and 

TD is the measured air temperature range, in 

degrees Celsius (Hargreaves and 

Samani, 1985).

Extraterrestrial radiation is estimated as a function 

of latitude and the day of year (Allen and others, 1998). 

Computation of soil moisture in equation 1 requires 

several intermediary values. First, PET is subtracted from 

precipitation (P) for all grid cells. If P<PET, then there is a 

potential deficiency of water and accumulated potential water 

loss (APWL) is computed as the running sum of daily P–PET 

values during times when P<PET. Soil moisture is estimated 

using current AWPL in the Thornthwaite-Mather relation that 

describes the nonlinear relation between soil moisture and 

APWL. AET is then equal to only the amount of water that 

can be extracted from the soil. If P > PET, then a potential 

surplus of water exists and AET is equal to PET. Soil moisture 

is then calculated by adding P–PET directly to the previous 

day’s soil-moisture value. If the new soil moisture value is 

less than the maximum water-holding capacity of the soil, 

which is estimated as the product of the root-zone depth and 

available soil water capacity, then the Thornthwaite-Mather 

relation is used to back-calculate a reduced APWL. If the new 

soil moisture value is greater than the maximum water-holding 

capacity of the soil, then excess soil-moisture becomes 

recharge, soil moisture is set to the maximum water-holding 

capacity, and AWPL is set to zero.

For Colorado River Basin SWB recharge simulations, all 

spatially gridded input datasets were resampled to the same 

cell size and geographic coordinate system as the 1/8° × 1/8° 

(latitude × longitude) climate data. Detailed descriptions of the 

sources, manipulation, and resampling of SWB model inputs 

for upper Colorado River Basin infiltration simulations are 

provided in Tillman (2015).

Climate change is expressed in SWB infiltration 

simulations (equation 1) through the computation of AET 

(mean temperature) and through precipitation input. The 

Colorado River Basin SWB model does not include changes in 

land use that may take place in the future or simulate changes 

in stomatal conductance or leaf area in a carbon dioxide 

enriched atmosphere (Eckhardt and Ulbrich, 2003; Holman 

and others, 2012). Only direct effects of climate change 

are evaluated in the Colorado River Basin SWB infiltration 

results. 

Although the SWB infiltration model has been shown 

to provide reasonable basin-scale estimates of groundwater 

infiltration, SWB assumptions and limitations should be 

considered when evaluating simulation results (Westenbroek 

and others, 2018). The daily time step of the SWB model 

allows short-term surpluses of water to become groundwater 

infiltration, but also necessitates that overland-flow routing 

of runoff either infiltrate in cells downslope or be routed out 
24 Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability

of the model domain on the same day in which it originated. 

In addition, depth to the top of the aquifer surface is not 

considered in SWB simulations, and there may be substantial 

time of travel through the unsaturated zone. Use of the NRCS 

curve number method to estimate runoff in SWB simulations 

introduces limitations, including that the method was 

developed to evaluate floods and was not designed to simulate 

daily flows of ordinary magnitude. Studies show that the curve 

number is not constant but may vary from event to event 

(Westenbroek and others, 2010). Finally, there are numerous 

methods for estimating evapotranspiration, each with its own 

benefits, limitations, uncertainties, and data requirements. This 

study uses the Hargreaves-Samani (Hargreaves and Samani, 

1985) method for PET, in which climate changes are reflected 

only in the daily air temperature range. More complex 

evapotranspiration relations require additional data that 

may include relative humidity, wind speed, and percentage 

of actual to possible daily sunshine hours, among others 

(Westenbroek and others, 2010).

References Cited

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errata 2006), Crop evapotranspiration—Guidelines for 

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Open-File Report 2015–1160, 17 p., https://doi.org/10.3133/

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Westenbroek, S.M., Kelson, V.A., Hunt, R.J., and Bradbury, 

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60 p., https://doi.org/10.3133/tm6A31.

Westenbroek, S.M., Engott, J.A., Kelson, V.A., and Hunt, 

R.J., 2018, SWB Version 2.0—A soil-water-balance code 

for estimating net infiltration and other water-budget 

components: U.S. Geological Survey Techniques and 

Methods, book 6, chap. A59, 118 p., https://doi.org/10.3133/

tm6A59.
Moffett Publishing Service Center, California 

Manuscript approved for publication September 16, 2020 

Edited by John Mark Brigham

Layout and design by Kimber Petersen

Illustration support by JoJo Mangano
Tillman and others—Trends in Historical and Projected Climate Data for the Colorado River Basin and Effects on Groundwater Availability—Scientific Investigations Report 2020–5107, 

September 2020

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205107