FRONT COVER: Playas of the High Plains at sunrise (Brian Slobe, photographer; 

published with permission).
Recharge Rates and Chemistry Beneath 

Playas of the High Plains Aquifer—

A Literature Review and Synthesis

By Jason J. Gurdak and Cassia D. Roe

Prepared in cooperation with Playa Lakes Joint Venture

Circular 1333

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Gurdak, J.J., and Roe, C.D., 2009, Recharge rates and chemistry beneath playas of the High Plains aquifer—A literature 

review and synthesis: U.S. Geological Survey Circular 1333, 39 p.
iii

Contents

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

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

Summary of Major Findings and Implications ..........................................................................................2

Methods...........................................................................................................................................................3

Background.....................................................................................................................................................5

High Plains Aquifer...............................................................................................................................5

Quantity of Recharge to the High Plains Aquifer....................................................................5

Quality of Recharge to the High Plains Aquifer......................................................................7

What Are Playas? .................................................................................................................................8

Hydrology of Playas—An Overview ................................................................................................10

Recharge Rates and Chemistry Beneath Playas....................................................................................12

Recharge Beneath Playas.................................................................................................................12

Water-Budget Studies...............................................................................................................12

Infiltration Studies .....................................................................................................................13

Unsaturated-Zone Studies ......................................................................................................14

Ground-Water Studies ..............................................................................................................15

Artificial Recharge.....................................................................................................................15

Sedimentation ............................................................................................................................16

Climate Change and Variability................................................................................................17

Recharge Chemistry Beneath Playas..............................................................................................18

Water Quality of Playa Lakes...................................................................................................18

Subsurface Processes Affecting Recharge Chemistry.......................................................20

Conclusion Regarding Conceptual Model of Recharge Beneath Playas...........................................21

Needs for Future Research........................................................................................................................22

Acknowledgments.......................................................................................................................................23

References Cited..........................................................................................................................................23

Appendix 1. Glossary ...............................................................................................................................33

Appendix 2. Infiltration estimates beneath playas of the southern High Plains............................36

Appendix 3. Recharge estimates for the southern High Plains........................................................38

Figures

1–3. Maps showing:

 1. A, Playas on the High Plains (or Ogallala) aquifer; B, index map

showing the northern, central, and southern High Plains subregions; 

and C, hydrogeologic units of the High Plains aquifer....................................................4

 2. Change in water-table elevation since before development.........................................6

 3. Locations of High Plains aquifer and U.S Geological Survey High Plains 

Unsaturated-Zone Research Network that monitors recharge ....................................7

4. Diagram showing chemical travel times from land surface to water table........................8

5. Block diagram showing principal physical features of playas ...........................................11

6. A, Diagram of recharge estimates listed in appendix 3; B, block diagram 

showing relative recharge rates..............................................................................................21
iv

Conversion Factors and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

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 0.004047 square kilometer (km2

)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

Flow rate

cubic foot per second (ft3

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

Abbreviations Used in This Report

μg/L micrograms per liter

CAFO	 confined animal feeding operation

Cl– chloride 

in. inch, inches

in./hr inches per hour

in./yr inches per year

ft feet

ft3 cubic foot, cubic feet

ft/day feet per day

mg/kg milligrams per kilogram

km kilometer

mg/L milligrams per liter

mi2 square miles

mm/yr millimeters per year

MCL maximum contaminant level

N nitrogen

NAWQA U.S. Geological Survey National Water-Quality Assessment Program

NO3

– nitrate

P phosphorus

PLJV Playa Lakes Joint Venture

SDWA Safe Drinking Water Act

TU tritium unit

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound
Abstract

Playas are ephemeral, closed-basin wetlands that are 

important zones of recharge to the High Plains (or Ogallala) 

aquifer and critical habitat for birds and other wildlife in the 

otherwise semiarid, shortgrass prairie and agricultural landscape. The ephemeral nature of playas, low regional recharge 

rates, and a strong reliance on ground water from the High 

Plains aquifer has prompted many questions regarding the 

contribution of recharge from playas to the regional aquifer. 

