Water Availability and Use Science Program

Groundwater Availability of the Northern High Plains Aquifer 

in Colorado, Kansas, Nebraska, South Dakota, and Wyoming

Saturated zone

Unsaturated zone

Surface water

Impermeable rock layer

Recharge from precipitation

10.1 million acre-feet per year

Recharge from canal seepage

1.2 million acre-feet per year

Net release from storage

3.9 million acre-feet per year

Irrigation wells

7.5 million acre-feet per year

Evapotranspiration

3.5 million acre-feet per year

Stream base flow

3.9 million acre-feet per year

Amount of storage in aquifer

2,428 million acre-feet

Municipal wells

68,000 acre-feet per year

EXPLANATION

Inflows Outflows

Drains

0.2 million acre-feet per year

EXPLANATION

Major river basins

Precipitation, in inches

3.5 to 4

4.1 to 6

6.1 to 8

8.1 to 10

10.1 to 12

12.1 to 14

14.1 to 16

16.1 to 18

18.1 to 20

20.1 to 22

22.1 to 24

Professional Paper 1864

U.S. Department of the Interior

U.S. Geological Survey
Cover. Upper illustration: Conceptual diagram showing average annual simulated water budget inflows and outflows by source and the 

amount of water in storage in the Northern High Plains aquifer for 2000–9. Example from figure 11B of this report. 

Lower right illustration: Map showing distribution of precipitation for the Northern High Plains aquifer for the 2009 growing season. 

Example from figure 2C of this report. 

Background photograph: Center-pivot irrigated cornfield near Ord, Nebraska, taken by J.P. Traylor, U.S. Geological Survey.

Back cover. Map of the conterminous United States showing the High Plains aquifer.
Groundwater Availability of the Northern 

High Plains Aquifer in Colorado, Kansas, 

Nebraska, South Dakota, and Wyoming

By Steven M. Peterson, Jonathan P. Traylor, and Moussa Guira

Water Availability and Use Science Program

Professional Paper 1864

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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U.S. Government.

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

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

Suggested citation:

Peterson, S.M., Traylor, J.P., and Guira, M., 2020, Groundwater availability of the Northern High Plains aquifer in 

Colorado, Kansas, Nebraska, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1864, 57 p., 

https://doi.org/10.3133/pp1864.

Associated data for this publication:

Peterson, S.M., Traylor, J.P., and Guira, M., 2020, MODFLOW–NWT groundwater flow model used to evaluate groundwater availability with five forecast scenarios in the Northern High Plains aquifer in Colorado, Kansas, Nebraska, 

South Dakota, and Wyoming: U.S. Geological Survey data release, https://doi.org/10.5066/P92UNY4F.

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

ISBN 978-1-4113-4361-0
iii

Foreword

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater 

is the Nation’s principal reserve of freshwater. It provides drinking water for half of the country, is essential to food production in the United States, and facilitates business and industrial 

activities. Groundwater also is an important source of water for sustaining the ecosystem health 

of rivers, wetlands, and estuaries throughout the country.

Groundwater-level declines resulting from large-scale development of groundwater resources, 

together with other effects of pumping, have led to concerns about the future availability of 

groundwater to meet our Nation’s needs. The compounding effects of recent droughts underscore the need for an updated status of the Nation’s groundwater resources. Assessments of 

groundwater resources provide the science and information needed by decision makers and the 

public to manage and use water resources responsibly. The potential future effects on groundwater resources from climate variability further exacerbate an already challenging situation, and 

the analysis of these potential effects adds to an already complex task. 

The U.S. Geological Survey’s Water Availability and Use Science Program is completing largescale multidisciplinary regional studies of groundwater availability, including the study of the 

Northern High Plains aquifer described herein. The regional studies are intended to inform citizens, communities, and natural resource managers of the condition of the Nation’s groundwater 

resources and how changes in land use, water use, and climate have affected those resources. 

The studies also are aimed at developing tools to enable scientists and managers to forecast 

how these resources may change in the future. Information from these individual groundwater 

assessments of principal aquifer systems will be used to inform national assessment of water 

availability and will help answer questions about the Nation’s ability to meet current and future 

demands for groundwater. 

Donald Cline 

Associate Director, 

U.S. Geological Survey

Water Resources Mission Area

v

Acknowledgments

We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for the Coupled Model Intercomparison Project (CMIP), and we thank 

the climate modeling groups (Geophysical Fluid Dynamics Laboratory, and Japanese Meteorological Research Institute) for producing and making available their model output. For CMIP the 

U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides 

coordinating support and led development of software infrastructure in partnership with the 

Global Organization for Earth System Science Portals.

This study was funded by the U.S. Geological Survey Water Availability and Use Science 

Program. The authors thank U.S. Geological Survey personnel John Masterson, Claudia Faunt, 

Howard Reeves, and Mike Fienen, whose suggestions and assistance directly contributed to this 

report and the work described therein.

vii

Contents

Foreword ........................................................................................................................................................iii

Acknowledgments.........................................................................................................................................v

Executive Summary.......................................................................................................................................1

Major Findings.......................................................................................................................................1

Resource Monitoring and Potential Improvements........................................................................2

Introduction.....................................................................................................................................................3

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

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

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

Hydrogeology................................................................................................................................5

Methods of Analysis....................................................................................................................................19

Future Forecasts .................................................................................................................................19

Observation Network Analysis.........................................................................................................23

Groundwater Availability of the Northern High Plains Aquifer............................................................23

Predevelopment Hydrologic System ...............................................................................................23

Description of the Predevelopment Hydrologic System.....................................................23

Predevelopment Water Budget...............................................................................................26

Postdevelopment Hydrologic System .............................................................................................38

Postdevelopment Water Budgets and Changes from Predevelopment...........................38

Response of the Hydrologic System to Estimated Future Conditions........................................39

Data Used to Forecast Future Conditions..............................................................................39

Soil-Water-Balance Model .............................................................................................39

Baseline Soil-Water-Balance Model.............................................................................39

Alternative Land Cover Soil-Water-Balance Models .................................................39

Alternative Climate Soil-Water-Balance Models........................................................42

Groundwater-Flow Model ........................................................................................................43

Baseline Forecast.............................................................................................................47

Human Stresses Forecast ...............................................................................................47

Environmental Stresses Forecast ..................................................................................48

Resource Monitoring and Potential Improvements......................................................................51

Importance of Continuing Ongoing Data Collection Program............................................51

Potential New Groundwater-Level Monitoring Locations ..................................................51

Resource Monitoring Improvements......................................................................................53

Challenges for Evaluation of Groundwater Availability................................................................53

References Cited..........................................................................................................................................54
viii

Figures

1. Map showing the Northern High Plains aquifer extent, study area, major river 

basins, U.S. Geological Survey streamgage locations, and weather stations used 

in analyses. ....................................................................................................................................4

2. Maps showing the distribution of precipitation for the Northern High Plains aquifer .....6

3. Maps showing the distribution of potential evapotranspiration for the Northern 

High Plains aquifer .....................................................................................................................10

4. Maps showing the distribution of potential evapotranspiration minus precipitation 

for the Northern High Plains aquifer .......................................................................................13

 5. Map showing locations of nonirrigated, groundwater-irrigated, and 

surface-water-irrigated lands in 2008 for the Northern High Plains aquifer, 

Colorado, Kansas, Nebraska, South Dakota, and Wyoming ...............................................16

 6. Map showing hydrogeologic units and aquifer base of the Northern High Plains 

aquifer...........................................................................................................................................17

7. Maps showing the Northern High Plains aquifer, 2000–9....................................................20

 8. Map showing locations where groundwater levels measured from May 2008 to 

April 2009 were used to calibrate the Northern High Plains aquifer groundwater 

model ............................................................................................................................................24

 9. Map showing locations across the Northern High Plains aquifer where potential 

new water-level observations were subjected to network analysis to indicate the 

usefulness for future groundwater availability monitoring. ................................................25

 10. Graph showing Nebraska registered irrigation wells and irrigated land for the 

Northern High Plains aquifer groundwater model, 1940–2009............................................26

 11. Conceptual diagrams comparing average annual simulated water budget inflows 

and outflows by source and the amount of water in storage in the Northern High 

Plains aquifer...............................................................................................................................27

 12. Graph showing net release from and net replenishment to groundwater in storage 

of the Northern High Plains aquifer for 1900–2009 and trends of change in storage 

for five forecasts representing 2009–49..................................................................................30

 13. Maps and graphs showing major river basins and corresponding simulated water 

budget inflows and outflows of the Northern High Plains aquifer.....................................31

14. Graphs showing average simulated 2000–9 base flow, and average 2040–49 

simulated base flow, for the baseline, GFDL, MRI, A2LC, and B2LC forecasts ................44

 15. Map showing standard deviation of simulated groundwater levels at selected 

locations across five alternate 2009–49 forecasts................................................................52

Tables

1. Summary of growing season (May 1–September 30) precipitation, potential 

evapotranspiration, and precipitation minus potential evapotranspiration, 2000–9, 

Northern High Plains aquifer......................................................................................................9

2. Generalized section of geologic units of the Northern High Plains aquifer.....................18

3. Summary of simulated average annual groundwater budgets for 1940–49 and 

2000–9, simulated by the Northern High Plains aquifer groundwater-flow model 

in Colorado, Kansas, Nebraska, South Dakota, and Wyoming ...........................................29

4. Summary of 2000–9 average annual precipitation and difference from 1940 to 2009 

average annual precipitation for selected weather stations in the Northern High 

Plains aquifer...............................................................................................................................40
ix

5. Summary of land cover classification for the Northern High Plains aquifer 

corresponding to the baseline, A2LC, and B2LC forecasts. ................................................41

6. Summary of downscaled global climate model precipitation and temperature data 

used as inputs for the Northern High Plains aquifer Soil-Water-Balance model for 

2009–49 for the baseline, GFDL, and MRI forecasts, compared with 2000–9 average 

measured data, and summary of corresponding output evapotranspiration, 

recharge, and irrigation requirements....................................................................................43

7. Summary of average annual simulated groundwater budgets for 2009–49 for the 

Northern High Plains aquifer, and 2049 groundwater in storage, for the baseline, 

A2LC, B2LC, GFDL, and MRI forecasts and differences from the baseline for the 

A2LC, B2LC, GFDL, and MRI forecasts. ...................................................................................49

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

)

acre 0.004047 square kilometer (km2

)

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2)

Volume

cubic foot (ft3

) 28.32 cubic decimeter (dm3)

cubic foot (ft3

) 0.02832 cubic meter (m3)

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

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

)

Flow rate

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

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

cubic foot per second (ft3

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

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Hydraulic gradient

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

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

°C = (°F – 32) / 1.8.
x

Datum

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

(NAVD 88).

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

Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations

A2LC forecast using A2 land cover and 2004 climate data

B2LC forecast using B2 land cover and 2004 climate data

CMIP5 Coupled Model Intercomparison Project phase 5

ESM Earth System Modelling

GCM General Circulation Model

GFDL forecast using 2009 land cover and Geophysical Fluid Dynamics Laboratory 

Earth System Model Second Generation downscaled daily outputs

MODFLOW–NWT Newton formulation of the U.S. Geological Survey’s modular finitedifference groundwater-flow model

MRI forecast using 2009 land cover and Japanese Meteorological Research 

Institute Coupled Global Climate Model Version 3 downscaled daily outputs

PET potential evapotranspiration

RCP Representative Concentration Pathway

SWB Soil-Water-Balance

USGS U.S. Geological Survey 
Groundwater Availability of the Northern High Plains 

Aquifer in Colorado, Kansas, Nebraska, South Dakota, and 

Wyoming

By Steven M. Peterson, Jonathan P. Traylor, and Moussa Guira

Executive Summary

The Northern High Plains aquifer underlies about 

93,000 square miles of Colorado, Kansas, Nebraska, South 

Dakota, and Wyoming and is the largest subregion of the 

nationally important High Plains aquifer. Irrigation, primarily 

using groundwater, has supported agricultural production sinc

before 1940, resulting in nearly $50 billion in sales in 2012. 

In 2010, the High Plains aquifer had the largest groundwater 

withdrawals of any major aquifer system in the United States.

Nearly one-half of those withdrawals were from the Northern 

High Plains aquifer, which has little hydrologic interaction 

with parts of the aquifer farther south. Land-surface elevation ranges from more than 7,400 feet (ft) near the western 

edge to less than 1,100 ft near the eastern edge. Major stream

primarily flow west to east and include the Big Blue River, 

Elkhorn River, Loup River, Niobrara River, Republican River

and Platte River with its two forks—the North Platte River 

and South Platte River. Population in the Northern High Plain

aquifer area is sparse with only 2 cities having a population 

greater than 30,000.

Droughts across much of the area from 2001 to 2007, 

combined with recent (2004–18) legislation, have heightened concerns regarding future groundwater availability and 

highlighted the need for science-based water-resource management. Groundwater models with the capability to provide 

forecasts of groundwater availability and related stream base 

flows from the Northern High Plains aquifer were published 

recently (2016) and were used to analyze groundwater availability. Stream base flows are generally the dominant component of total streamflow in the Northern High Plains aquifer, 

and total streamflows or shortages thereof define conjunctive 

management triggers, at least in Nebraska. Groundwater avail

ability was evaluated through comparison of aquifer-scale 

water budgets compared for periods before and after major 

groundwater development and across selected future forecasts. Groundwater-level declines and the forecast amount of 

groundwater in storage in the aquifer also were examined.

e 

s 

, 

s 

-

Major Findings

Major findings informing groundwater availability of the 

Northern High Plains aquifer include the following:

• Aquifer losses to irrigation withdrawals increased 

greatly from 1940 to 2009 and were the largest average 

2000–9 outflow (49 percent of total). Groundwater 

levels generally were rising from 1940 to 1949, but 

the increase in losses to wells, combined with climatic 

differences between 1940–49 and 2000–9, resulted in 

groundwater-level declines from 2000 to 2009. Average 2000–9 groundwater in storage was 2,428 million 

acre-feet (acre-ft), or about 28 million acre-ft less than 

for 1940–49.

• Basin to basin groundwater flows were not a large part 

of basin water budgets. This means that the dominant 

processes affecting water budgets happened within 

the basins, not as groundwater flows to or from other 

basins.

• Development of irrigated land and associated withdrawals were not uniform across the Northern High 

Plains aquifer, and different parts of the Northern 

High Plains aquifer responded differently to agricultural development. For the Big Blue and Republican 

River Basins, 2000–9 aquifer losses to irrigation wells 

were larger than all other gains and losses for either 

1940–49 or 2000–9. The 2000–9 Republican River 

Basin water-level declines were nearly one-half of the 

declines of the Northern High Plains aquifer, thought 

to be primarily caused by the large losses to irrigation 

wells, resulting from a deficit between precipitation 

and potential evapotranspiration. For other stream 

basins containing fewer irrigated lands, such as the 

Loup and Niobrara River Basins, the largest aquifer 

outflows were to streams, sustaining base flow.
2 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

• For the Northern High Plains aquifer, areas with high 

recharge and low evapotranspiration had the most 

streamflow, and most streams only remove water from 

the aquifer. In some other principal aquifers, such as 

the Mississippi Alluvial Plain, streams provide an 

important source of recharge to the aquifer. 

• Results of a baseline future forecast indicated that 

groundwater levels declined overall, indicating an 

overdraft of the aquifer when climate was about average and agricultural development was held at the same 

state as 2009. However, forecasted 2009–49 declines 

(at a rate of 1.6 million acre-feet per year [acre-ft/yr]) 

were at less than one-half the rate than that of 2000–9, 

a drier than average period and, therefore, indicated 

that by 2049, the amount of groundwater in storage in 

the aquifer would be 2,372 million acre-ft, or 56 million acre-ft less than the 2000–9 average.

• Results of two human stresses future forecasts indicated that increases of 13 percent or 23 percent in 

agricultural development, mostly near areas of previous development, caused increases in groundwater 

pumping of 8 percent or 11 percent, and resulted in 

continued groundwater-level declines, at rates 0.3 or 

0.5 million acre-ft/yr larger than the baseline forecast. However, both forecasts caused about the same 

decrease in stream base flow of 100,000 acre-ft/yr.

• Results of environmental stresses forecasts (generated from two downscalings of global climate model 

outputs) compared with the baseline forecast indicated 

that even though annual precipitation was nearly the 

same, differences in temperature and a redistribution 

of precipitation from the spring to the growing season 

(from about May 1 through September 30), created 

a large (12–15 percent) decrease in recharge to the 

aquifer.

• For the two environmental stresses forecasts, temperature and precipitation were distributed about the same 

among basins of the Northern High Plains aquifer, but 

the amounts were different. The largest effects of these 

forecasts were in two basins: a northern basin that had 

not had previous groundwater-level declines and a 

southern basin where groundwater levels have declined 

previously. In the baseline forecast, the largest outflow 

of the northern basin had been to streams, and the 

decreased recharge created a 15-percent decline in that 

flow; however, in the southern basin, which had had 

far less base flow in the baseline forecast, base flow 

declined by a larger part (35 percent).

Resource Monitoring and Potential 

Improvements

The importance of resource monitoring and potential 

improvements is summarized below:

• Streamflow and groundwater-level data provide 

fundamental data describing water availability and 

will continue to be primary sources of information 

on the health of the aquifer (as well as model calibration data) and, therefore, are critical to the success of 

future groundwater availability forecasts. Collection 

of additional such data at more locations, or at higher 

temporal frequency, may further reduce water availability uncertainty and model uncertainty.

• Additional geologic data, collected through test hole 

drilling, airborne resistivity, or other methods, could 

aid in improving future models. Aquifer property 

data directly affect simulations of groundwater flow 

and related uncertainty; the collection of additional 

geologic data describing aquifer thickness and other 

properties would likely directly benefit future groundwater models and studies. Previous work has indicated 

that the addition of such data reduced simulation 

uncertainty.

• The simulation described in this report also can be used 

as a tool to evaluate the benefit of collecting additional physical or hydrologic data. As an example, an 

analysis to evaluate where the collection of additional 

groundwater-level data would provide the most benefit 

across select candidate locations was completed using 

the simulation. These results indicated that the largest 

variations were spatially biased, generally along the 

drainage divide between the South Platte River and the 

Republican River Basin in southwest Nebraska, close 

to the northeast corner of Colorado. Other locations 

with large variations were commonly closer to drainage divides between simulated streams in the area and 

near the downstream end of the South, Middle, and 

North Loup Rivers.

• Groundwater withdrawals for irrigation have the single 

largest anthropogenic effect on the groundwater-flow 

system of the Northern High Plains aquifer and are the 

largest outflow component of the groundwater system, yet one that is poorly defined by data. Therefore, 

additional groundwater withdrawal data would likely 

improve model calibration and reduce uncertainty. 

By 2016, groundwater withdrawal meters had been 

installed in more locations across the area, yet it is 
Introduction 3

unclear if enough data have been collected to describe 

aquifer-scale groundwater withdrawals, or if data have 

been collected over a sufficient period. In addition to 

the groundwater withdrawal data, ancillary related 

data, such as irrigation application methods and crops 

grown at each groundwater withdrawal site, would provide needed context to improve estimated groundwater 

withdrawals.

In general, although the model is considered accurate at a 

regional scale, it is not suitable for evaluating local-scale problems nor for short periods of less than a few months. Readers 

who are interested in more local-scale analysis could use data 

and results of this study as a starting point for further refinement. Readers are cautioned that future forecast base flows 

and groundwater budgets included in the analysis described in 

this report are not comprehensive and may not be the average nor span the range of effects caused by potential future 

climate or land cover changes. These forecasts are provided 

as examples of how the groundwater-flow model can be used 

as a tool to evaluate aquifer and stream base flow responses to 

potential future conditions.

Introduction

The High Plains aquifer (fig. 1) is a nationally important 

water resource underlying about 175,000 square miles (mi2) 

in parts of eight States: Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming (Qi, 

2010). Irrigation, primarily using groundwater, has supported 

agricultural production since before 1940, resulting in nearly 

$50 billion in sales in 2012 (U.S. Department of Agriculture, 

2019). In 2010, the High Plains aquifer had the largest groundwater withdrawals of any major aquifer system in the United 

States (15.1 million acre-feet [acre-ft]; Maupin and others, 

2014). At the onset of this study (2008), team members and 

program coordinators identified that it would be beyond scope 

to study all parts of the aquifer in detail and determined that 

the most value would be added by focusing on the Northern 

High Plains aquifer.

The Northern High Plains aquifer (labeled “study area” 

on fig. 1) is a distinct region that has little hydrologic interaction with parts of the aquifer farther south and is the largest 

subregion of the aquifer. In 2010, groundwater withdrawals 

from the Northern High Plains aquifer were about 42 percent 

of the total groundwater withdrawals from the High Plains 

aquifer (Maupin and others, 2014). In 2010, groundwater 

withdrawals for irrigation, public supply, livestock, and other 

purposes were 95, 2, 2, and 1 percent, respectively, of the 

groundwater withdrawals from the Northern High Plains aquifer (Maupin and others, 2014). In 2008, about 8,379,000 acres 

(13,092 mi2

 or 13 percent of the area) of the Northern High 

Plains aquifer were developed for agriculture with irrigation 

(U.S. Department of Agriculture, 2019); of that area, about 

11 percent was served by surface-water diversions and the 

remainder was irrigated with groundwater. Droughts across 

much of the area from 2001 to 2007, combined with recent 

legislation (Nebraska Department of Natural Resources, 

2004), have heightened concerns regarding future groundwater 

availability and highlighted the need for additional information to support science-based water-resource management. For 

example, although surface-water development is managed at 

the State level and groundwater is managed at a regional level 

in Nebraska, shortages of surface water can trigger integrated 

management of surface water and groundwater (Nebraska 

Department of Natural Resources, 2004). Groundwater models 

with the capability to provide forecasts of groundwater availability and related stream base flows from the Northern High 

Plains aquifer were recently published (Peterson and others, 

2016a). Using a groundwater-flow model of the Northern High 

Plains aquifer, future groundwater availability for various 

potential future conditions can be simulated and can provide 

information about the effects of potential future changes in 

climate or land cover on the aquifer and related groundwater 

discharge and how simulated groundwater budgets differ for 

subregions.

Purpose and Scope

This report documents the analysis of groundwater availability for the Northern High Plains aquifer using a calibrated 

model that simulates from presettlement to April 30, 2009 

(Peterson and others, 2016a), and a select set of 2009–49 

forecast conditions. The report describes the methods of 

analysis and the models used as tools for the analysis that can 

be revised to evaluate water availability under other potential future conditions. For the included forecasts, the report 

also describes aquifer-scale groundwater budgets and the 

amount of groundwater in the aquifer, basin-scale groundwater 

budgets, simulated base flows of major streams, and aspects 

related to hydrologic monitoring.

Study Area Description

Information on the study area presented in this section 

is from Peterson and others (2016a) unless cited otherwise. 

The Northern High Plains aquifer (study area, fig. 1) underlies about 62 million acres of the States of Colorado, Kansas, 

Nebraska, South Dakota, and Wyoming. Land-surface elevation ranges from more than 7,400 feet (ft) near the western 

edge to less than 1,100 ft near the eastern edge. Major streams 

primarily flow west to east and include the Big Blue River, 

Elkhorn River, Loup River, Niobrara River, Republican River, 

and Platte River with its two forks—the North Platte River 

and South Platte River (fig. 1). Population in the study area 

is sparse with only 2 cities having a population greater than 

30,000 (Cheyenne, Wyoming, and Grand Island, Nebraska; 

U.S. Geological Survey, 2004).
4 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. White River Merritt Reservoir Box Butte Reservoir Calamus ReservoirDavis Creek Reservoir Sherman Reservoir Lake McConaughy Harry Strunk Lake Hugh Butler Lake Swanson Lake Harlan County Lake

LoupLoup

River

South

Loop

River South

Loup

River

North Loop

River

North Loup

River

Middle Loup River

Elkhorn RiverElkhorn River

Platte River

North Platte River

South Platte River

N oi brara River N oi brara River

Lodgepole Creek

Republican RiverRepublican River

Big Blue

River

Little White RiverLittle White River

96° 98° 100° 102° 104° 106°

42°40°

Base modified from U.S. Geological Survey digital data, 2008and U.S. Geological Survey, 2019b.Albers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

Utica

W.B. Heilig Field Airport, Scottsbluff

Miller Field Airport, Valentine

Karl Stefan Memorial Airport, Norfolk

Broken Bow

McCook

SOUTH DAKOTA

WYOMING

IOWA

NEBRASKA

COLORADO

KANSAS

MINNESOTA

MISSOURI

06881000

00549760 00549760

00500860 005

06853500

00026460 00026460

Grand Island

enneyehC enneyehC

!

!

!

!

!

!

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

U.S. Geological Survey streamgage 

with identifier

EXPLANATION

Dune sandRepublican River Basin

Niobrara River BasinPlatte River Basin

Loup River BasinBig Blue River BasinElkhorn River BasinStudy areaHigh Plains aquiferFault

06800500McCook

! Weather station used to evaluate average 

precipitation year, with identifier

, Geological Survey (U.S. Geological Survey streamgage locations , U.S. (Watermolen, 2005) The Northern High Plains aquifer extent, study area, major river basins Figure 1.

2019a), and weather stations used in analyses.
Introduction 5

Climate

Precipitation generally increases west to east and ranges 

from less than 12 inches (in.) in the western part of the 

Northern High Plains aquifer to almost 31 in. in the eastern 

part (Peterson and others, 2016a; fig. 2A). The average annual 

precipitation from 2000 to 2009 was 21.1 in. More than 

one-half of the precipitation falls during the growing season, 

or from about May 1 through September 30; for example, 

average annual 2000–9 Northern High Plains aquifer growing 

season precipitation ranged from 8.8 (2002) to 15.7 in. (2009),

with an average of 13.1 in. (table 1). The general distribution of precipitation in the growing season is the same as for 

annual or decadal periods, but some subregional variations 

exist; for example, in 2002, growing season precipitation on 

the Nebraska side of the north-east corner of Colorado was 

more than 16 in., around twice that of the surrounding region 

where it was about 6–10 in. (fig. 2B). In 2009, growing season

precipitation was more than 14 in. for most of the central 

Northern High Plains aquifer but was atypically less in the 

lower Republican River Basin and Big Blue River Basin, in 

the southeast part of the area (fig. 2C).

Average annual (1981–2010) temperatures generally 

are highest in the southern part of the Northern High Plains 

aquifer (Trego County, Kansas, average of 54 degrees Fahrenheit [°F]; High Plains Regional Climate Center, 2015) and 

decrease to the north (Jackson County, South Dakota, average 

of 47 °F) and with increasing elevation near the western edge 

(Laramie County, Wyo.; average of 45 °F). Higher temperature directly relates to higher potential evapotranspiration 

(PET), so the largest PET rates were in the southern and western parts of the area, at more than 51 inches per year (in/yr), 

except at the highest elevation area in Wyoming (fig. 3). The 

smallest PET rates were in the northeastern part of the area, 

at around 41 in/yr, and the 2000–9 average annual PET rate 

was 45.8 in/yr. As with precipitation, annual PET rates follow 

the decadal regional distribution and PET rates are highest 

during the growing season (May 1–September 30). However, 

some annual variations exist; for example, although average 

PET rates for the growing season of 2002 (32.6 in.) and 2009 

(30 in.) were fairly similar (table 1), in 2002, the western onehalf of the area had a PET rate of more than 32 in., whereas 

for 2009, only part of western Nebraska and the upper end of 

the Republican River Basin exceeded 32 in. (fig. 3B, C).

Average annual 2000–9 PET exceeded precipitation by 

about 10 in. for the eastern part of the area and by more than 

36 in. for the western part of the area (fig. 4A). This means 

that, on average for 2000–9, all precipitation would have been 

consumed by PET annually. PET also exceeded precipitation 

for every 2000–9 growing season (table 1), with the average 

deficit within the range from 14.3 in. (2009) to 23.8 in. (2002).

The distribution of the deficit was different for different 

years; for example, for the 2002 growing season, the deficit 

was larger than 18 in. except in the eastern part of the area, 

whereas for the 2009 growing season, only the northwestern 

part of the area had a deficit exceeding 18 in. (fig. 4B, C).

The economy of lands overlying the Northern High 

Plains aquifer depends on agriculture, and irrigation has 

sustained agricultural production for parts of the area and 

years where precipitation was less than crops needed. Hence 

the area of irrigated land has increased from about 1890 until 

at least 2015. Early (circa 1890) irrigation was through the 

diversion of surface water (State Board of Irrigation, 1899), 

and most surface-water irrigation projects, covering 1.7 percent of the area or 13 percent of irrigated land in the study 

area, were substantially in place by 1955. Development of 

groundwater was meager before 1940 (Weeks and others, 

1988), covering less than 0.5 percent of the study area. By 

1949, about 827,000 acres of the Northern High Plains aquifer 

were irrigated (1.7 percent of the area), expanding primarily 

through groundwater development to 8,379,000 acres by 2008 

(U.S. Department of Agriculture, 2019). In 2008, most of the 

1,085,000 acres irrigated with surface water used diversions 

from the Platte River system or reservoirs therein (fig. 1), 

whereas 7,294,000 acres of groundwater-irrigated areas 

(11.7 percent of the area) were most intensive in the eastern 

one-half of the Northern High Plains aquifer (fig. 5). Additional information on land cover data and changes through 

time are available in Houston and others (2013) and Peterson 

and others (2016b).

Hydrogeology

Information on the study area presented in this section is 

from Peterson and others (2016a) unless cited otherwise. The 

general hydrogeology of the High Plains aquifer was described 

in Gutentag and others (1984); a summary of that description 

is provided herein as a convenience to the reader. The High 

Plains aquifer consists of hydraulically connected deposits 

of late Tertiary and Quaternary age (fig. 6, table 2; Gutentag 

and others, 1984). Late Tertiary-age deposits, from oldest to 

youngest, include the Chadron Formation of the White River 

Group (Bartos and others, 2014), Brule Formation of the 

White River Group, Arikaree Group, Ogallala Formation, and 

Broadwater Formation (Gutentag and others, 1984; Diffendal, 

1995, Diffendal and others, 2008). Quaternary-age deposits 

include alluvial, valley-fill, dune sand, and glacial deposits 

(fig. 6). The Ogallala Formation composes most of the Northern High Plains aquifer (fig. 6, table 2; Gutentag and others, 

1984). Generally, low-permeability geologic units of midTertiary age or older underlie the High Plains aquifer (Weeks 

and Gutentag, 1981), forming a gently eastwardly dipping 

(5–7 feet per mile) paleosurface where the High Plains aquifer 

units were deposited (fig. 6; Peterson and Traylor, 2016). 

Local variations in paleosurface elevation form buried valleys. 

Groundwater flow between the High Plains aquifer and the 

underlying units is minimal. Groundwater flow in the Northern 

High Plains aquifer corresponds to the aquifer base slope and 

flows from west to east, except where it discharges to streams 

or is removed by evapotranspiration.

The Chadron and Brule Formations of the White River 

Group, together with the younger Arikaree Group, are 
6 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

A

EXPLANATION

Average annual precipitation, 

Major river basins

2000–9, in inches

11.7 to 1212.1 to 1414.1 to 1616.1 to 1818.1 to 2020.1 to 2222.1 to 2424.1 to 2626.1 to 2828.1 to 3030.1 to 3232.1 to 34

Study areaHigh Plains aquifer

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

, precipitation for the 2002 growing season 

B , average annual precipitation for 2000–9; 

A. The distribution of precipitation for the Northern High Plains aquifer Figure 2.

(May 1–September 30); 

C, precipitation for the 2009 growing season.
Introduction 

7

B

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

EXPLANATION

Major river basins

3.5 to 44.1 to 66.1 to 88.1 to 1010.1 to 1212.1 to 1414.1 to 1616.1 to 1818.1 to 20

Precipitation, in inches

, precipitation for the 2002 growing season B , average annual precipitation for 2000–9; A The distribution of precipitation for the Northern High Plains aquifer. Figure 2.

 1–September 30); C, precipitation for the 2009 growing season.—Continued (May
8 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

C

EXPLANATION

Precipitation, in inches

Major river basins

3.5 to 44.1 to 66.1 to 88.1 to 1010.1 to 1212.1 to 1414.1 to 1616.1 to 1818.1 to 2020.1 to 2222.1 to 24

, precipitation for the 2002 growing season 

B , average annual precipitation for 2000–9; 

A The distribution of precipitation for the Northern High Plains aquifer. Figure 2.

 1–September 30); 

C, precipitation for the 2009 growing season.—Continued (May
Introduction 9

Table 1. Summary of growing season (May 1–September 30) precipitation, potential evapotranspiration, and precipitation minus 

potential evapotranspiration, 2000–9, Northern High Plains aquifer (Peterson and others, 2016b).

PPT, precipitation; PET, potential evapotranspiration]

Year Growing season (May 1–September 30)

PPT, in inches PET, in inches PPT minus PET, in inches

2000 10.1 32.8 −22.7

2001 13.6 31.6 −18.0

2002 8.8 32.6 −23.8

2003 11.1 31.6 −20.5

2004 14.1 30.6 −16.5

2005 14 31.8 −17.8

2006 12.1 32.3 −20.2

2007 14.7 31.5 −16.8

2008 16.4 30.9 −14.5

2009 15.7 30.0 −14.3

Average 13.1 31.6 −18.5

generally fine-grained, low-permeability units except for a 

few areas of high permeability and areas where permeability 

has been increased by fractures. These are the oldest geologic 

units of the High Plains aquifer and are along the northwestern 

margins of the Northern High Plains aquifer (fig. 6, table 2). 

The Brule Formation is considered part of the High Plains 

aquifer only where the permeability has been increased by secondary porosity such as joints, fractures, and solution openings 

(Gutentag and others, 1984). Where it has not been enhanced 

through secondary porosity, the top of the Brule Formation 

forms the base of the High Plains aquifer. In the western part 

of the Northern High Plains aquifer, the Brule Formation is 

overlain by the younger Arikaree Group, mainly composed 

of very fine to fine-grained sandstone (fig. 6). The Arikaree 

Group has a maximum thickness of about 1,000 ft in western 

Nebraska and eastern Wyoming.

The Ogallala Formation is generally coarser and more 

permeable than the older underlying units and extends over 

most of the study area (fig. 6, table 2). The Ogallala Formation 

is a heterogeneous deposit of interlayered stream sediments; 

lakebeds; and eolian sand, silt, and clay. The Ogallala Formation varies greatly in particle size and physical character over 

short distances (Cannia and others, 2006). The maximum 

thickness of the Ogallala Formation is about 984 ft (Hobza 

and others, 2012; Flynn and Stanton, 2018). Sediments of 

the Ogallala Formation form the thickest part of the Northern 

High Plains aquifer. Sediments of the Ogallala Formation are 

less coarse than the overlying Quaternary alluvial and valleyfill deposits; gravel is not abundant within the Ogallala Formation (Lawton, 1984).

Unconsolidated Quaternary-age sedimentary deposits 

overlie the older aquifer units. The oldest Quaternary alluvial 

deposits are largely to the east where the Ogallala Formation 

is absent (fig. 6, table 2). The next oldest are the dune sands 

that overlie the central part of the study area (fig. 1), followed 

by the Quaternary glacial deposits to the northeast (fig. 6). 

The youngest deposits are the alluvial deposits associated 

with the modern river basins (labeled “Quaternary valley-fill 

deposits” in fig. 6). Unconsolidated Quaternary-age alluvial 

gravel, sand, silt, and clay overlie and are in hydrologic connection with the Ogallala Formation in the eastern parts of 

the Northern High Plains aquifer. Unconsolidated Quaternary 

alluvial deposits generally are coarser and more permeable 

than those of the Ogallala Formation and other older underlying units. Eastward, where the Ogallala Formation is absent, 

Quaternary alluvial and valley-fill deposits directly overlie 

poorly permeable bedrock. Where the aquifer consists mainly 

of Quaternary-age alluvial deposits, it generally is thinner 

than in areas dominated by the Ogallala Formation; maximum 

thicknesses of Quaternary alluvium are around 300 ft (Gutentag and others, 1984).