Considerable scientific debate has led to more than 175 publications about the potential for water to infiltrate the relatively

impermeable playa floors and subsequently recharge the High

Plains aquifer. Since the early 1900s, many conceptual models 

about recharge beneath playas have been proposed. Some 

early conceptual models indicate that playas are evaporative 

pans that do not allow recharge beneath playas, whereas other 

more recent models indicate that playas are effective recharge 

basins. A variety of data supports various aspects of these 

competing conceptual models. 

The competing conceptual models have developed 

because of the sporadic nature of rainfall to the region, the 

large number of playas in the region (more than 66,000), a 

range of physical characteristics in playas, the relatively thick 

unsaturated zones (often greater than 100 feet) separating 

most playas from the regional water table, and the inherently 

uncertain nature of most methods used to estimate recharge. 

An accurate understanding of recharge rates beneath playas 

is important from the perspective of ground-water management and the sustainability of rural agricultural economies, 

particularly in light of the substantial water-level declines in 

the High Plains aquifer. Other environmental concerns, such 

as erosion and transport of sediment and contaminants from 

surrounding land and modification of playas to allow artificial

recharge, also have made accurate understanding of recharge 

an important priority from the perspective of wetland function 

and habitat health, protecting ground-water quality, and the 

substantial costs associated with land and water management, 

conservation, and regulation.

To address these questions and concerns, the 

U.S. Geological Survey, in cooperation with the Playa Lakes 

Joint Venture, present a review and synthesis of the more than 

Recharge Rates and Chemistry Beneath Playas of the 

High Plains Aquifer—A Literature Review and Synthesis

By Jason J. Gurdak and Cassia D. Roe

175 publications about recharge rates and chemistry beneath 

playas and interplaya settings. Although a number of questions 

remain regarding the controls on recharge rates and chemistry 

beneath playas, the results from most published studies indicate that recharge rates beneath playas are substantially (1 to 

2 orders of magnitude) higher than recharge rates beneath 

interplaya settings. The synthesis presented here supports the 

conceptual model that playas are important zones of recharge 

to the High Plains aquifer and are not strictly evaporative 

pans. The major findings of this synthesis yield science-based

implications for the protection and management of playas and 

ground-water resources of the High Plains aquifer and directions for future research.

Introduction

Playas are ephemeral, closed-basin wetlands that have 

been hypothesized by some researchers to be important zones 

of recharge to the High Plains (or Ogallala) aquifer (note:

see glossary terms in appendix 1). Playas are critical for maintaining biodiversity (Tsai and others, 2007) and are wetlands 

unique to the Great Plains physiographic province (fig. 1A) 

because they are zones of recharge and do not receive groundwater discharge as do prairie potholes and many other types of 

wetlands. The floors of most playas are lined with relatively

impermeable clay soils and are commonly separated from the 

regional water table by tens to hundreds of feet of unsaturated 

zone (vadose zone), which have generally confounded a 

detailed understanding of the role that playas have in recharging the High Plains aquifer.

 Although numerous studies have investigated the role 

of playas in recharging the High Plains aquifer, relatively 

few have directly measured water and chemical movement 

beneath playas and interplaya settings. Most studies rely on 

indirect methods to estimate water and chemical movement 

beneath playas. Although results from these studies indicate 

that playas enhance recharge at rates higher than rates in 

interplaya settings (Scanlon and Goldsmith, 1997), the water 

fluxes beneath playas are highly variable in both space and

time. No studies to date have systematically characterized all 
2 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

the factors that control spatial or temporal variability of water 

and chemical movement within and beneath playas. A more 

detailed understanding of these controls is needed for best 

management of the ground-water resources of the High Plains 

aquifer and of the ecosystems and wetland habitat within each 

playa. In 2008, the U.S. Geological Survey, in cooperation 

with the Playa Lakes Joint Venture, began a study to gain more 

understanding by reviewing and synthesizing scientific literature related to the playas of the High Plains aquifer.

The purpose of this report is to present previous information from investigations of playas in the High Plains aquifer 

and to synthesize the existing knowledge about the rates and 

chemistry of recharge beneath playas and interplaya settings. 

The information presented in this report is designed to inform 

and assist ground-water resource managers and partners, 

such as the Playa Lakes Joint Venture, responsible for playa 

management and conservation.