Eolian dune sand deposits of Quaternary age overlie the 

Ogallala Formation in the central part of the Northern High 

Plains aquifer (fig. 1, table 2). The largest contiguous area, 

known as the Nebraska Sand Hills (in north-central Nebraska, 

labeled as “dune sand” on fig. 1), covers about 20,000 mi2

 of 

the Northern High Plains aquifer and was undergoing dune 

formation and migration as recently as about 700 years ago 

(Miao and others, 2007). The dune sands range from very fine 

to medium sand and, where saturated, are considered part of 

the High Plains aquifer (Gutentag and others, 1984). The dune 

sand deposits are as much as 300 ft thick, but on average, their 

thickness is 100 to 150 ft, and therefore dune sands mostly 

exist as a thin veneer on top of the underlying deposits of the 
10 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

EXPLANATION

Study areaHigh Plains aquiferMajor river basins

41.1 to 4242.1 to 4343.1 to 4444.1 to 4545.1 to 4646.1 to 4747.1 to 4848.1 to 4949.1 to 50

50.1 to 51

51.1 to 52

Average annual potential 

evapotranspiration,in inches

A

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

, potential , average annual potential evapotranspiration for 2000–9; 

B

A. The distribution of potential evapotranspiration for the Northern High Plains aquifer

 1–September 30); 

C, potential evapotranspiration for the 2009 growing season.

Figure 3.evapotranspiration for the 2002 growing season (May
Introduction 11

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106° B

EXPLANATION

Major river basins

29.5 to 3030.1 to 3131.1 to 3232.1 to 3333.1 to 3434.1 to 3535.1 to 36

Potential evapotranspiration, 

in inches

, potential , average annual potential evapotranspiration for 2000–9; B A The distribution of potential evapotranspiration for the Northern High Plains aquifer. 

 1–September 30); C, potential evapotranspiration for the 2009 growing season.—Continued

Figure 3.evapotranspiration for the 2002 growing season (May
12 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

C

EXPLANATION

Potential evapotranspiration, 

Major river basins

in inches

27.1 to 2828.1 to 2929.1 to 3030.1 to 3131.1 to 3232.1 to 3333.1 to 3434.1 to 35

, potential , average annual potential evapotranspiration for 2000–9; 

B

A The distribution of potential evapotranspiration for the Northern High Plains aquifer. 

 1–September 30); 

C, potential evapotranspiration for the 2009 growing season.—Continued

Figure 3.evapotranspiration for the 2002 growing season (May
Introduction 13

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106° A

EXPLANATION

Average annual potential 

Study areaHigh Plains aquiferMajor river basins

evapotranspiration minus precipitation, in inches

10.2 to 1212.1 to 1515.1 to 1818.1 to 2121.1 to 2424.1 to 2727.1 to 3030.1 to 33

33.1 to 3636.1 to 39

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

, for the 2002 growing , average annual for 2000–9; B A. The distribution of potential evapotranspiration minus precipitation for the Northern High Plains aquifer Figure 4.

season (May 1–September 30); C, for the 2009 growing season.
14 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

B

EXPLANATION

Potential evapotranspiration 

Major river basins

minus precipitation,in inches

5.3 to 66.1 to 99.1 to 1212.1 to 1515.1 to 1818.1 to 2121.1 to 2424.1 to 2727.1 to 30

, for the 2002 growing , average annual for 2000–9; 

B

A The distribution of potential evapotranspiration minus precipitation for the Northern High Plains aquifer. 

, for the 2009 growing season.—Continued

Figure 4.season (May 1–September 30); 

C
Introduction 15

C

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

EXPLANATION

Potential evapotranspiration 

Major river basins

minus precipitation,in inches

5.3 to 66.1 to 99.1 to 1212.1 to 1515.1 to 1818.1 to 2121.1 to 2424.1 to 2727.1 to 30

, for the 2002 growing , average annual for 2000–9; B A The distribution of potential evapotranspiration minus precipitation for the Northern High Plains aquifer. 

, for the 2009 growing season.—Continued

Figure 4.season (May 1–September 30); C
16 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. 96° 98° 100° 102° 104° 106° 42° 40° SOUTH DAKOTA WYOMING IOWA NEBRASKA COLORADO KANSAS MINNESOTA MISSOURI Base from U.S. Geological Survey digital data, variously dated, various scales Albers Equal-Area Conic projection Standard parallels 29o50' N. and 45o50' N. Central meridian 96o00' W.

North American Datum of 1983

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

EXPLANATION

Nonirrigated croplandIrrigated with surface waterIrrigated with groundwaterStudy areaHigh Plains aquifer

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

, Colorado, Kansas, Nebraska, -irrigated lands in 2008 for the Northern High Plains aquifer -irrigated, and surface-water Locations of nonirrigated, groundwater

yoming (data from Peterson and others, 2016a).

Figure 5.South Dakota, and W
Introduction 17

3,600

3,800

4,000

3,200

3,600

4,200

3,800

3,600

2,200

2,000

5,000

3,800

2,200

3,400

1,800

1,800

1,400

3,400

4,000

2,000

3,000

2,600

4,400

3,800

3,000

1,600

4,400

1,400

4,200

4,800

2,600

2,800

3,400

1,400

2,200

3,600

2,000

3,000

3,200

4,200

4,800

2,400

4,600

1,400

4,400

2,400

3,200

1,800

4,000

3,800

2,400

4,600

3,000

4,600

1,400

5,400

1,600

4,200

2,600

1,600

5,200

5,200

5,000

6,600

5,200

4,800

2,000

4,400

6,200

6,000

4,600

5,600

1,200

4,400

5,400

4,200

4,000

3,200

2,800

2,800

2,600

2,400

2,200

1,800

1,400

2,400

2,400

2,800

2,600

3,200 3,400

3,600

2,800

EXPLANATION

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

Quaternary valley-fill depositsQuaternary glacial deposits

Quaternary alluvial deposits

White River and Arikaree Groups

Northern High Plains aquiferHigh Plains aquiferSelected aquifer base 

elevation contour andvalue—Contour interval 200 feet. Datum isNorth American Vertical Datum of 1988. Elevation contours depict the base unit elevation of the Northern High Plains aquifer, from Peterson and Traylor (2016)

96° 98° 100° 102° 104° 106°

42°40°

SOUTH DAKOTA

WYOMING

IOWA

NEBRASKA

COLORADO

KANSAS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

MINNESOTA

MISSOURI

Late Tertiary deposits

Ogallala Formation

Hydrogeologic units modified from Gutentag 

and others (1984) and Soller and others (2012)

5,400

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

Figure 6. Hydrogeologic units and aquifer base of the Northern High Plains aquifer (Peterson and Traylor, 2016; Peterson and others, 2016a).
18 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. Table 2. Generalized section of geologic units of the Northern High Plains aquifer (modified from Weeks and Gutentag, 1981; Peterson, 2009; and Hobza and others, 2012). Physical character Stream-laid deposits of gravel, sand, silt, and clay associated with the most recent cycle of erosion and deposition along present streams. Usually hydraulically connected to underlying Quaternary and Tertiary deposits. Fine to medium sand with small amounts of clay, silt, and coarse sand formed into hills and ridges by the wind. Silt with lesser amounts of very fine sand and clay deposited as windblown dust. Where thickest, generally only the lowest 100 feet is below the water table. Stream-laid deposits of gravel, sand, silt, and clay locally cemented by calcium carbonate into caliche or mortar beds. Usually hydrau- lically connected to underlying Quaternary and Tertiary deposits. Poorly sorted clay, silt, sand, and gravel generally unconsolidated; forms caliche layers or mortar beds when cemented by calcium carbonate. Most of the saturated thickness of the Northern High Plains aquifer, though absent from the western and eastern ends. Predominantly massive very fine to fine-grained sandstone with lo- calized beds of volcanic ash, silty sand, siltstone, claystone, sandy clay, limestone, marl, and mortar beds. Upper unit, Brule Formation, predominantly massive siltstone containing sandstone beds and channel deposits of sandstone. Included where it contains saturated sandstones or interconnected fractures, mainly limited to western Nebraska. Otherwise, the top of the Brule Formation is considered the base of the Northern High Plains aquifer.

Shales, chalks, limestones, or other poorly permeable deposits, the 

top of which form the base of the Northern High Plains aquifer. Most often, Pierre Shale, Niobrara Chalk, or Niobrara Shale. 

Thickness, in feet

0 to 1800 to 3000 to 370

0 to 5000 to 980

0 to 1,000

0 to 700

Various

Geologic unit

Valley-fill deposits

Dune sand

Loess

Unconsolidated alluvial 

deposits

Ogallala Formation

Arikaree Group 

White River Group

Undifferentiated

Series

Pleistocene 

and 

Holocene

Pleistocene 

and 

Holocene

Miocene

Lower Miocene and 

upper Oligocene

Oligocene

Various

System

Quaternary Tertiary Cretaceous or older
Methods of Analysis 19

Ogallala Formation (Lawton, 1984). Though the dune sands 

compose only a minor part of the aquifer, they serve as an 

important surficial feature enhancing aquifer recharge. Ogallala Formation deposits underlie all Quaternary dune sands in 

the High Plains aquifer (Muhs, 2007).

Glacial deposits overlie the eastern end of the Northern 

High Plains aquifer (fig. 6; Condra and others, 1950), forming a region of generally lower aquifer permeability but with 

poorly defined subsurface character and continuity. Although 

glacial deposits have been eroded in major stream valleys, 

glacial till remains in intervalley areas in the northeastern part 

of the Northern High Plains aquifer (Soller and others, 2012). 

The glacial deposits consist of till and outwash overlain by 

eolian loess with possible buried valley-fill deposits of sand 

and gravel. The distribution of buried valley-fill deposits 

within or underlying the till is not well known.

Surficial deposits of eolian loess overlying parts of the 

Northern High Plains aquifer (Muhs and Bettis, 2000) are 

important because their fine texture limits the maximum infiltration rate and, therefore, the rate of groundwater recharge. 

Loess is defined as eolian deposits of primarily silt-sized 

particles that can be as thick as 370 ft (Johnson and Brennan, 

1960; Swinehart and others, 1994; Pye, 1995; Condon, 2005; 

Peterson, 2009). Quaternary-age valley-fill deposits are similar 

in character and deposition to the Quaternary-age alluvial 

deposits and are distinguished because the valley-fill deposits 

are related to erosion and deposition by modern-day stream 

systems rather than ancient streams. These valley-fill deposits 

are as much as 180 ft thick (Peterson, 2009) and occupy most 

major river valleys that cross the Northern High Plains aquifer.

The water table is the upper surface of the saturated part 

of the unconfined aquifer, and it slopes gently west to east, 

although local variations exist (Gutentag and others, 1984); 

for example, simulated 2000–9 groundwater levels (Peterson 

and others, 2016a) indicate deflections around regional stream 

systems superimposed on the general west to east gradient (fig. 7A). Some steep gradients are indicated in western 

Nebraska, though in these areas the aquifer is thin and poorly 

conductive. Saturated thickness is another important aspect 

defining groundwater availability because it directly relates 

to the amount of water in storage in the aquifer. The average 

2000–9 saturated thickness calculated using the simulated 

water table (Peterson and others, 2016a) and aquifer base 

elevation (Peterson and Traylor, 2016) ranged from less than 

50 to more than 1,100 ft, with an average thickness of 249 ft 

(fig. 7B). The aquifer is thickest in north-central Nebraska and 

thinnest in the east, west, and south.

Depth to groundwater (or the water table) is an important 

characteristic influencing irrigation well pumping lift, interaction with surface water, and evapotranspiration. Based on 

average simulated water table elevations from 2000 to 2009 

(Peterson and others, 2016a), depth to groundwater ranges 

from less than 10 ft near major rivers (such as the Platte River) 

and in the upstream end of the Elkhorn River Basin to more 

than 500 ft in uplands in the western part of the area (fig.7C). 

Depth to water calculated from the simulated water table 

elevation indicates the same general patterns as published in 

Stanton and others (2011). The median depth to groundwater 

for the study area is 84 ft.

Methods of Analysis

Two process-based methods contributed to the analysis of 

groundwater availability described in this report: a Soil-WaterBalance (SWB) model (Westenbroek and others, 2010) was 

used to estimate recharge and groundwater withdrawals for 

irrigation, and a Newton formulation of the U.S. Geological 

Survey (USGS) modular finite-difference groundwater-flow 

model (MODFLOW–NWT, Niswonger and others, 2011) 

was used to evaluate forecasted stream base flow and simulated groundwater budgets (Peterson and others, 2016a). As 

in Peterson and others (2016a), recharge and groundwater 

withdrawals for irrigation were estimated with SWB, then 

used as inputs to the MODFLOW–NWT groundwater model; 

therefore, for the analysis documented in this report, the 

SWB and MODFLOW–NWT models were used to simulate 

2009–49 conditions for five future forecasts. 

Future Forecasts

The forecasts are a baseline forecast, a forecast using A2 

land cover and 2004 climate data (A2LC), a forecast using 

B2 land cover and 2004 climate data (B2LC), a forecast using 

2009 land cover and Geophysical Fluid Dynamics Laboratory Earth System Model Second Generation downscaled 

daily outputs (GFDL), and a forecast using 2009 land cover 

and Japanese Meteorological Research Institute Coupled 

Global Climate Model Version 3 downscaled daily outputs 

(MRI). The baseline forecast was constructed to represent a 

static repeating condition to evaluate effects of current (as of 

2009) practices and average climate continuing into the future. 

The A2LC and B2LC forecasts also used 2004 climate data 

combined with A2 land cover and B2 land cover (modified 

from Houston and others, 2013), respectively, to evaluate the 

effects of possible land development patterns. A2 land cover 

of irrigated agricultural area increases by 23 percent from 

2009 to 2049, as compared with 13 percent for B2 land cover. 

The GFDL and MRI forecasts were constructed to evaluate the 

effects of potentially different future climate, and used 2009 

land cover combined with alternative climate data derived 

from downscalings of global climate model data (Bureau of 

Reclamation, 2013). Additional details are in the “Data Used 

to Forecast Future Conditions” section.

Adjustments to recharge estimated during calibration 

(Peterson and others, 2016a) also were applied to the forecast 

period (2009–49) recharge. Use of different SWB inputs to 

represent changes in land cover or climate changed the SWB 

and corresponding groundwater model outputs. To characterize the effects, aquifer and basin-scale groundwater budgets 

for the baseline forecast were compared against 2000–9 
20 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

, , saturated thickness from average simulated groundwater levels; 

C

B , average simulated groundwater levels; 

A , 2000–9. The Northern High Plains aquifer

, calculated from average simulated water levels (Peterson and others, 2016a).

Figure 7.depth to groundwater

3 4, 00

3,600

4, 6 00

5,400

4,60 0

5,200

4

5,000

,800

1,400

6,400

6,100

4,700

1 3, 0 0

6,000

5,900 , 5 006

5,500

4,600

3,900

4,5 0 0

5,400

5,300

4,500

5,200

1,400

1 5, 00

4,400

5,100

4,800

5,000 4 9, 00

4,4 00

4,700

2,400

4,300

4,600

4,300

4,200

4,100

3,700

8, 3 00

4,000

3,600

3,900

4,500

4,300

3 5, 00

1,500

1 6, 00

1,700

3 9, 00

1,800

1 9, 00

4,000

2,200

3,600

2 1, 00

3,500

, 3 8 00 3,700

2,000

23, 00

4,100

2,400

4 2, 00

3,400

3,200

2,500

3 3, 00

2,600

27, 00

2,900

3,000

2,800

4,100

96° 98° 100° 102° 104° 106°

42°40°

SOUTH DAKOTA

WYOMING

IOWA

NEBRASKA

COLORADO

KANSAS

MINNESOTA

MISSOURI

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

EXPLANATION

Study areaHigh Plains aquifer

Average 100-foot 

groundwater-level contour—Showsaverage altitude atwhich simulatedwater levels wouldhave stood in tightlycased wells, for 2000–9

1,400

A

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD
Methods of Analysis 21

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106° B

EXPLANATION

Average saturated thickness 

Major river basins

from simulated 2000–9 groundwater levels, in feet

Less than 5051 to 100101 to 250251 to 500501 to 750751 to 1,105

, saturated thickness from average simulated groundwater levels; B , average simulated groundwater levels; , 2000–9. A The Northern High Plains aquifer Figure 7.

, depth to groundwater, calculated from average simulated water levels (Peterson and others, 2016a).—Continued C
22 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

, saturated thickness from average simulated groundwater levels; 

B , average simulated groundwater levels; , 2000–9. 

A The Northern High Plains aquifer Figure 7.

, depth to groundwater, calculated from average simulated water levels (Peterson and others, 2016a).—Continued C

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

C

EXPLANATION

Average 2000–9 depth

to groundwater, in feet

Major river basins

Less than 1010 to 5051 to 100101 to 250251 to 500Greater than 500
Groundwater Availability of the Northern High Plains Aquifer 23

average conditions. Next, four alternative forecasts were compared against baseline (no change from 2009) outputs, and for 

most basins, simulated flow from major streams was compared 

against basin-scale flow. Groundwater model outputs also 

were used to analyze potential improvements to the (2009) 

groundwater monitoring network, by evaluating the variability 

in simulated water levels across all five forecasts, at a random 

sampling of locations where groundwater levels were not 

monitored (as of 2009).

Observation Network Analysis

To select locations to evaluate the worth of new potential 

water-level observations, a vector representation of model cells 

was used in tandem with measured groundwater levels used to 

calibrate the 1940–2009 model (Peterson and others, 2016a). 

First, the water levels used to calibrate the last year (May 1, 

2008, to April 30, 2009) of the Northern High Plains aquifer 

groundwater model were extracted from the dataset (fig. 8) 

because these water levels represent the most developed state of 

monitoring of the aquifer; the largest number of wells generally 

was near the end of the calibration period. These water-level 

locations were joined to the active cells of the groundwater 

model that were more than 5 miles (mi) inside the area boundary. Cells within 2 mi of existing water levels in 2009 were 

removed on the assumption that only cells that were more than 

2 mi from existing water levels would be considered as potentially valuable new observed water levels. This removal resulted 

in 107,205 cells where potential new observations could be considered, and each cell was assigned a random number between 0 

and 107,204. Experimentation indicated that selecting the highest 2,462 cells as potential sampling locations gave a distribution across the model domain yet was still practical to evaluate 

(fig. 9). Though the 2-mi distance was arbitrary, the effect of 

that selection was minimized by the process of culling potential 

locations using the random grid. Using a larger distance would 

have resulted in fewer potential locations subjected to culling 

but probably would not have affected the general results of the 

analysis.

The groundwater-level observation network was analyzed 

for these locations (fig. 9) using groundwater levels extracted 

from the five future forecasts simulated with the numerical 

model (referred to as baseline, A2LC, B2LC, GFDL, and MRI). 

The standard deviation of groundwater levels across all seasons 

of the five future forecasts was taken as an indication of larger 

variability in the future forecasts, similar to the approach of 

Masterson and others (2016), who used variability in future 

water levels as an indicator of sensitivity to climate inputs. For 

this study, variability in future forecast water levels was taken as 

an indicator of where additional data collection would be most 

valuable for improving the model calibration by either confirming and measuring the variability or refuting it and providing 

additional data to constrain the model calibration.

Groundwater Availability of the

Northern High Plains Aquifer

This section of the report describes the groundwater 

availability of the Northern High Plains aquifer, during predevelopment (defined as 1940–49) and postdevelopment (defined 

as 2000–9) conditions, as well as across five alternative future 

forecasts representing 2009–49. Groundwater availability was 

evaluated by comparing regional and subregional simulated 

groundwater budgets for the different periods. Simulated 

stream base flows at select USGS streamgages were compared between forecasts when they indicated results different 

from those indicated by water budgets of the containing river 

basin. This section of the report also describes an evaluation 

of potential monitoring improvements important for informing 

groundwater availability.

Predevelopment Hydrologic System

This section of the report describes the predevelopment 

hydrologic system in terms of the processing causing gains 

and losses to the groundwater system before major groundwater withdrawals for irrigation, frequently defined as around 

1940 or 1950, and defined in this report as 1940–49. This section also describes the corresponding 1940–49 simulated water 

budget components of the calibrated Northern High Plains 

aquifer model (Peterson and others, 2016a).

Description of the Predevelopment Hydrologic 

System

The predevelopment hydrologic system is defined herein 

to mean before major groundwater withdrawals for irrigation, 

frequently considered to be around 1940 (Weeks and others, 1988) or 1950 (Peterson, 2009). Groundwater had been 

developed primarily for irrigation on a local scale before 1950, 

mostly where depth to water was small, such as near Grand 

Island, Nebr. (figs. 1, 7C, 10). The largest predevelopment 

hydrologic processes included recharge from precipitation, 

groundwater discharge to streams, and groundwater discharge 

to evapotranspiration. Groundwater has been presumed to 

have discharged to (or been captured by) evapotranspiration 

only near major rivers and in other places where the depth to 

groundwater is less than several feet below land surface.

The earliest human-instigated changes to the hydrologic 

system were the diversion of streams and rivers to irrigate 

adjacent lands before the 20th century (State Board of Irrigation, 1899). At that time, surface-water irrigation was limited 

primarily to areas within 10 mi of major rivers and streams, 

starting with cooperation of neighbors and hand-dug ditches 

and later being developed at a larger scale in western Nebraska 

by the Bureau of Reclamation (Autobee, 1996) and in central 

Nebraska through the Public Works Administration (Central 

Nebraska Public Power and Irrigation District, 2018). The 
24 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

groundwater model Locations where groundwater levels measured from May 2008 to April 2009 were used to calibrate the Northern High Plains aquifer Figure 8.

(Peterson and others, 2016b).

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

EXPLANATION

Groundwater levels 

measured in 2008–9

Study area

High Plains aquifer

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD
Groundwater Availability of the Northern High Plains Aquifer 25 -level observations were subjected to network analysis to indicate the Locations across the Northern High Plains aquifer where potential new water Figure 9. usefulness for future groundwater availability monitoring.

SOUTH DAKOTA

WYOMING

IOWA

MISSOURI

MINNESOTA

NEBRASKA

COLORADO

KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

EXPLANATION

Locations for evaluation of 

potential new water-level observations

Study area

High Plains aquifer

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD
26 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

8,000,000

9,000,000

10,000,000

11,000,000

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

Irrigated land, in acres

Number of registered irrigation wells in Nebraska

Year

1930 1940 1950 1960 1970 1980 1990 2000 2010

EXPLANATION

Count of registered wells

Irrigated land

Figure 10. Nebraska registered irrigation wells and irrigated land for the Northern High Plains aquifer 

groundwater model, 1940–2009 (data from Nebraska Department of Natural Resources, 2015; Peterson and 

others, 2016b).

diversion, transmittal, and application of surface-water irrigation created large local changes to the groundwater system 

because of the influx of a new large amount of water and 

related seepage into the subsurface. Most surface-water irrigation projects were in place by 1955, covering 1.7 percent of 

the study area (also equivalent to 13 percent of irrigated land 

within the study area; Peterson and others, 2016a).

Irrigated agricultural land on the Northern High Plains 

aquifer expanded more than tenfold from only 857,000 acres 

in 1949 to 8,379,000 acres by 2008 (fig. 10; Peterson and 

others, 2016a). Most surface-water irrigation was substantially in place by 1955 and remained through 2008 (fig. 5). 

The remainder of irrigation expansion was through development of groundwater; for example, the cumulative number of 

irrigation wells registered overlying the High Plains aquifer in 

Nebraska increased from only a few thousand in 1940 to more 

than 70,000 in 1980 and 100,000 by 2009 (fig. 10; Nebraska 

Department of Natural Resources, 2015).

Predevelopment Water Budget

For the Northern High Plains aquifer, average 1940–49 

simulated gains (inflows) were predominantly from recharge 

from precipitation (93 percent), followed by a far smaller 

amount from recharge from canal seepage (7 percent; fig. 11A; 

table 3). Average 1940–49 simulated losses (outflows) were 

largest for changes in storage resulting in groundwater-level 

increases (45 percent), with 6.8 million acre-ft per year leaving the flow system and replenishing groundwater in storage 

(fig. 11A). It was not possible to assess the uncertainty associated with the net replenishment to storage for this period; 

there were not large outflows to a net replenishment to storage 

in any other parts of the simulated period (fig. 12), and data 

available to calibrate the 1940–49 part of the simulation were 

relatively sparse compared with later parts of the simulation, 

such as 2000–9 (Peterson and others, 2016a). The net replenishment to groundwater storage for this period correlates to 
Groundwater Availability of the Northern High Plains Aquifer 27

Saturated zone

Unsaturated zone

Saturated zone

Unsaturated zone

Surface water

Surface water

Impermeable rock layer

Impermeable rock layer

B. 2000–9

Recharge from precipitation

10.1 million acre-feet per year

Recharge from canal seepage

1.2 million acre-feet per year

Net release from storage

3.9 million acre-feet per year

Irrigation wells

7.5 million acre-feet per year

Evapotranspiration

3.5 million acre-feet per year

Stream base flow

3.9 million acre-feet per year

A. 1940–49

Recharge from precipitation

14.1 million acre-feet per year

Recharge from canal seepage

1.1 million acre-feet per year

Net replenishment to storage

6.8 million acre-feet per year

Irrigation wells

1.0 million acre-feet per year

Evapotranspiration

3.4 million acre-feet per year

Stream base flow

3.8 million acre-feet per year

Amount of storage in aquifer

2,456 million acre-feet

Amount of storage in aquifer

2,428 million acre-feet

Drains

0.2 million acre-feet per year

Municipal wells

47,000 acre-feet per year

Municipal wells

68,000 acre-feet per year

EXPLANATION

Inflows Outflows

EXPLANATION

Inflows Outflows

Drains

0.2 million acre-feet per year

Figure 11. Conceptual diagrams comparing average annual simulated water budget inflows and outflows by source and the amount of 

water in storage in the Northern High Plains aquifer. A, for 1940–49; B, for 2000–9; C, for five forecasts representing 2009–49.
28 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

Saturated zone

Unsaturated zone

Surface water

Impermeable rock layer

EXPLANATION

Inflows

Outflows

C. 2009–49 baseline and forecasts

Recharge from canal seepage 

1.1 million acre-feet per year

1.1 million acre-feet per year

1.1 million acre-feet per year

1.1 million acre-feet per year

1.1 million acre-feet per year

Recharge from precipitation

11.6 million acre-feet per year

11.6 million acre-feet per year

11.7 million acre-feet per year

10.2 million acre-feet per year

9.8 million acre-feet per year

Evapotranspiration

3.4 million acre-feet per year

3.3 million acre-feet per year

3.4 million acre-feet per year

3.3 million acre-feet per year

3.3 million acre-feet per year

Irrigation wells

6.7 million acre-feet per year

7.5 million acre-feet per year

7.2 million acre-feet per year

6.3 million acre-feet per year

6.4 million acre-feet per year

Drains

0.2 million acre-feet per year

0.2 million acre-feet per year

0.2 million acre-feet per year

0.2 million acre-feet per year

0.2 million acre-feet per year

Municipal wells

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Outflows to reservoirs

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Stream base flow

4.0 million acre-feet per year

3.8 million acre-feet per year

3.9 million acre-feet per year

3.6 million acre-feet per year

3.6 million acre-feet per year

1.6

2.1

1.9

2.5

2.0

A2LC forecast

Baseline

B2LC forecast

GFDL forecast

MRI forecast

Amount of storage in aquifer

2,372 million acre-feet

2,350 million acre-feet

2,359 million acre-feet

2,355 million acre-feet

2,335 million acre-feet

Inflows from west

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

Less than 0.1 million acre-feet per year

[A2LC, forecast using A2 land cover (25-percent increase in irrigated agriculture) and 2004 climate data;

B2LC, forecast using B2 land cover (12-percent increase in irrigated agriculture and 50-percent decrease in dryland agriculture) and 2004 climate data;

GFDL, forecast using 2009 land cover and Geophysical Fluid Dynamics Laboratory Earth System Model Second Generation downscaled daily outputs;

MRI, forecast using 2009 land cover and Japanese Meteorological Research Institute Coupled Global Climate Model Version 3 downscaled daily outputs]

Net release from storage

1.6 million acre-feet per year

2.2 million acre-feet per year

1.9 million acre-feet per year

2.0 million acre-feet per year

2.5 million acre-feet per year

Figure 11. Conceptual diagrams comparing average annual simulated water budget inflows and outflows by source and the amount of 

water in storage in the Northern High Plains aquifer. A, for 1940–49; B, for 2000–9; C, for five forecasts representing 2009–49.—Continued
Groundwater Availability of the Northern High Plains Aquifer 29

groundwater-level increases and probably occurred because of 

a combination of climatic conditions, as well as the introduction of substantial additional recharge from leakage because of 

canal systems that began operations around that time.

The next largest simulated loss was to streams (25 percent), followed by losses to evapotranspiration (22 percent; 

table 3). Losses to irrigation wells were only 7 percent of the 

total outflows and about the same magnitude as the gains of 

recharge from canal seepage. Other outflow components were 

much smaller, such as outflows to municipal wells, reservoirs, 

and drains (seeps and springs near the edges of the aquifer). 

The amount of groundwater in storage was about 2,456 million acre-ft (fig. 11A).

Average predevelopment (1940–49) simulated water budgets for the six major basins of the Northern High Plains aquifer generally followed the same distribution as the regional 

Northern High Plains aquifer (fig. 13A); however, in the Elkhorn, Platte, and Republican River Basins, evapotranspiration 

was larger than losses to streams (creating base flow), contrary 

to the whole. This difference was caused by the evapotranspiration rate in the Platte and Republican River Basins, which 

is larger in the southwest than in the rest of the area (fig. 3A). 

In addition, evapotranspiration was limited to groundwater 

within 7 ft of the land surface, and much of the Elkhorn River 

Basin has shallow depths to water, as does much of the Platte 

River Basin (fig. 7C).

In contrast, the Niobrara and Loup River Basins had 

smaller evapotranspiration and larger 1940–49 recharge than 

other areas, and together, gained nearly half of the recharge of 

the Northern High Plains aquifer (fig. 13A, table 3). Recharge 

is likely larger than in other areas because of cooler temperatures resulting in decreased loss to evapotranspiration, as 

well as sandy soils facilitating infiltration of precipitation to 

become groundwater recharge. The Niobrara and Loup River 

Basins also had larger simulated losses to streams than other 

areas, and together constituted more than half of the losses to 

Table 3. Summary of simulated average annual groundwater budgets for 1940–49 and 2000–9, simulated by the Northern High Plains 

aquifer groundwater-flow model in Colorado, Kansas, Nebraska, South Dakota, and Wyoming (Peterson and others, 2016a).

[Parenthetical entries are MODFLOW package codes from Niswonger and others (2011); <, less than; --, zero]

Water budget component 

(MODFLOW package 

in parentheses)

1940–49 2000–9

Quantity average 

(cubic feet 

per second)

Relative quantity 

(percentage of total 

budget)

Quantity average 

(cubic feet 

per second)

Relative quantity 

(percentage of total 

budget)

Inflows

Recharge (RCH) 20,960 100 15,638 74

–from canal seepage 1,489 7 1,641 8

–from precipitation 19,472 93 13,997 66

Groundwater inflows simulated as specified water levels 

(BAS)

47 <1 54 <1

Head dependent bounds representing reservoirs (GHB)17 <1 -- --

Change in storage: groundwaterlevel decreases (UPW)-- -- 5,366 25

Total inflows 21,025 100 21,058 100

Outflows

Change in storage: groundwaterlevel increases (UPW)9,371 45 -- --

Evapotranspiration (EVT) 4,646 22 4,873 23

Irrigation wells (WEL) 1,416 7 10,288 49

Drains (DRN) 254 1 301 1

Head dependent bounds representing reservoirs (GHB)-- -- 71 <1

Municipal wells (MNW2) 65 <1 95 <1

Stream base flow (SFR) 5,273 25 5,437 26

Total outflows 21,025 100 21,065 100
30 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

1900 1920 1940 1960 1980 2000 2020 2040

Year

−10

−8

−6

−4

−2

0

2

4

6

8

10

Net release from groundwater in storage, 

in millions of acre-feet per year

Net replenishment to groundwater in storage, 

in millions of acre-feet per year

EXPLANATION

1900–2009

Net replenishment to storage

Baseline forecast

A2LC

B2LC

GFDL

MRI

Net release from storage

2009–49 forecast trends

[A2LC, forecast using A2 land cover and 2004 climate data; 

B2LC, forecast using B2 land cover and 2004 climate data;

GFDL, forecast using 2009 land cover and Geophysical Fluid 

Dynamics Laboratory Earth Systems Model Second Generation 

downscaled daily outputs; MRI, forecast using 2009 land cover 

and Japanese Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Figure 12. Net release from and net replenishment to groundwater in storage of the Northern High Plains aquifer for 

1900–2009 and trends of change in storage for five forecasts representing 2009–49.
Groundwater Availability of the Northern High Plains Aquifer 31

SOUTH DAKOTA

WYOMING

IOWA

MINNESOTA

NEBRASKA

COLORADO

KANSAS

Base map from U.S. Geological Survey digital dataNorth American Datum of 1983

A. Inflows/outflows, 1940–9

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

TX NM

CO

KS

IA

NE

OK

WY

MO

AR

LA

SD

Study areaHigh Plains aquifer

96° 98° 100° 102° 104° 106°

42°40°

EXPLANATION

Dune sandPlatte River BasinRepublican River BasinNiobrara River BasinLoup River BasinBig Blue River BasinElkhorn River BasinInflows, in millions of 

acre-feet per year

Inflow from rechargeNet release from storageInflow from fixed water levelsInflow from other areas

Outflows, in millions of 

acre-feet per year

Outflow to stream base flowOutflow to wellsOutflow to evapotranspirationOutflow to drains

Net replenishment to storageOutflow to municipal withdrawalsOutflow to other areas

4,672 2,207 1,269

777

349

10 134 208

2,388

1,883

893

109 22

531

5,793

37

5,511 2,620 2,101

651

90 7

40

2,080

762

396

548

33

332

7 2

1,168 723

294

91

67

16

2

24

Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows

16

1,749

640

618 500

7

27

44

C, baseline , for 2000–9; B , for 1940–49; A ajor river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

 MRI forecast. , G

M

, A2LC forecast; E, B2LC forecast; F, GFDL forecast; 

Figure 13.

D forecast; 
32 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. Base map from U.S. Geological Survey digital data North American Datum of 1983 B. Inflows/outflows, 2000–9 SOUTH DAKOTA WYOMING IOWA MINNESOTA NEBRASKA COLORADO KANSAS 0 50 100 150 200 MILES 0 50 100 150 200 KILOMETERS 96° 98° 100° 102° 104° 106° 42° 40° EXPLANATION Dune sand Platte River Basin Republican River Basin Niobrara River Basin Loup River Basin Big Blue River Basin Elkhorn River Basin Inflows, in millions of acre-feet per year Inflow from recharge Net release from storage Inflow from fixed water levels Inflow from other areas Outflows, in millions of acre-feet per year Outflow to stream base flow Outflow to wells Outflow to evapotranspiration Outflow to drains Net replenishment to storage Outflow to municipal withdrawals Outflow to other areas 2,379 4,295 2,520 1,006 878 184 75 8 1,235 852 890 258 713 140 12 7 41 1,0794,175 1,701 2,442 2,242 2,903 2,103 473 205

7

3 12 19

126

6

27

1,303

1,708

806

245

868

43 163 75

37

285 448

973

680

1,788

12

Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows

12

C, baseline , for 2000–9; 

B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; 

E, B2LC forecast; 

F, GFDL forecast; 

Figure 13.

D forecast; 
Groundwater Availability of the Northern High Plains Aquifer 33 Base map from U.S. Geological Survey digital data North American Datum of 1983

C. Inflows/outflows, baseline

SOUTH DAKOTA

WYOMING

IOWA

MINNESOTA

NEBRASKA

COLORADO

KANSAS

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

96° 98° 100° 102° 104° 106°

42°40°

EXPLANATION

Dune sandPlatte River BasinRepublican River BasinNiobrara River BasinLoup River BasinBig Blue River BasinElkhorn River BasinInflows, in millions of 

acre-feet per year

Inflow from rechargeNet release from storageInflow from fixed water levelsInflow from other areas

Outflows, in millions of 

acre-feet per year

Outflow to stream base flowOutflow to wellsOutflow to evapotranspirationOutflow to drains

Net replenishment to storageOutflow to municipal withdrawalsOutflow to other areas

4,976 1,028

709

482

12

1,158

808

236

746

7

2,145

242

34

1,562 1,258

71

4,331

448

43

2,228

1,541

760

190 43

2, 134

34 58

2,760

961

997

168 81 8

8

2,048

213

117

19

24

2,193

451

208

6

2,416

379

149 15

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

10

C, baseline , for 2000–9; B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; E, B2LC forecast; F, GFDL forecast; 

Figure 13.