The Playa Lakes Joint Venture (http://www. pljv.org/)

is a nonprofit partnership of Federal and State wildlife agencies, conservation groups, private industry, and 

landowners dedicated to conserving bird habitat in the Great 

Plains. The mission of the Playa Lakes Joint Venture is to 

conserve playas, other wetlands, and associated landscapes 

through partnerships for the benefit of birds, other wildlife,

and people. There are approximately 66,000 playas throughout the southern Great Plains, most of which are located 

within the joint venture’s boundary (fig. 1A) (McLachlan, 

2008). Approximately 61,000 playas are on the High Plains 

aquifer and have the highest density in the southern High 

Plains (or Llano Estacado) aquifer in Texas and in part of 

the central and northern High Plains aquifer in Kansas and 

Nebraska (fig. 1B) (Smith, 2003; LaGrange, 2005; McLachlan, 

2008). The playas of the Playa Lakes Joint Venture region 

are essential habitat in one of the most important inland 

migratory corridors in North America for many waterfowl, 

shorebirds, and waterbirds, and for many other migratory and 

resident birds.

Summary of Major Findings 

and Implications

Understanding how playas affect the quantity and quality 

of recharge to the High Plains aquifer has important implications for the sustainability of the High Plains aquifer, human 

and ecosystem health, the sustainability of rural agricultural 

economies, and the substantial costs associated with land and 

water management, conservation, and regulation. The major 

findings of the literature synthesis are outlined in this section

and yield science-based implications for assessing and managing playas and ground-water resources of the High Plains.

Movement of recharge and chemicals to the water 

table follows fast and slow pathways. Different pathways 

are available for recharge and chemical transport to reach the 

water table, and some paths are relatively faster than others. 

In locations that represent diffuse recharge (slow paths), 

estimated time of chemical transport from land surface to the 

water table exceeds the period of agricultural activity (more 

than 100 years in some locations) and imply that agricultural 

chemicals should not be present at the water table yet. In fact, 

agricultural chemicals are commonly detected in ground water. 

This apparent discrepancy is explained by local fast paths that 

may enable water and chemicals from the land surface to reach 

the water table in months to decades. By comparison, slow 

paths may enable water and chemicals from the land surface to 

reach the water table in centuries to millennia.

Ground-water quality is changing with time. Changes 

in water quality are occurring with time that may affect the 

sustainability of the High Plains aquifer. Understanding 

ground-water quality is important because it directly affects 

how water can be used. Studies show that at some local and 

subregional scales, particularly where pumping is intense or 

where environmental and topographic settings are conducive 

to fast-path recharge and chemical transport, water quality 

may be a limiting factor for some intended uses such as drinking water or irrigation water.

The High Plains aquifer has a limited ability to naturally attenuate contaminants. The High Plains aquifer is 

limited in its ability to naturally attenuate contaminants, such 

as nitrate (NO3

–

) by means of denitrification, and it generally

has slow recharge rates—both of which suggest that once 

the aquifer is contaminated it will remain so for decades and 

even millennia. Denitrification rates are slow and would take

between 250 to 14,000 years to lower nitrate concentrations by 

1 milligram/liter (mg/L) as nitrogen (N) in ground water of the 

High Plains aquifer. Additionally, because transport times to 

the water table are generally long—decades to millennia along 

slow paths—the amount of chemical mass reaching the aquifer 

will most likely increase with time. These results highlight the 

importance of managing land use to minimize contaminants 

in recharge.

Playas help recharge the High Plains aquifer. Most 

playas represent fast pathways for recharge and provide an 

important component of recharge to the High Plains aquifer. 

Although the exact amount of recharge to the High Plains 

Playas are an integral component of resource management in the 

High Plains (Brian Slobe, photographer; published with permission).
Methods 3

aquifer from any individual playa or group of playas is 

unknown without detailed investigation, substantial evidence 

in the literature shows that some portion of water that is stored 

seasonally in playas is able to infiltrate and eventually intercept the High Plains aquifer as recharge.

Recharge from interplaya settings is relatively low 

compared with playa settings. Interplaya settings generally represent slow paths for recharge and chemical transport 

because of high evapotranspiration and low precipitation rates 

in the southern High Plains. Reported interplaya recharge rates 

average 1 to 2 orders of magnitude smaller than most estimated recharge rates beneath playas.