D forecast; 
34 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. Base map from U.S. Geological Survey digital data North American Datum of 1983 D. Inflows/outflows, A2LC forecast SOUTH DAKOTA WYOMING IOWA MINNESOTA NEBRASKA COLORADO KANSAS 0 50 100 150 200 MILES 0 50 100 150 200 KILOMETERS 96° 98° 100° 102° 104° 106° 42° 40° EXPLANATION [A2LC, forecast using A2 land cover (25-percent increase in irrigated agriculture) and 2004 climate data] Dune sand Platte River Basin Republican River Basin Niobrara River Basin Loup River Basin Big Blue River Basin Elkhorn River Basin Inflows, in millions of acre-feet per year Inflow from recharge Net release from storage Inflow from fixed water levels Inflow from other areas Outflows, in millions of acre-feet per year Outflow to stream base flow Outflow to wells Outflow to evapotranspiration Outflow to drains Net replenishment to storage Outflow to municipal withdrawals Outflow to other areas 4,965 1,152 801 235 787

7

2,162

379

26

1,568 1,524

69

4,329

508

43

2,319

1,525

736

197

10

43

2,152

37

228

2,734

952

1067

119 85 8

6

2,275

164

102

19

23

2,511

427

187

6

2,405

415

148 16

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

1,301

675

435

11

C, baseline , for 2000–9; 

B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; 

E, B2LC forecast; 

F, GFDL forecast; 

Figure 13.

D forecast; 
Groundwater Availability of the Northern High Plains Aquifer 35 Base map from U.S. Geological Survey digital data North American Datum of 1983

E. Inflows/outflows, B2LC forecast

SOUTH DAKOTA

WYOMING

IOWA

MINNESOTA

NEBRASKA

COLORADO

KANSAS

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

96° 98° 100° 102° 104° 106°

42°40°

EXPLANATION

Dune sandPlatte River BasinRepublican River BasinNiobrara River BasinLoup River BasinBig Blue River BasinElkhorn River BasinInflows, in millions of 

acre-feet per year

Inflow from rechargeNet release from storageInflow from fixed water levelsInflow from other areas

Outflows, in millions of 

acre-feet per year

Outflow to stream base flowOutflow to wellsOutflow to evapotranspirationOutflow to drains

Net replenishment to storageOutflow to municipal withdrawalsOutflow to other areas

[B2LC, forecast using B2 land cover 

(12-percent increase in irrigated agriculture and 

50-percent decrease in dryland agriculture) 

and 2004 climate data]

4,992 1,156

806

235

773

7

2,174

327

23

1,587 1,416

71

4,347

477

43

2,294

1,530

746

194

10

43

2,186

35

159

2,746

956

1054

145 82 8

7

2,208

182

109

19

23

2,408

437

191

6

2,422

393

148 16

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

1,233

687

453

12

C, baseline , for 2000–9; B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; E, B2LC forecast; F, GFDL forecast; 

Figure 13.

D forecast; 
36 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. Base map from U.S. Geological Survey digital data North American Datum of 1983 F. Inflows/outflows, GFDL forecast SOUTH DAKOTA WYOMING IOWA MINNESOTA NEBRASKA COLORADO KANSAS 0 50 100 150 200 MILES 0 50 100 150 200 KILOMETERS 96° 98° 100° 102° 104° 106° 42° 40° EXPLANATION Dune sand Platte River Basin Republican River Basin Niobrara River Basin Loup River Basin Big Blue River Basin Elkhorn River Basin Inflows, in millions of acre-feet per year Inflow from recharge Net release from storage Inflow from fixed water levels Inflow from other areas Outflows, in millions of acre-feet per year Outflow to stream base flow Outflow to wells Outflow to evapotranspiration Outflow to drains Net replenishment to storage Outflow to municipal withdrawals Outflow to other areas 4,119 209 1,137 797 230 651

6

1,776

222

11

1,348 1,447

70

4,137

562

42

2,158

1,548

748

186

11

43

36

2,043

2,392

849

1007

78 8

6

1,666

206

113

19

24

2,261

411

154

6

2,335

330

14514

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

871

695

498

12 4

[GFDL, forecast using 2009 land cover 

and Geophysical Fluid Dynamics Laboratory 

Earth System Model Second Generation 

downscaled daily outputs]

C, baseline , for 2000–9; 

B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; 

E, B2LC forecast; 

F, GFDL forecast; 

Figure 13.

D forecast; 
Groundwater Availability of the Northern High Plains Aquifer 37 Base map from U.S. Geological Survey digital data North American Datum of 1983

G. Inflows/outflows, MRI forecast

SOUTH DAKOTA

WYOMING

IOWA

MINNESOTA

NEBRASKA

COLORADO

KANSAS

0 50 100 150 200 MILES0 50 100 150 200 KILOMETERS

96° 98° 100° 102° 104° 106°

42°40°

EXPLANATION

Dune sandPlatte River BasinRepublican River BasinNiobrara River BasinLoup River BasinBig Blue River BasinElkhorn River BasinInflows, in millions of 

acre-feet per year

Inflow from rechargeNet release from storageInflow from fixed water levelsInflow from other areas

Outflows, in millions of 

acre-feet per year

Outflow to stream base flowOutflow to wellsOutflow to evapotranspirationOutflow to drains

Net replenishment to storageOutflow to municipal withdrawalsOutflow to other areas

4,015

333

1,179

823

239

634

7

1,693

346

17

1,152

1,638

69

4,137

562

42

2,183

1,496

708

181

11

43

37

2,049

30

2,394

853

1,013

84 8

6

1,740

180

111

19

22

2,304

387

133

6

2,382

347

141 15

Inflows Outflows Inflows Outflows Inflows Outflows

Inflows Outflows Inflows Outflows Inflows Outflows

887

720

500

12

[MRI, forecast using 2009 land cover 

and Japanese Meteorological Research Institute 

Coupled Global Climate Model Version 3 

downscaled daily outputs]

C, baseline , for 2000–9; 

B for 1940–49; , A Major river basins and corresponding simulated water budget inflows and outflows of the Northern High Plains aquifer. 

, MRI forecast.—Continued G , A2LC forecast; 

E, B2LC forecast; 

F, GFDL forecast; 

Figure 13.

D forecast; 
38 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

streams of the Northern High Plains aquifer. In these basins, 

losses to streams were also larger than to evapotranspiration. 

Average 1940–49 simulated losses to irrigation wells 

were largest in the Platte and Republican River Basins but 

were still a small part of total groundwater leaving the aquifer 

system (losses). Just as for the Northern High Plains aquifer 

(fig. 12), every basin had simulated net replenishment to storage, corresponding to groundwater-level rises. Groundwaterlevel rises were caused in part by canal systems that were 

emplaced or expanded from 1940 to 1949; but because gains 

from canal seepage recharge were only 7 percent of aquifer 

gains, the remainder of the difference must be attributed to 

simulated 1940–49 recharge from precipitation being larger 

than that of preceding periods. Also, basin to basin groundwater flows were not a large part of basin water budgets (fig. 13A, 

labeled “inflow from other areas”). This means that the dominant processes affecting water budgets happened within the 

basins, not as groundwater flows to or from other basins.

Postdevelopment Hydrologic System

This section of the report describes the postdevelopment hydrologic system in terms of processes causing gains 

and losses to the groundwater system during major groundwater withdrawals for irrigation (2000–9). This section also 

describes the corresponding 2000–9 simulated water budget 

components of the calibrated Northern High Plains aquifer 

model (Peterson and others, 2016a) and differences in 2000–9 

simulated groundwater budget components from 1940 to 1949, 

for the Northern High Plains aquifer and across the six major 

stream basins.

Postdevelopment Water Budgets and Changes 

from Predevelopment

Average postdevelopment (2000–9) gains were dominated by recharge from precipitation (fig. 11B; table 3) though 

recharge was much smaller than for 1940–49. The decrease 

is assumed to have been caused primarily by differences in 

climate, because other factors controlling recharge were the 

same. A decrease in recharge agrees with Stanton and others 

(2011), who estimated decreased recharge for 2000–9 as compared with 1940–49, though to a lesser extent than reported 

here. Recharge from canal seepage was about the same for 

2000–9 as for 1940–49, but the amount of land irrigated with 

surface water is nearly the same because most surface-water 

irrigation projects were developed by the 1940s.

The largest average 2000–9 outflow was to irrigation 

wells, followed by losses to streams and to evapotranspiration, and the largest single difference between 1940–49 and 

2000–9 was the large increase in losses to irrigation wells 

(fig. 11B; table 3). Because this is a closed system and closed 

water budget, increases in an outflow (or loss) must be balanced by either increases in gains or decreases to other losses. 

In this case, increased losses to irrigation wells were largely 

balanced by the eradication of outflows to a net replenishment 

of aquifer storage, and further by the induction of gains from 

a net release from storage (fig. 12). Gains from a net release 

from storage, corresponding to water-level declines, were consequently larger than those from 1940 to 1949 (0 acre-ft/ yr), 

as well as being more persistent than any decade from 1949 

to 1999, and were 25 percent of the 2000–9 gains, though 

these were not uniformly distributed across the Northern High 

Plains aquifer. Average 2000–9 groundwater in storage was 

2,428 million acre-ft, or about 28 million acre-ft less than for 

1940–49.

Differences among average 2000–9 simulated water budgets from basin to basin and from the Northern High Plains 

aquifer are driven by basin characteristics and corresponding 

hydrologic processes; for example, gains from recharge were 

the largest inflow at the aquifer scale, and for five of the six 

basins (but not the Republican River Basin; fig. 13B). Basins 

with the most gains from recharge (in descending order, Loup, 

Platte, and Niobrara River Basins) had the most simulated 

stream base flow. The Platte River Basin had the second 

most gain from recharge but the third most loss to streams, 

likely because losses to evapotranspiration in the Platte River 

Basin removed water that otherwise might have discharged 

to streams. Loss to evapotranspiration was the third largest 

aquifer-scale outflow, but was limited to areas of shallow 

water table, and was largest for the Platte River Basin. Readers 

are reminded that evapotranspiration can remove either soil 

moisture or groundwater, which exist in nature in a continuum in areas of shallow groundwater but are represented 

in this study by separate simulation models. Evapotranspiration results in this report are presented as simulated in the 

groundwater model and represent groundwater discharge. Soil 

moisture evapotranspiration was simulated with the SWB 

model (Peterson and others, 2020) and those discharges are 

not presented in this report.

The Republican River Basin 2000–9 gains from a net 

release from storage made up a larger part of the gains than for 

any other basin in the Northern High Plains and, at more than 

2,200 cubic feet per second (ft3/s), net release from storage 

also was larger than that basin’s 1940–49 average gain from 

recharge and was larger than the total groundwater losses of 

the Elkhorn River Basin for 2000–9 (fig. 13B). Further, this 

rate was nearly one-half of the net release from storage of the 

Northern High Plains aquifer (table 3). Groundwater levels 

declined across more of the Republican River Basin than in 

the rest of the Northern High Plains aquifer (McGuire, 2017), 

and simulated net release from storage corresponds to those 

declines. The Republican River Basin also had the largest 

loss to (irrigation) wells, followed by the Platte River, Big 

Blue River, and Elkhorn River Basins. Not surprisingly, the 

Big Blue River, Elkhorn River, Platte River, and Republican 

River Basins also contain the most irrigated land (fig. 5). The 

upper Republican River Basin in particular also has a large 

deficit between potential evapotranspiration and precipitation (fig. 4A). Though the average simulated 2000–9 losses 

to (irrigation) wells were the largest outflow of the Northern 
Groundwater Availability of the Northern High Plains Aquifer 39

High Plains aquifer, they were not the largest loss in the Loup 

and Niobrara River Basins, because those basins contain fewer 

irrigated acres than other areas (fig. 5) and have a smaller deficit between precipitation and potential evapotranspiration.

The largest 2000–9 loss of the Loup and Niobrara River 

Basins was to streams (creating base flow; fig. 13B). Loss to 

streams was the second largest outflow of the Northern High 

Plains aquifer (fig. 11B), most of which was in the Loup and 

Niobrara River Basins rather than being spread uniformly 

across the aquifer. Average 2000–9 loss to streams in the Loup 

River Basin was nearly one-half the total loss to streams for 

the Northern High Plains aquifer (table 3). The Republican 

River Basin had the least 2000–9 loss to streams.

Response of the Hydrologic System to Estimated

Future Conditions

As described in the “Future Forecasts” section, 2009–49 

groundwater model outputs were evaluated for five future 

forecasts using recharge and groundwater withdrawals for 

irrigation estimated with the SWB model. This section of the 

report describes the data used to forecast future conditions, as 

well as response of the simulated groundwater system to the 

five alternative forecasts.

Data Used to Forecast Future Conditions

Data input to the SWB models for various future (2009–

49) forecasts are described in this section. The inputs were 

selected to represent (1) a baseline forecast, with average 

climate and 2009 land cover; (2) two forecasts of alternate patterns of land cover representing different levels of expansion 

of groundwater irrigation; and (3) two forecasts representing 

alternative climate conditions, using climate data downscaled 

from a global climate model. As described in the “Future 

Forecasts” section, SWB outputs of recharge and groundwater withdrawals for irrigation were inputs to corresponding 

groundwater models, so changes to SWB inputs caused the 

differences in future forecasts.

Soil-Water-Balance Model

The Northern High Plains aquifer SWB model (Peterson 

and others, 2020) was used to estimate 2009–49 recharge 

from precipitation and groundwater withdrawals for irrigation 

for subsequent use in a corresponding groundwater model. 

Recharge from precipitation and groundwater withdrawals for irrigation were estimated for five 2009–49 forecasts: 

(1) a baseline forecast; (2) a land cover forecast referred 

to as A2LC; (3) a land cover forecast referred to as B2LC; 

(4) a climate forecast referred to as GFDL; and (5) a climate 

forecast referred to as MRI. The baseline forecast is meant 

to represent no change from the end of the calibration model 

(2009), the land cover forecasts were used to evaluate the 

effects of potential future land cover changes (in particular, 

changes in agricultural usage), and the climate forecasts were 

used to evaluate the effects of potentially different future 

climate. Readers are cautioned that there is no certainty that 

any of these future forecasts are necessarily a “true” future 

forecast, nor do they necessarily span the possible range of 

conditions in the future period. Rather, these forecasts are used 

to evaluate the sensitivity of the Northern High Plains aquifer, 

groundwater flows, and related stream base flows to potential 

changes in future conditions.

Baseline Soil-Water-Balance Model

For the baseline forecast, the SWB input land cover 

was held at the 2009 land cover used in Peterson and others (2016b). Input climate data consisting of precipitation, 

minimum temperature, and maximum temperature were taken 

from 2004 because precipitation from this year was nearly the 

same as the average of 1940–2009, based on an analysis of six 

weather stations from around the Northern High Plains aquifer 

(National Oceanic and Atmospheric Administration, 2010; 

fig. 1, table 4). Precipitation was considered the most important indicator of average climatic conditions because recharge 

and irrigation requirements are heavily dependent on the 

amount and timing of precipitation (Stanton and others, 2011). 

The average precipitation year (2004) was selected as the year 

from 2000 to 2009 that had annual precipitation closest to 

average annual 1940–2009 precipitation (table 4). The average difference between 2004 and average annual 1940–2009 

precipitation of all the stations was less than 0.1 in. for all 

six stations, though individual station data indicate that 2004 

was from about 2.6 in. wetter to about 3.1 in. drier than the 

1940–2009 average (for Broken Bow, Nebr., and Scottsbluff, 

Nebr., respectively).

The 2009 land cover dataset was used for the baseline 

forecast. Estimating 2009–49 recharge and irrigation rates 

under 2009 land cover conditions represented no change in 

irrigated land or dry cropland, and provided a static repeating 

condition against which to compare alternative forecasts.

Alternative Land Cover Soil-Water-Balance Models

SWB land cover forecasts used two alternative land 

cover datasets, referred to as A2 land cover and B2 land cover 

(Houston and others, 2013), with the same climate data used 

in the baseline SWB forecast (precipitation, minimum temperature, and maximum temperature were taken from 2004). 

The A2 forecast land cover changes can be summarized as a 

substantial increase in irrigated agriculture, from 8.4 million 

acres in 2009 to 10.3 million acres in 2049, primarily through 

increases in irrigated corn and irrigated soybeans (table 5). 