Playa recharge varies in space and time. Large variations in estimated recharge rates beneath playas indicate that 

recharge is controlled, in part, by the spatial and temporal 

patterns in the physical characteristics of the playas, in climate, 

and in surrounding land-use practices. The physical characteristics of playas that have apparent influence on recharge rates

are the drainage area, playa volume, depth of the playa floor,

vertical extent of shrink-and-swell clay that lines playa floors,

depth of sediment overlying clay-lined floors, unsaturated-zone

sediments underlying the playa, and depth to the water table. 

Climate factors that affect the shrink-and-swell characteristics 

of the playa floors are likely to have important controls on

changes in recharge with time. Some land-use practices, such as 

cultivation, increase sedimentation to playas and thus affect the 

physical characteristics that influence infiltration and recharge

beneath playas. However, existing studies do not provide data 

to support the development of a reliable predictive model (or 

models) of recharge beneath any individual playa or group of 

playas. Future studies are needed to develop models that predict 

the recharge rates beneath playas.

The terms infiltration and recharge are not equivalent 

but are commonly used interchangeably in the literature. 

Our literature search indicates that many authors commonly 

use the terms infiltration and recharge interchangeably. However, the two terms are not synonymous. Infiltration of water

from playa or interplaya settings into the subsurface does not 

necessarily guarantee that the infiltrating water will intercept

the water table as recharge. 

Cost-benefit analyses of artificial recharge need to 

consider natural infiltration rates beneath playas. Given 

the ecological importance of unmodified playa wetlands to

the biodiversity of the Great Plains region and the substantial 

infiltration rates reported for some natural playas, cost-benefit

analyses for artificial recharge need to consider any added 

improvements that playa modification may have on rates of

infiltration and recharge that exceed the rates reported for

unmodified playas. Therefore, considerations of playa modification for artificial recharge need to weigh the costs associated

with the difference between the estimated recharge rate under 

modified playas and the recharge rate under natural playas.

Methods used to estimate recharge have inherent and 

unavoidable uncertainty. The same is true for the methods 

used by studies to estimate recharge beneath playas. However, 

these studies rarely report errors or uncertainties associated 

with recharge estimates. Furthermore, many studies use only 

a single method to estimate recharge. Recent research has 

shown that the use of many different methods can help constrain recharge estimates and reduce uncertainty. Thus, future 

studies that use as many different approaches as logistically 

and financially possible to estimate recharge will likely help

answer important remaining questions about recharge rates 

and chemistry beneath playas. 

Important questions remain about the role of playas 

recharging the High Plains aquifer. The existing literature 

does not bring data to bear on important questions that include 

the following:

1. What are the effects of current and future rates of sedimentation on infiltration and recharge beneath playas?

2. How much of the water that infiltrates beneath playas is

lost to lateral subsurface flow and subsequent evapotranspiration before reaching the water table, and how do such 

processes affect the results of studies that assume that all 

water infiltrated beneath playas becomes recharge?

3. Are innovative and wetland-friendly approaches for 

artificial recharge beneath playas available?

4. How much contamination reaches the ground water 

beneath playas, and does playa modification that increases

artificial recharge also increase transport of contaminants

to the water table?

5. How important are playas for recharge to the northern 

High Plains aquifer, for which comparatively little 

research has been reported?

6. How will climate change and climate variability affect 

recharge beneath playas?

These and other questions may be answered using interdisciplinary studies of water and movement of chemicals through 

the playa-wetland system to the High Plains aquifer as recharge 

Methods

The synthesis of previous work that is outlined in this 

report is based on an extensive literature search of databases 

such as Cambridge Scientific Abstracts, First Search, Web

of Science, and Dissertation Abstracts. In addition, keyword 

and citation searches were conducted in library catalogues 

(U.S. Geological Survey and Texas Tech University) and on 

the World Wide Web using http://www.google.com. Many 

hundreds of publications have been written on the topic of 

playas in the High Plains. This report presents a synthesis of 

findings from more than 175 publications that are related to

recharge (quantity and quality) beneath playas to the High 

Plains aquifer. Because the vast majority of these publications 

describe playas of the southern High Plains, the following 

synthesis focuses on recharge beneath playas of the southern 

High Plains. Approximately 40 larger saline lakes, which 

are sites of ground-water discharge, exist in the High Plains 

(Wood and Osterkamp, 1987) but are not included in the 

following synthesis.
4 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

Figure 1. A, Playas on the High Plains (or Ogallala) aquifer. Approximately 92 percent of the more than 66,000 playas of the southern 