Dryland agriculture remained about the same throughout this 

time, though the mix changed slightly through decreases in 

dryland corn and soybeans and increases in dryland small 

grains. Grassland decreased by more than 3 million acres to 

accommodate the increase in irrigated agriculture. The B2 

forecast land cover changes can be summarized as a more 

moderate increase in irrigated agriculture (than that of A2), 
40 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. her stations in the Northern High Summary of 2000–9 average annual precipitation and difference from 1940 to 2009 average annual precipitation for selected weat Table 4. Plains aquifer (National Oceanic and Atmospheric Administration, 2010). ., Nebraska] [Nebr Weather station 1940–2009 average annual precipitation, in inches 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Annual precipitation, in inches Karl Stefan Memorial Airport, Norfolk, Nebr. 25.63 23.54 27.52 19.98 22.06 27.35 26.17 26.78 37.81 28.79 24.96 Broken Bow (2W), Nebr. 22.64 23.27 23.79 10.91 18.54 25.19 19.38 16.02 31.78 25.74 24.39 Miller Field, Valentine, Nebr. 18.82 19.48 20.56 11.22 15.76 17.28 26.25 14.61 25.14 20.74 21.79 W.B. Heilig Field Airport, Scottsbluff, Nebr. 15.18 14.79 13.02 7.77 10.46 12.04 19.64 12.03 9.36 14.27 19.42 McCook, Nebr. 21.40 19.63 19.27 10.87 22.34 23.2 23.12 24.91 28.61 15.34 28.30 Utica, Nebr. 27.55 24.43 24.29 22.94 25.99 25.83 25.36 29.33 35.66 40.40 26.02 Average 21.87 20.86 21.41 13.95 19.19 21.82 23.32 20.61 28.06 24.21 24.15 Difference from 1940 to 2009 average annual precipitation, in inches Karl Stefan Memorial Airport, Norfolk, Nebr. 25.63 −2.09 1.89 −5.65 −3.57 1.72 0.54 1.15 12.18 3.16 −0.67 Broken Bow (2W), Nebr. 22.64 0.63 1.15 −11.73 −4.10 2.55 −3.26 −6.62 9.14 3.10 1.75 Miller Field, Valentine, Nebr. 18.82 0.66 1.74 −7.60 −3.06 −1.54 7.43 −4.21 6.32 1.92 2.97 W.B. Heilig Field Airport, Scottsbluff, Nebr. 15.18 −0.39 −2.16 −7.41 −4.72 −3.14 4.46 −3.15 −5.82 −0.91 4.24 McCook, Nebr. 21.40 −1.77 −2.13 −10.53 0.94 1.80 1.72 3.51 7.21 −6.06 6.90 Utica, Nebr. 27.55 −3.12 −3.26 −4.61 −1.56 −1.72 −2.19 1.78 8.11 12.85 −1.53

Average 21.87 −1.01 −0.46 −7.92 −2.68 −0.05 1.45 −1.26 6.19 2.34 2.28
Groundwater Availability of the Northern High Plains Aquifer 41

from 8.4 to 9.5 million acres from 2009 to 2049, again primarily through increases in irrigated corn and soybeans. At the 

same time, B2 dryland agriculture decreased substantially, 

from 8.1 million acres in 2009 to 4.4 million acres in 2049, 

primarily through decreases to dryland corn, dryland small 

grains, and dryland soybeans, converted to grassland; therefore, grassland increased from 36.1 million acres in 2009 to 

39.3 million acres in 2049.

Because the estimated land cover data from Houston and 

others (2013) did not distinguish irrigated from nonirrigated 

agricultural land, a separate process was applied to differentiate these lands. First, all lands classified as irrigated in 2008 

from Peterson and others (2016b) were classified as irrigated 

in 2009, and any other agricultural lands were classified as 

nonirrigated. Second, various agencies such as the Nebraska 

Natural Resources Districts and the States of Colorado, South 

Dakota, Wyoming, and Kansas were contacted to determine 

if post-2009 expansion of irrigated lands would be allowed 

because some of these areas were subject to moratoriums or 

limits on additional irrigation wells or irrigated acres.

The Northern High Plains aquifer then was divided into 

94 zones, correlated to the maximum amount of additional 

irrigated agricultural development allowed each year from 

2009 to 2049. The A2 and B2 datasets for each year from 2009 

to 2049 were evaluated to determine if the legally allowed 

Table 5. Summary of land cover classification for the Northern High Plains aquifer Soil-Water-Balance (SWB) model for 2009 and 2049, 

for SWB models corresponding to the baseline, A2LC, and B2LC forecasts.

[A2LC, forecast using A2 land cover and 2004 climate data (purple in figures 12 and 14); B2LC, forecast using B2 land cover and 2004 climate data (green in 

figures 12 and 14)]

Land cover 

classification Forecast

Baseline, in millions of 

acres

A2LC, in millions of acres B2LC, in millions of acres

2008, 2009 2049 2009 2049 2009 2049

Irrigated Corn 5.2 5.2 5.2 6.5 5.2 5.9

Soybeans 1.8 1.8 1.8 2.3 1.8 2.2

Sorghum 0.1 0.1 0.1 0.1 0.1 0.1

Small grains 0.8 0.8 0.8 0.8 0.8 0.8

Alfalfa and hay 0.5 0.5 0.5 0.6 0.5 0.5

Dryland Corn 3.1 3.1 3.1 2.9 3.0 1.2

Soybeans 1.2 1.2 1.2 1.0 1.2 0.7

Sorghum 0.5 0.5 0.5 0.5 0.5 0.4

Small grains 3.0 3.0 3.0 3.5 3.0 1.7

Alfalfa and hay 0.4 0.4 0.4 0.4 0.4 0.3

Nonagricultural Developed 2.2 2.2 2.2 2.9 2.2 2.4

Grassland 36.0 36.0 36.0 32.9 36.1 39.3

Other 4.7 4.7 4.7 5.1 4.7 4.0

Total agricultural, irrigated 8.4 8.4 8.4 10.3 8.4 9.5

Total agricultural, dryland 8.2 8.2 8.3 8.3 8.1 4.4

Total other land cover 42.9 42.9 42.8 40.9 43.0 45.7

irrigated agricultural areas were larger than those of A2 or B2, 

and if so, additional areas were reclassified as irrigated. The 

location of the reclassification was selected as the areas closest 

to those irrigated in 2008 on the assumption that areas closest 

to existing irrigation were the most likely to be developed. In 

some cases, legal restrictions on irrigated acres were meant to 

reduce or limit the number of irrigated acres in future years, 

and in these or any other cases where the A2 or B2 irrigated 

areas were larger than the legally allowed area, irrigated areas 

were reclassified as nonirrigated, again preferentially preserving the irrigated areas that were closest to 2008 irrigation.

Lastly, the A2 and B2 land cover data from Houston 

and others (2013) were generated at a different time than the 

1950–2008 land cover data from Peterson and others (2016b), 

and the estimation method was slightly different. The different estimation methods resulted in an artificial mismatch 

between classifications for 2008 and 2009. The land cover data 

from Peterson and others (2016b) more closely resembled the 

legally allowed irrigated areas, and the transition to the less 

constrained 2009–49 data (Houston and others, 2013) created 

an abrupt and unrealistic shift in the land cover classification 

between 2008 and 2009. Therefore, the trends of annual land 

cover changes were extracted from the 2009–49 data described 

in the previous paragraph and applied successively to the 2008 

land cover dataset (Peterson and others, 2016b), generating 
42 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

a new 2009–49 land cover dataset and eliminating the abrupt 

shift. The trends were applied to agricultural classifications 

and developed (or urban) parcels. Grassland is the dominant 

land cover of the Northern High Plains aquifer (table 5); 

therefore, proximity to existing classification was used to preferentially place “new” land covers on locations classified as 

grassland, closest to the previously existing land cover of the 

same type, combined with a random factor to prevent spatial 

bias when more than one location was equally close. When 

a particular land cover decreased, that cell was converted to 

grassland. The land cover data processed as described in the 

preceding paragraphs were used as the SWB inputs for the 

groundwater model forecasts referred to as A2LC and B2LC 

(Peterson and others, 2020).

Alternative Climate Soil-Water-Balance Models

Climate forecasts used two alternative sets of climate 

data with the same land cover as the baseline SWB forecast 

(the 2009 land cover used in Peterson and others [2016b]). 

Alternative climate data included precipitation and minimum 

and maximum temperature generated by two downscaled 

Earth System Models from the World Climate Research 

Programme’s Coupled Model Intercomparison Project phase 5 

(CMIP5) multimodel ensemble that couples global atmospheric circulation and oceanic circulation models (Taylor 

and others, 2012). The CMIP5 multimodel ensemble used 

four emissions scenarios called Representative Concentration Pathways (RCPs), named after their respective radiative forcing target levels for 2100 (as Watts per square meter 

[Watts/m2

]). The four scenarios are RCP2.6, RCP4.5, RCP6.0, 

and RCP8.5, each with an increased radiative forcing of 2.6, 

4.5, 6.0, and 8.5 Watts/m2, respectively. The RCPs are used 

to predict future climate under different greenhouse gas and 

air pollution emissions and land cover conditions. RCP2.6 

simulates a peak in human emissions of greenhouse gases 

and decline by 2030 with the least amount of warming over 

the next century, whereas RCP8.5 predicts a steep increase in 

greenhouse gas emissions and, subsequently, the most extreme 

amount of warming over the next century. The Earth System 

Modelling (ESM) community of climate modeling teams from 

many agencies around the world downscale the CMIP5 multimodel ensemble General Circulation Model (GCM) outputs 

using two downscaling techniques: monthly bias correction 

with spatial disaggregation and daily bias correction with 

constructed analogs. Different downscaling methods for a specific RCP produce gridded climate projections of a finer grid 

resolution than the GCMs but tend to produce a wide range of 

results/projections. Based on this information, this study used 

two daily bias corrections with constructed analogs downscaled climate projections from the RCP6.0 scenario because 

it forecasts moderate warming and was not an extreme case. 

It was considered that using different downscaling versions of 

the same RCP scenario might provide insight on how forecasts 

of future groundwater availability might be affected by different GCM downscaling methods.

Climate forecast 1 used the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory Earth System Model Second Generation downscaled daily 

outputs (referred to herein as the “GFDL forecast”; Bureau 

of Reclamation, 2013). Climate forecast 2 used the Japanese 

Meteorological Research Institute Coupled Global Climate 

Model Version 3 downscaled daily outputs (referred to herein 

as the “MRI forecast”; Bureau of Reclamation, 2013). The 

GFDL and MRI datasets were used because their June-JulyAugust (approximate growing season) average temperature 

and precipitation had the lowest average bias compared to 

observed 1979–2005 temperature and precipitation in central 

North America (Sheffield and others, 2013).

Average 2009–49 climate data for the GFDL forecast 

were slightly wetter and warmer than 2000–9 (table 6). The 

2009–49 MRI forecast was slightly wetter and cooler than 

2000–9. GFDL 2009–49 average precipitation was 0.04 in/

yr higher than the baseline, whereas MRI 2009–49 average 

precipitation was about 0.04 in/yr lower than the baseline. 

However, the timing of the precipitation also is important 

because precipitation that falls during the growing season 

(herein defined as May 1 through September 30) can be used 

consumptively by evapotranspiration, whereas precipitation 

that falls during the nongrowing season has a better chance of 

becoming recharge to the aquifer because, for most areas of 

the Northern High Plains aquifer, potential evapotranspiration 

is larger than precipitation (fig. 4B, C). GFDL average growing season precipitation was 0.79 in. larger than that of the 

baseline, meaning that for the GFDL data, 0.79 in. of precipitation was in effect retimed from the nongrowing season to the 

growing season as compared with the baseline (table 6). The 

MRI 2009–49 precipitation also included a redistribution of 

about 0.4 in. of precipitation from the nongrowing season into 

the growing season as compared with the baseline.

Increases in temperature also directly increase the rate 

of evapotranspiration (Allen and others, 1998). The GFDL 

2009–49 average daily temperature was about 1 °F higher 

than for the baseline (2009–49), whereas MRI 2009–49 average daily temperature was 0.16 °F lower than for the baseline (table 6). As with precipitation, the time of year of the 

temperature changes also matters. Maximum growing season 

daily temperatures for 2009–49 were 3.13 °F higher than the 

baseline for GFDL and 2.09 °F higher than the baseline for 

MRI. For GFDL, maximum nongrowing season daily temperatures for 2009–49 were 3.29 °F higher than the baseline, and 

for MRI, were 1.21 °F higher than the baseline. The average 

daily temperature of MRI is lower than the baseline and indicated the bias of that dataset towards lower temperatures, even 

though the average of the MRI minimum and maximum daily 

temperatures is larger than the baseline. The GFDL and MRI 

forecasts also include a warming trend from 2009 to 2049 that 

is not in the baseline temperature data taken from 2004 (used 

for 2009–49).
Groundwater Availability of the Northern High Plains Aquifer 43

Groundwater-Flow Model

The groundwater-flow model ending April 30, 2009 

(Peterson and others, 2016b), was used as the initial model 

for the work described in this report. For the future forecasts, 

the model was revised to simulate May 1, 2009, to December 

31, 2049, and is referred to hereafter as the “forecast model.” 

The “forecast model” used the same model grid, configuration, 

and all calibrated model inputs as the initial model, except 

recharge and groundwater withdrawals for irrigation, for 

forecasts described later in this section. The seasonal temporal 

discretization was maintained, thereby each year of the “forecast model” also used an irrigation season from May 1 to September 30 each year as one stress period and a nonirrigation 

stress period from October 1 through April 30 of the following 

year. The only exception was that the last nonirrigation season, 

for 2049, was truncated to end on December 31 for simplicity 

in reporting, rather than covering the first 4 months of 2050. 

This approach to temporal discretization resulted in 82 stress 

periods representing May 2009 through December 2049.

For 2009–49, recharge and groundwater withdrawals for 

irrigation were estimated with the SWB model as described in 

the “Soil-Water-Balance Model” section, for forecasts referred 

to as the baseline, A2LC, B2LC, GFDL, and MRI. For each 

forecast, recharge and groundwater withdrawals for irrigation 

estimated with SWB were input to the 2009–49 groundwater 

model. Recharge was modified using calibration factors and 

addends after Peterson and others (2016a) and was summed 

with the canal seepage recharge from 2004 (identified as 

an “average year” as described in the “Soil-Water-Balance 

Model” section). Municipal groundwater withdrawals were 

similarly set at the 2004 rates. Only one instream reservoir 

(Lake McConaughy; fig. 1) was simulated directly in the 

1940–2009 groundwater model, and reservoir releases for the 

forecast period (2009–49) were held at the 2004 rates documented in Peterson and others (2016b).

Primary model outputs evaluated across forecasts 

included aquifer-scale simulated water budgets, and basinscale simulated water budgets, for the six major stream basins 

of the Northern High Plains aquifer: the Niobrara River Basin, 

Table 6. Summary of downscaled global climate model precipitation and temperature data used as inputs for the Northern High Plains 

aquifer Soil-Water-Balance model for 2009–49 for the baseline, GFDL, and MRI forecasts, compared with 2000–9 average measured 

data, and summary of corresponding output evapotranspiration, recharge, and irrigation requirements.

[GFDL, forecast using 2009 land cover and Geophysical Fluid Dynamics Laboratory Earth System Model Second Generation downscaled daily outputs (blue in 

figures 12 and 14); MRI, forecast using 2009 land cover and Japanese Meteorological Research Institute Coupled Global Climate Model Version 3 downscaled 

daily outputs (red in figures 12 and 14); in/yr, inch per year; in/season, inch per season; °F, degrees Fahrenheit]

Input/output

Measured

Forecast

Baseline GFDL MRI

2000–9 2009–49 2009–49 2009–49

Soil-Water-Balance model inputs

Average precipitation, in/yr 21.10 21.28 21.32 21.24

Average growing season 

precipitation, in/season

13.07 14.13 14.92 14.53

Average nongrowing season 

precipitation, in/season

8.06 7.15 6.40 6.71

Minimum growing season daily temperature, (°F)54.45 53.19 54.80 54.40

Minimum nongrowing season daily 

temperature, (°F)

24.11 23.25 24.91 23.98

Average daily temperature, (°F) 50.40 50.32 51.34 50.16

Maximum growing season daily temperature, (°F)82.15 80.52 83.65 82.61

Maximum nongrowing season daily 

temperature, (°F)

50.78 48.92 52.21 50.13

Soil-Water-Balance model outputs

Average evapotranspiration, in/yr 18.50 19.69 20.07 20.35

Average recharge, in/yr 3.50 2.80 2.43 2.18

Average irrigation requirement, in/yr 12.07 10.90 10.13 10.48
44 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

the Loup River Basin, the Elkhorn River Basin, the Platte 

River Basin, the Republican River Basin, and the Big Blue 

River Basin (fig. 1). River basin extents are mostly constant 

over time and are expected to be more familiar to readers of 

this report and, therefore, were used as boundaries to summarize model results for the same six major basins of the Northern High Plains aquifer. River basins also were considered 

more useful for summarizing than groundwater basins because 

the latter are less familiar or perhaps unknown to some readers; further, groundwater basins can expand, contract, or 

change shape, depending on the balance and distribution of 

inflows and outflows (also referred to as gains and losses), 

such as recharge and groundwater losses to streams or wells.

Results for the baseline forecast were characterized first 

by describing the water budget of the Northern High Plains 

area averaged across the forecast period (2009–49) and 

comparing that budget to what was simulated for the postdevelopment (2000–9) aquifer-scale simulated water budgets 

to examine the effects of conditions different from 2000–9. 

Basin-scale simulated water budgets were compared to the 

aquifer-scale simulated water budgets and against other basins 

to examine basins that differ from the aquifer-scale conditions, 

or basins that contributed a predominant part of aquifer-scale 

flows of a certain term. Subsequent forecasts were compared 

against the baseline forecast to isolate the effects of specific 

anthropogenic or climate data input changes on the groundwater system.

Streamflows are a critical indicator of groundwater availability because shortages in streamflow can trigger conjunctive (surface-water and groundwater) management actions 

(Nebraska Department of Natural Resources, 2004). Simulated 

stream base flow passing the downstream-most streamgage 

was evaluated for five of the six major streams of the Northern 

High Plains aquifer: the Niobrara River near Norden, Nebr. 

(USGS streamgage 06462000); the Loup River at Columbus, 

Nebr. (USGS streamgage 06794500); the Elkhorn River at 

Waterloo, Nebr. (USGS streamgage 06800500); the Republican River near Hardy, Nebr. (USGS streamgage 06853500); 

and the Big Blue River near Crete, Nebr. (USGS 

streamgage 06881000) (figs. 1, 14A–E). Simulated base 

flows at the downstream end of the major basins represent an 

integrated measure of all the upstream simulated flows and 

are another demonstration of the effects of basin- and aquiferscale simulated flows. Simulated average 2040–49 stream 

base flows for each forecast were compared to the average 

2000–9 simulated base flows (fig. 14A–E; Peterson and others, 

2016a), and against 2040–49 baseline forecast base flows, to 

indicate the effect of the alternate conditions represented in 

the forecast on the stream. Simulated 2040–49 base flows at 

the streamgages could be different from the average forecast 

Simulated stream base flow, in cubic feet per second

Niobrara River near Norden, Nebraska (06462000)

0

100

200

300

400

500

600

700

800

900

1,000

A

[A2LC, forecast using A2 land cover and 2004 

climate data; B2LC, forecast using B2 land cover 

and 2004 climate data; GFDL, forecast using 2009 

land cover and Geophysical Fluid Dynamics 

Laboratory Earth System Model Second 

Generation downscaled daily outputs; MRI, 

forecast using 2009 land cover and Japanese 

Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Average 2000–9

Average 2040–49, for the forecast

A2LC

Baseline

B2LC

GFDL

MRI

EXPLANATION

Figure 14. Average simulated 2000–9 base flow (Peterson and others, 2016b), and average 2040–49 simulated base flow, for the 

baseline, GFDL, MRI, A2LC, and B2LC forecasts. The baseline used 2004 climate data and 2009 land cover inputs, and the A2LC 

and B2LC forecasts used A2 land cover and B2 land cover data (modified from Houston and others, 2013). A, Niobrara River near 

Norden, Nebraska (06462000); B, Loup River at Columbus, Nebraska (06794500); C, Elkhorn River at Waterloo, Nebraska (06800500); 

D, Republican River near Hardy, Nebraska (06853500); E, Big Blue River near Crete, Nebraska (06881000).
Groundwater Availability of the Northern High Plains Aquifer 45

B

0

500

1,000

1,500

2,000

2,500

3,000

Loup River at Columbus, Nebraska (06794500)

Simulated stream base flow, in cubic feet per second

[A2LC, forecast using A2 land cover and 2004 

climate data; B2LC, forecast using B2 land cover 

and 2004 climate data; GFDL, forecast using 2009 

land cover and Geophysical Fluid Dynamics 

Laboratory Earth System Model Second 

Generation downscaled daily outputs; MRI, 

forecast using 2009 land cover and Japanese 

Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Average 2000–9

Average 2040–49, for the forecast

A2LC

Baseline

B2LC

GFDL

MRI

EXPLANATION

C

0

100

200

300

400

500

600

700

Elkhorn River at Waterloo, Nebraska (06800500)

Simulated stream base flow, in cubic feet per second

[A2LC, forecast using A2 land cover and 2004 

climate data; B2LC, forecast using B2 land cover 

and 2004 climate data; GFDL, forecast using 2009 

land cover and Geophysical Fluid Dynamics 

Laboratory Earth System Model Second 

Generation downscaled daily outputs; MRI, 

forecast using 2009 land cover and Japanese 

Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Average 2000–9

Average 2040–49, for the forecast

A2LC

Baseline

B2LC

GFDL

MRI

EXPLANATION

Figure 14. Average simulated 2000–9 base flow (Peterson and others, 2016b), and average 2040–49 simulated base flow, for the 

baseline, GFDL, MRI, A2LC, and B2LC forecasts. The baseline used 2004 climate data and 2009 land cover inputs, and the A2LC 

and B2LC forecasts used A2 land cover and B2 land cover data (modified from Houston and others, 2013). A, Niobrara River near 

Norden, Nebraska (06462000); B, Loup River at Columbus, Nebraska (06794500); C, Elkhorn River at Waterloo, Nebraska (06800500); 

D, Republican River near Hardy, Nebraska (06853500); E, Big Blue River near Crete, Nebraska (06881000).—Continued
46 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

Republican River near Hardy, Nebraska (06853500)

Simulated stream base flow, in cubic feet per second

D

[A2LC, forecast using A2 land cover and 2004 

climate data; B2LC, forecast using B2 land cover 

and 2004 climate data; GFDL, forecast using 2009 

land cover and Geophysical Fluid Dynamics 

Laboratory Earth System Model Second 

Generation downscaled daily outputs; MRI, 

forecast using 2009 land cover and Japanese 

Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Average 2000–9

Average 2040–49, for the forecast

A2LC

Baseline

B2LC

GFDL

MRI

EXPLANATION

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

Big Blue River near Crete, Nebraska (06881000)

Simulated stream base flow, in cubic feet per second

E

[A2LC, forecast using A2 land cover and 2004 

climate data; B2LC, forecast using B2 land cover 

and 2004 climate data; GFDL, forecast using 2009 

land cover and Geophysical Fluid Dynamics 

Laboratory Earth System Model Second 

Generation downscaled daily outputs; MRI, 

forecast using 2009 land cover and Japanese 

Meteorological Research Institute Coupled Global 

Climate Model Version 3 downscaled daily outputs]

Average 2000–9

Average 2040–49, for the forecast

A2LC

Baseline

B2LC

GFDL

MRI

EXPLANATION

Figure 14. Average simulated 2000–9 base flow (Peterson and others, 2016b), and average 2040–49 simulated base flow, for the 

baseline, GFDL, MRI, A2LC, and B2LC forecasts. The baseline used 2004 climate data and 2009 land cover inputs, and the A2LC 

and B2LC forecasts used A2 land cover and B2 land cover data (modified from Houston and others, 2013). A, Niobrara River near 

Norden, Nebraska (06462000); B, Loup River at Columbus, Nebraska (06794500); C, Elkhorn River at Waterloo, Nebraska (06800500); 

D, Republican River near Hardy, Nebraska (06853500); E, Big Blue River near Crete, Nebraska (06881000).—Continued
Groundwater Availability of the Northern High Plains Aquifer 47

period (2009–49) base flows of the basin because of the difference in the averaged period, or if there were streams within 

the basin that were not tributaries to the stream being measured at the streamgage. Stream base flow was not evaluated 

for the Platte River Basin because of the effect of 2004 Lake 

McConaughy (fig. 1) reservoir release rates on the Platte River 

simulated stream base flow.

Readers are cautioned that forecast base flows and 

groundwater budgets included in these analyses are not 

comprehensive and may not be the average nor span the range 

of effects caused by potential future climate or land cover 

changes. These forecasts are provided as examples of how 

the groundwater-flow model can be used as a tool to evaluate aquifer and stream base flow responses to potential future 

conditions, such as different land cover or different climate. 

Forecast models generated during this study are available as a 

USGS data release (Peterson and others, 2020). Altering input 

data, such as selecting a different year from 2004 to provide 

average climate data inputs for three of the forecasts, likely 

would generate different results.

Baseline Forecast

Differences in simulated groundwater-flow terms 

between the baseline forecast and 2000–9 simulated conditions are directly attributable to climate because that was the 

only different input. Average baseline forecast flows among 

components were similar to that of 2000–9 (fig. 11B, C); 

the largest gains were of recharge from precipitation and the 

largest losses (or outflows) were to irrigation wells. However, 

the balance of the flows was different. For instance, baseline 

forecast gains of recharge from precipitation were, on average, 

about 1.4 million acre-feet per year (acre-ft/yr) larger than for 

2000–9, and losses to irrigation wells were 0.8 million acre-ft/

yr smaller. As a result, baseline groundwater-level declines 

(gains from net release from storage) were 2.3 million acre-ft/

yr smaller than for 2000–9. In other words, groundwater 

levels declined more slowly for the baseline forecast than 

during 2000–9. This is because the average precipitation used 

in the baseline forecast was larger than actual precipitation 

from 2000 to 2009, leading to the increased recharge from 

precipitation and decreased losses to irrigation wells; less 

water needed to be withdrawn to meet crop needs. However, 

the baseline forecast net release from storage still indicates 

average declining groundwater levels of the aquifer at 1.6 million acre-ft/yr; hence, the amount of water in storage at the 

end of the baseline forecast (2049) was 56 million acre-ft 

less than the 2000–9 average. In addition, the rate at which 

groundwater was released from storage declined from 2009 to 

2049 (fig. 12), indicating that the aquifer was moving toward 

equilibrium with the stresses imposed in the baseline forecast.

Baseline forecast simulated gains and losses were not 

distributed uniformly among basins of the Northern High 

Plains aquifer but were distributed proportionately in the same 

way as for 2000–9 (fig. 13B, C). For example, for 2000–9 and 

the baseline forecast, recharge from precipitation was largest 

in the Loup and Platte River Basins and smallest for the 

Republican River Basin. Baseline forecast recharge was larger 

for the aquifer and in every basin than it was for 2000–9, 

correlating with the average climate inputs that represented 

wetter conditions than 2000–9. Wetter conditions also resulted 

in baseline forecast losses to irrigation wells being smaller 

than for 2000–9 for five of the six basins, other than the 

Elkhorn River Basin. Because gains from recharge increased 

but losses to irrigation wells also increased, more precipitation fell in the Elkhorn River Basin during the nongrowing 

season in the baseline forecast than for 2000–9. The increase 

in Elkhorn River Basin baseline forecast recharge above the 

amount gained in 2000–9 was more than sufficient to balance the increase in losses to wells, and also to increase base 

flow slightly, and to reduce the rate of water-level decline 

(net release from storage) from 285 to 58 ft3/s. Gains from a 

net release from storage were largest in the Republican River 

Basin, which is the same as for 2000–9, followed by the Platte 

River Basin and Big Blue River Basin, corresponding to the 

areas where the deficit between precipitation and potential 

evapotranspiration is largest (fig. 4A).

Also, though Northern High Plains aquifer losses to 

streams were the same for 2000–9 and the baseline forecast, 

the contributions of each basin were different (figs. 13B–C

and 14A–E); for example, baseline forecast losses to streams 

in the Loup River Basin were 240 ft3/s larger than for 2000–9 

but were smaller in the Platte River, Niobrara River, and Big 

Blue River Basins. Baseline forecast losses to streams were 

only slightly larger than for 2000–9 for the Elkhorn River 

Basin and were about the same for the Republican River 

Basin. Simulated base flow declined the most (relative to 

2000–9) for the Big Blue River near Crete, Nebr. (USGS 

streamgage 06881000), where the baseline forecast 2040–49 

stream base flow was about 30 percent of the 2000–9 average 

(fig. 14E). However, the Big Blue River also had the smallest average 2000–9 simulated base flow as compared with the 

other major rivers of the Northern High Plains, and declines in 

the Big Blue River, driven by a dry climate in 2004 near Utica 

(table 4), were balanced by increases in other Big Blue River 

Basin streams (such as the Little Blue River and Big Sandy 

Creek). Therefore, the Big Blue River Basin baseline forecast 

stream base flow was only 32 ft3/s smaller than that of 2000–9.

Human Stresses Forecast

The effects of potential human stresses on the Northern 

High Plains aquifer were evaluated through this study using 

alternative potential land cover maps A2 and B2; corresponding groundwater model forecasts are referred to herein as 

A2LC and B2LC. As described in more detail in the “Future 

Forecasts” section, the land cover for the A2LC forecast can 

be summarized as a substantial (23 percent) increase in irrigated agriculture, from 8.4 million acres in 2009 to 10.3 million acres in 2049, primarily through increases in irrigated 

corn and irrigated soybeans (table 5). Dryland agriculture 

remained about the same throughout this time, and grassland 
48 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

decreased by more than 3 million acres to accommodate the 

increase in irrigated agriculture. The land cover for the B2LC 

forecast can be summarized as a more moderate (13 percent) 

increase in irrigated agriculture, from 8.4 to 9.5 million acres 

from 2009 to 2049, again primarily through increases in 

irrigated corn and soybeans, accompanied by a substantial 

decrease in dryland agriculture, from 8.1 million acres in 2009 

to 4.4 million acres in 2049 (table 5). The A2LC and B2LC 

forecasts used the same measured 2004 climate data as the 

baseline forecast.

To isolate the effect of only the land use changes on aquifer gains and losses and eliminate those caused by climate, 

A2LC and B2LC water budgets were compared against the 

baseline forecast (fig. 13C–E; table 7). The main differences of 

the A2LC forecast compared with the baseline were for losses 

to irrigation wells that increased 0.7 million acre-ft/yr (11 percent) and respondent gains from a net release from storage that 

increased 0.5 million acre-ft/yr (32 percent). The increased 

irrigation well outflow was necessary to accommodate the 

expanded irrigated area of 1.9 million acres for A2LC. This 

was a little more moderate for the more limited expansion represented in the B2LC forecast, having losses to irrigation wells 

that were only 0.5 million acre-ft/yr (8 percent) larger and 

gains from a net release from storage that were only 0.3 million acre-ft/yr (19 percent) larger. However, increases in the 

net release from storage correlate with groundwater-level 

declines, and groundwater levels declined more quickly for 

the A2LC and B2LC forecasts than for the baseline forecast. 

The trend of net release from storage was nearly the same 

from 2009 to 2049 (fig. 12), which indicates that the aquifer 

flows were not in balance with the stresses imposed under the 

A2LC and B2LC forecasts. A2LC and B2LC losses to stream 

base flow, and losses to evapotranspiration, were 0.1 million 

acre-ft/yr smaller than for the baseline forecast. To restate, for 

A2LC, a 0.7 million acre-ft/yr increase in losses to irrigation 

wells above that of the baseline forecast was balanced by an 

increase in the net release from storage of 0.5 million acre-ft/

yr and decreases of 0.1 million acre-ft/yr each in evapotranspiration and stream base flow. As a result of these changes, 

the amount of groundwater in storage at the end of the A2LC 

forecast was 22 million acre-ft (0.9 percent) less than that of 

the baseline forecast. B2LC forecast declines were smaller, 

and the B2LC forecast had 13 million acre-ft (0.5 percent) less 

groundwater in storage than that of the baseline forecast.

The proportion of stresses in individual basins for the 

A2LC and B2LC forecasts was about the same as in the baseline forecast. In every basin, B2LC losses to irrigation wells 

were larger than for the baseline forecast, and A2LC losses to 

irrigation wells were more than for B2LC (fig. 13C–E). Basins 

with the most irrigated lands (fig. 5) had the largest increases 

in losses to irrigation wells relative to the baseline forecast; 

in descending order, the Republican River Basin, the Elkhorn 

River Basin, and the Big Blue River Basin. These same basins, 

in the same order, had the largest corresponding increase in 

gains from a net release from storage corresponding to water 

level declines.

Differences in the A2LC base flow for the Big Blue River 

near Crete, Nebr. (USGS streamgage 06881000; fig. 14E), 

from the aquifer loss to streams of the Big Blue River Basin, 

indicated how much nontributary streamflow was in the basin. 

Specifically, A2LC base flow for the Big Blue River near 

Crete was only about 4 ft3/s and was 32 ft3/s smaller than that 

of the baseline forecast; however, A2LC loss to streams in the 

Big Blue River Basin was 164 ft3/s (fig. 13D), about 49 ft3/s 

smaller than the baseline. Therefore, most of the decrease in 

base flow in the Big Blue River Basin was for the Big Blue 

River upstream from Crete, Nebr., and other streams in the 

basin (the Little Blue River and Big Sandy Creek) had A2LC 

base flows that were more similar to those of the baseline 

forecast. Further, the precipitation near Utica, Nebr. (in the 

Big Blue River Basin), was a more important factor causing base flow declines for the Big Blue River upstream from 

Crete, Nebr., but the precipitation was not less everywhere 

in the Big Blue River Basin. In contrast, simulated 2000–9 

base flow for the Republican River near Hardy, Nebr. (USGS 

streamgage 06853500; fig. 14D), was about the same size as 

the Big Blue River but declined less in the baseline, A2LC, 

and B2LC forecasts, probably because the 2004 climate used 

for all three forecasts was closer to average across the Republican River Basin.

Environmental Stresses Forecast

The effects of potential environmental stresses on the 

Northern High Plains aquifer were evaluated through this 

study using alternative future climate data that are referred 

to herein as GFDL and MRI. As described in more detail in 

the “Data Used to Forecast Future Conditions” section, the 

average 2009–49 GFDL climate data can be summarized 

as warmer and slightly wetter conditions than 2000–9 and 

the baseline forecast, whereas 2009–49 MRI climate data 

indicated slightly wetter and cooler conditions than 2000–9 

(table 6). The GFDL and MRI forecasts indicated a redistribution of precipitation from the spring to the summer as compared to the baseline forecast. The GFDL and MRI forecasts 

used the same land cover data as the baseline forecast (that of 

2009). It should further be noted that because the SWB model 

used growing degree days to calculate crop-water demand, any 

temporal change in irrigation withdrawals caused by temperature differences would have been automatically adjusted 

by that model. However, forecast results in this report are 

presented on decadal or longer time scales and would probably 

not reveal temperature-based irrigation onset differences that 

were on the scale of days to weeks.

To isolate the effect of only climatic changes and 

eliminate those caused by land cover, GFDL and MRI water 

budgets were compared against the baseline forecast (fig. 11C, 

table 7). The largest difference was in gains of recharge from 

precipitation, which were 1.4 million acre-ft/yr (12 percent) 

smaller for GFDL, and 1.7 million acre-ft/yr (15 percent) 

smaller for MRI. This decrease was apparently caused by the 

retiming of precipitation from the spring to the growing season 
Groundwater Availability of the Northern High Plains Aquifer 49

, and 2049 groundwater in storage, for the baseline, A2LC, 

A2 land cover and 2004 climate data (purple in figures 12 and 

Summary of average annual simulated groundwater budgets for 2009–49 for the Northern High Plains aquifer Table 7.

B2LC, GFDL, and MRI forecasts and differences from the baseline for the A2LC, B2LC, GFDL, and MRI forecasts.

A2LC, forecast using , millions of acre feet per year; [baseline, baseline forecast using 2004 climate data and 2009 land cover inputs; MAF/yr

14); B2LC, forecast using B2 land cover and 2004 climate data (green in figures 12 and 14); GFDL, forecast using 2009 land cover and Geophysical Fluid Dynamics Laboratory Earth System Model Second 

ersion 3 downscaled Generation downscaled daily outputs (blue in figures 12 and 14); MRI, forecast using 2009 land cover and Japanese Meteorological Research Institute Coupled Global Climate Model 

V

daily outputs (red in 14); --, zero; <, less than]

Gain/loss

Baseline, in MAF/yr

A2LC, in MAF/yr

A2LC difference from 

baseline forecast, 

in MAF/yr

Percent difference

B2LC, in MAF/yr

B2LC difference from 

baseline forecast, 

in MAF/yr

Percent difference

GFDL, in MAF/yr

GFDL difference from 

baseline forecast, 

in MAF/yr

Percent difference

MRI, in MAF/yr

MRI difference from 

baseline forecast, 

in MAF/yr

Percent difference

Gains (inflows)

From recharge from 

precipitation

11.6 11.6 <0.1 -- 11.7 0.1 1 10.2 –1.4 12 9.8 –1.7 15

From recharge from 

canal seepage

1.1 1.1 -- -- 1.1 -- -- 1.1 -- -- 1.1 -- --

From inflows from 

west 

<0.1 <0.1 -- -- <0.1 -- -- <0.1 -- -- <0.1 <0.1 2

From net release from 

storage

1.6 2.2 0.5 32 1.9 0.3 19 2.0 0.4 22 2.5 0.9 55

Losses (outflows)

To evapotranspiration 3.4 3.3 –0.1 2 3.4 >–0.1 1 3.3 –0.1 1 3.3 –0.1 4

To irrigation wells 6.7 7.5 0.7 11 7.2 0.5 8 6.3 –0.5 7 6.4 –0.3 5

To drains 0.2 0.2 -- -- 0.2 <0.1 1 0.2 <0.1 1 0.2 <0.1 --

To outflows to reservoirs <0.1 <0.1 >–0.1 2 <0.1 >–0.1 2 <0.1 >–0.1 8 <0.1 >–0.1 8

To municipal wells 0.1 0.1 -- -- 0.1 -- -- 0.1 -- -- 0.1 -- --

To stream base flow 4.0 3.8 −0.1 3 3.9 −0.1 2 3.6 –0.3 8 3.6 –0.3 9

Groundwater in storage 

in Northern High Plains aquifer, 2049

2,372.0 2,350.0 −22.0 −0.9 2,359.0 −13.0 −0.5 2,355 −17.0

−0.7 2,335.0 −37.0

−2
50 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

(table 6); that is, rather than precipitation falling in the spring, 

when vegetative growth is minimal, and resulting in recharge 

to the aquifer, it fell more during the summer, when crops and 

native vegetation are actively consuming water. Hence, GFDL 

and MRI losses to irrigation wells were 0.5 million acre-ft/

yr (7 percent) smaller and 0.3 million acre-ft/yr (5 percent) 

smaller than the baseline forecast, respectively. That is, more 

precipitation fell during the growing season, so less groundwater needed to be withdrawn to supply crop irrigation; however, 

the decrease in groundwater withdrawn for irrigation was 

smaller than the decrement in gains from recharge, resulting in 

a net negative effect for the aquifer as compared with the baseline forecast. The decrease in recharge was directly affected 

by the distribution of land cover, whereas native vegetation 

(rangeland) was far and away the dominant land cover of the 

Northern High Plains aquifer (fig. 5); therefore, even moderate 

increases in consumption of precipitation by native vegetation have the potential to have a large effect on the aquifer 

water budget. GFDL and MRI losses to streams were 0.3 million acre-ft/yr smaller than for the baseline forecast, probably 

because of the decrease in gains from recharge.

The other result of the decreased gains of recharge was 

that gains from a net release from storage were 0.4 million 

acre-ft/yr (22 percent) to 0.9 million acre-ft per year (55 percent) larger than the baseline for GFDL and MRI, respectively 

(fig. 11C, table 7). Again, gains from a net release from storage correlate with groundwater-level declines, and groundwater levels declined more quickly for the GFDL and MRI 

forecasts than for the baseline forecast; however, the trends of 

net release from storage were different for GFDL than for MRI 

(fig. 12). The decrease in the GFDL net release from storage 

from 2009 to 2049 indicates that the aquifer was coming to 

equilibrium with the GFDL forecast stresses. Conversely, the 

upward trend of the MRI net release from storage indicates 

that the rate of groundwater declines was still increasing at 

2049 in the MRI forecast.

As a result of these changes, the amount of groundwater 

in storage at the end of the GFDL and MRI forecasts was 17 to 

37 million acre-ft (0.5 to 2 percent) less than that of the baseline forecast (table 7). GFDL forecast declines were smaller 

than those of the MRI forecast, and though these datasets 

originated from the same global climate model, the alternate 

methods of downscaling the data produced different resulting 

effects on groundwater gains and losses. Additional details on 

differences in input climate data are described in the “Alternative Climate Soil-Water-Balance Models” section.

The largest difference in basin water budgets for the 

GFDL and MRI forecasts was gains from recharge that 

decreased mainly in the Loup and Republican River Basins. 

GFDL gains from recharge were 857 ft3/s (or 21 percent) 

smaller than the baseline forecast in the Loup River Basin 

(fig. 13C, F, and G). Gains also decreased in the Republican 

River Basin, where GFDL recharge was 214 ft3/s (16 percent) 

less than the baseline forecast and MRI recharge was 410 ft3/s 

(36 percent) less than the baseline forecast. In these basins, 

the decline in recharge caused an increase in gains from a 

net release from storage, correlating to groundwater-level 

declines. These basins also had a resulting GFDL forecast 

decline in losses to streams, and though the rate in the Loup 

River Basin declined by more (368 ft3/s) than that of the 

Republican River Basin (54 ft3/s), the Republican River Basin 

had a larger relative decline (35-percent decline from baseline 

forecast, as opposed to 15-percent decline for the Loup River 

Basin). In other words, the Loup River Basin losses to streams 

declined more, but there was more available to lose; declines 

in the Republican River Basin, though smaller, represent a 

larger part of the baseline forecast base flow. The MRI forecast 

affected the Loup and Republican River Basins in the same 

fashion as the GFDL forecast, but to a larger extent; therefore, even though the different downscaling methods used to 

produce the GFDL and MRI climate data produced a different 

magnitude of recharge, they did not produce a different distribution of recharge.

One other difference between the baseline forecast and 

the GFDL or MRI forecasts was that losses to irrigation wells 

were smaller in the Big Blue River Basin than for the baseline forecast (by 382 ft3

/s for GFDL and by 308 ft3/s for MRI; 

fig. 13C, F, and G). Losses were also smaller in the Elkhorn 

River Basin, where the losses to irrigation wells were 157 ft3/s 

smaller for GFDL and 141 ft3/s smaller for MRI. This indicates that the GFDL and MRI forecasts that produced more 

growing season precipitation did so mainly in the Big Blue 

River and Elkhorn River Basins. Conversely, GFDL losses to 

irrigation wells were 105 ft3/s larger than the baseline for the 

Niobrara River Basin and 68 ft3/s larger than the baseline for 

the Republican River Basin. Increases in losses to irrigation 

wells are likely correlated with increased temperatures because 

growing season precipitation increased overall (table 6).

The GFDL stream base flows were smaller than those of 

the baseline forecast, except for the Elkhorn River, and MRI 

base flows were smaller than those of GFDL (fig. 14A–E). 

The GFDL 2040–49 base flow for the Elkhorn River at 

Waterloo, Nebr. (USGS streamgage 06800500), was about 

100 ft3/s larger than for the baseline forecast. Because this 

was much larger than the increase in the GFDL forecast base 

flow for the Elkhorn River Basin, and most of the streams in 

that basin are tributary to the selected streamgage location, 

the difference was probably caused by the different period 

being averaged (last 10 years of the forecast as opposed to 

2009–49). In other words, the flow of those last 10 years was 

larger than the average of the forecast period. Similarly, the 

largest decrease was for the Loup River at Columbus, Nebr. 

(USGS streamgage 06794500), where 2040–49 base flows 

were 300 ft3/s (GFDL) or 500 ft3/s (MRI) smaller than those 

of the baseline forecast; however, the forecast (2009–49) Loup 

River Basin flows declined by 366–368 ft3/s. The difference 

indicates that the GFDL and MRI forecasts were biased differently from 2040–49; therefore, different climate downscaling methods also produced different timing of recharge at a 

decadal time scale.
Groundwater Availability of the Northern High Plains Aquifer 51

Resource Monitoring and Potential 

Improvements

This section of the report describes the importance of 

continuing existing data collection efforts, provides context 

for uses of such data, and describes an evaluation of the variance of simulated groundwater levels to indicate where new 

groundwater-level monitoring may provide the most benefit. 

This section also describes additional data that could be collected to benefit future resource monitoring.

Importance of Continuing Ongoing Data

Collection Program

Streamflow and groundwater-level data provide fundamental data describing water availability and will continue to 

be primary sources of model calibration data; therefore, these 

data are critical to the success of future modeling efforts used 

to evaluate groundwater availability. Collection of additional 

such data at more locations, or in some cases higher frequency 

such as monthly or seasonal rather than annual, may further 

reduce model uncertainty; for example, only 91 streamgages 

had data within the calibration period of this model, or about 

1 streamgage for every 1,100 mi2 if they were evenly spaced 

across the aquifer. Adding new streamgages for the largest 

ungaged tributaries of currently gaged streams could provide 

valuable data informing model calibration and surface-water 

interactions with groundwater and provide additional spatial 

context for existing streamgage data. In addition, groundwater 

quality and tracer data could be collected to determine pathways and timescales of environmental processes (Cook and 

Böhlke, 2000). Pathways and travel times could be simulated 

with the groundwater-flow model using particle tracking, such 

as MODPATH (Pollock, 2016), and compared to results of 

tracer sampling to further improve the model.

In addition, as of 2009, in some areas, considerable 

distance exists between test holes used to define thickness and 

character of the Northern High Plains aquifer (Houston and 

others, 2013). Aquifer property data directly affect simulations 

of groundwater flow and related uncertainty; therefore collection of additional geologic data describing aquifer thickness 

and other properties will directly benefit future groundwater 

availability studies and probably reduce simulation uncertainty. In addition, from 2010 to present (2019), considerable 

effort has been put into airborne resistivity data collection 

and, in some cases, these data have been used to improve the 

understanding of aquifer characteristics (Hobza and others, 

2014). The earliest parts of airborne resistivity data from 

Abraham and others (2012) helped revise the current base of 

aquifer map (Peterson and Traylor, 2016); however, the rest 

of the airborne resistivity data were not available in time for 

this study and could be used to refine aquifer properties and 

surfaces for the Northern High Plains and other future groundwater models. Incorporation of additional geologic data may 

improve model calibration and reduce model uncertainty.

Potential New Groundwater-Level Monitoring 

Locations

The simulation described in this report can be used as a 

tool to evaluate the benefit of collecting additional physical 

or hydrologic data. As an example, an analysis was completed using the simulation to evaluate groundwater-level 

data. Because of the large size of the Northern High Plains 

aquifer, and the relative sparsity of measurements in some 

parts of the area (fig. 8), new groundwater-level monitoring could be done at many potential locations. Given that 

resources are not sufficient to measure local water levels 

uniformly throughout the aquifer, some relative prioritization could help indicate priority areas for monitoring. Model 

simulations of future conditions indicated areas where one 

would expect the largest changes in groundwater levels, and 

that information can be used to guide future data collection 

efforts to monitor for these potential changes; therefore, the 

standard deviation of simulated seasonal groundwater levels 

at 2,462 selected locations (fig. 15) was used as an indication of variability in future simulations that was not informed 

by existing observation data. As described in the “Observation Network Analysis” section, 2009–49 standard deviations were calculated from seasonal simulated water levels 

that were retrieved from outputs of the five future forecasts 

described in this report (baseline, A2LC, B2LC, GFDL, and 

MRI). The threshold of 5 ft was selected because it approximated the 75th percentile of the standard deviations; that 

is, 75 percent of the standard deviations were less than 5 ft, 

whereas only 25 percent were larger.

The largest standard deviations seemed to be spatially biased, generally along the drainage divide between 

the South Platte River and the Republican River Basin, 

in southwest Nebraska, close to the northeast corner of 

Colorado (fig. 15). Several other locations with standard 

deviations larger than 20 ft are scattered about the area but 

most were not close to simulated streams because streams 

moderate groundwater-level responses to changing hydrologic conditions. Locations with standard deviations from 5 

to 20 ft were commonly closer to drainage divides between 

simulated streams in the area. Near the downstream end 

of the South, Middle, and North Loup Rivers, the standard 

deviation of water levels was larger than 5 ft for nearly every 

location evaluated, indicating more groundwater-level variability across the forecast results. It must be noted that these 

results are strictly subject to the construction and calibration 

of the groundwater model and the range of conditions used 

in the forecasting procedure (five forecasts, as described in 

the “Future Forecasts” section). Sampling of variability from 

other forecasts, alterations of the model construction or calibration, or other methods of network analysis could produce 

different results.
52 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo. SOUTH DAKOTA WYOMING IOWA MISSOURI MINNESOTA NEBRASKA COLORADO KANSAS

00

50

50

100

100

150

150

200 MILES

200 KILOMETERS

Base from U.S. Geological Survey digital data, variously dated, various scalesAlbers Equal-Area Conic projectionStandard parallels 29o50' N. and 45o50' N.Central meridian 96o00' W.North American Datum of 1983

42°40°

96° 98° 100° 102° 104° 106°

Study area

TX

NM

CO KS

IA

NE

OK

WY

MO

AR

SD

EXPLANATION

Standard deviation of forecast

groundwater levels acrossfive forecasts, in feet

Study area

High Plains aquifer

0 to 5.05.1 to 10.010.1 to 20.020.1 to 30.0

Major river basins

Figure 15. Standard deviation of simulated groundwater levels at selected locations across five alternate 2009–49 forecasts.
Groundwater Availability of the Northern High Plains Aquifer 53

Resource Monitoring Improvements

Groundwater withdrawals for irrigation are the single 

largest anthropogenic effect and largest outflow component 

of the groundwater-flow system of the Northern High Plains 

aquifer, but insufficient withdrawal data have been collected 

to precisely characterize the amount and timing of the withdrawals. In 2009, groundwater withdrawals for irrigation in 

the Northern High Plains aquifer were measured only in a few 

areas and for a small number of seasons. The sparse amount of 

data represented only a fraction of a percentage of the groundwater withdrawals in the study area, and the few data that did 

exist may not represent groundwater withdrawal rates well at 

the scale of the aquifer. These issues preclude the use of existing groundwater withdrawal data for calibration or verification of groundwater withdrawal estimates produced with the 

SWB model in this study, though the SWB-produced estimates 

were within a similar range to average annual precipitation 

minus crop-water use. The SWB model used in this study is 

described in more detail in the “Future Forecasts” section, 

as well as in the “Data Used to Forecast Future Conditions” 

section, and in Peterson and others (2016a). It is assumed that 

the SWB model correctly represented soil-zone processes and 

that it was sufficiently calibrated by adjustment of the SWB 

recharge during calibration of the groundwater-flow model 

(Peterson and others, 2016a). However, if many more groundwater withdrawal data had existed, they could have been used 

as an initial calibration step on the SWB model outputs before 

input to the groundwater-flow model. By 2016, groundwater withdrawal meters had been installed in more locations 

across the area, yet additional compilation and review would 

be required to indicate if sufficient data have been collected 

to relate to aquifer-scale groundwater withdrawals, or if data 

have been collected over enough time to sample the normal 

range of climatic variability. In addition to the groundwater 

withdrawal data, ancillary related data, such as irrigation 

application methods and crops grown at each groundwater 

withdrawal site, would provide needed context to improve 

estimated groundwater withdrawals. Groundwater withdrawals 

for irrigation are the largest outflow component of the groundwater system; therefore, reduction in uncertainty in groundwater withdrawal data and estimates thereof should lead directly 

to reductions in uncertainty in groundwater model results and 

forecasts.

Challenges for Evaluation of Groundwater 

Availability

The model and dependent analyses described in this 

report that were used to analyze groundwater availability of 

the Northern High Plains aquifer are deemed adequate in that 

the calibration results indicated agreement between calibration targets and simulated equivalents and, as such, that the 

model correctly reproduced regional patterns of groundwater 

levels and flow and interaction with other features, such as 

groundwater discharge to streams. Several assumptions and 

limitations specific to the model developed for this study were 

described in Peterson and others (2016a). In general, although 

the model is considered accurate at a regional scale, it is not 

suitable for use in evaluating local-scale problems, nor for 

periods less than a few months. Readers who are interested in 

more local-scale analysis could use the data and results of this 

study as a starting point for additional refinement.

In addition, as noted in the “Resource Monitoring and 

Potential Improvements” section, many additional data could 

be collected that would improve the understanding of water 

availability in the Northern High Plains aquifer and would 

bolster the data used to calibrate the groundwater models 

developed as part of this study and other groundwater models. 

Such data include but are not limited to irrigation withdrawal 

data, maps describing irrigation application practices (such 

as pivot, sprinkler, or high-efficiency sprinkler), hydrologic 

data such as groundwater levels or streamflows at additional 

locations or increased frequency, or physical data describing 

aquifer properties or properties affecting interflows between 

the aquifer and related features.

In addition to assumptions and limitations of the models, some difficulties arose that were specific to development 

of a regional model, such as when attempting to simulate a 

principal aquifer. For the Northern High Plains aquifer simulations, problems arose for input file processing, file sharing, 

and communication. Original Python programs were written to 

handle the complexity of incorporating model parameters and 

adjustments to recharge or other model inputs that varied spatially and temporally. In addition, most commercially available 

graphical user interfaces cannot handle the large amounts of 

data such as were used for calibration of this model (Peterson 

and others, 2016a). Therefore, postprocessing was handled 

through original programs written in Python (Python Software 

Foundation, 2017), wherein summary statistics and graphical representations of model calibration were written into a 

commonly available file format (Adobe Portable Document 

Format) for review. The programs generate hundreds of plots 

or other summary statistics and metrics in seconds, much 

faster than if the outputs were postprocessed manually using 

spreadsheet or other user-driven processes. Readers planning 

other efforts with large amounts of data could benefit from 

similar approaches.

Readers are cautioned that future forecast base flows and 

groundwater budgets included in the analysis described in this 

report are not comprehensive and may not be the average nor 

span the range of effects caused by potential future climate or 

land cover changes. These forecasts are provided as examples 

of how the groundwater-flow model can be used as a tool to 

evaluate aquifer and stream base flow responses to potential 

future conditions. Alteration of input data, such as the selection of a different year from 2004 to provide average climate 

data inputs for three of the forecasts, likely would generate different results. For example, a large suite of downscaled global 

climate model data could be evaluated through the SWB 

model and groundwater-flow model to determine if the various 
54 Groundwater Availability of the Northern High Plains Aquifer in Colo., Kans., Nebr., S. Dak., and Wyo.

versions have a central tendency of effects on groundwater and 

related flows, or to reveal the range of variation caused by the 

suite of forecasts. For the two environmental stresses forecasts 

provided in this report, no context is available to characterize 

how these particular forecasts might be similar or different 

from forecasts using other climate data.

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For more information about this publication, contact:

Director, USGS Nebraska Water Science Center 

5231 South 19th Street

Lincoln, NE 68512

402–328–4100

For additional information, visit: https://www.usgs.gov/centers/

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