Great Plains and Playa Lakes Joint Venture (PLJV) region are located on the High Plains (or Ogallala) aquifer (modified from 

McLachlan, 2008); B, northern, central, and southern High Plains subregions; and C, hydrogeologic units of the High Plains aquifer 

(modified from McMahon and others, 2007). Playas in southeastern Wyoming are not shown in figure 1A because these playas are 

not within the PLJV boundary (Mike Carter, PLJV, written commun., 2008).

108° 104° 100° 98°

34°

38°

42°

A

MO

IA

SD

WY

CO

NE

KS

Southwest 

playas

Central Table 

playas

Todd

Valley

playas

Rainwater Basin

NM

OK

TX

0 50 100 150 MILES

0 10050 150 KILOMETERS

Base information from U.S. Geological Survey

digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30',

central meridian –96°

Playas

Playa Lakes Joint Venture (PLJV) boundary

High Plains aquifer

Great Plains physiographic province

108° 106° 104° 102° 100° 98°

36°

40°

42°

38°

32°

34°

108° 106° 104° 102° 100° 98°

32°

34°

36°

40°

42°

38°

0 50 100 MILES

0 50 100 KILOMETERS

CO

KS

NE

OK

WY

SD

IA

TX

NM

C

TX

CO

KS

NM

NE

OK

WY

SD

IA Northern 

High

Plains

Central 

High

Plains

Southern 

High

Plains

B

EXPLANATION EXPLANATION

Alluvial systems

Aquifer not 

 present

Arikaree/Brule 

 Formations

Ogallala 

 Formation

Plio-Pleistocene 

 deposits

Sand Hills eolian 

 deposits

Quaternary 

 deposits

Hydrogeologic units

Base information from U.S. Geological Survey

digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30',

central meridian –96°
Background 5

Background

High Plains Aquifer

The High Plains (or Ogallala) aquifer underlies about 

174,000 square miles (mi2

) in parts of eight States (Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming) (fig. 1B). The aquifer includes six 

primary hydrogeologic units (fig. 1C), of which, the Ogallala 

Formation is the largest (McMahon and others, 2007). In 

2000, the aquifer had an estimated 2,980 million acre feet of 

water in storage (McGuire and others, 2003), thus making it 

one of the largest aquifers in the world.

Use of ground water from the aquifer as a source of irrigation water has transformed the High Plains into one of the 

largest and most productive agricultural regions in the United 

States, earning it the nickname “breadbasket of the world” 

(Opie, 2000). Ground-water withdrawals from the High Plains 

aquifer account for about 20 percent of total ground water 

withdrawn in the United States, of which 97 percent is for 

irrigation (Maupin and Barber, 2005). In 1989, the economic 

value of the aquifer was estimated to be 20 billion dollars 

(Moody, 1990); this value was based largely on the agricultural production that relies on water from the aquifer. Although 

public and domestic uses account for a relatively small percentage of the total ground-water use, these two uses provide 

drinking water for about 82 percent of the 2.3 million people 

who live within the High Plains (Maupin and Barber, 2005).

The sustainability of the High Plains aquifer is in question for a number of communities that rely on this aquifer 

as their principal source of water for irrigated agriculture 

and for public and domestic drinking supplies (Dennehy and 

others, 2002). The agricultural productivity of the region 

has come at the cost of declining water tables and nonpointsource contamination. Since the 1940s, aquifer development 

has lowered the water-table more than 150 feet (ft) in parts 

of the region (McGuire and others, 2003). Water tables have 

declined (fig. 2) substantially since predevelopment times 

because ground-water withdrawals, largely for irrigated agriculture, have greatly exceeded recharge throughout much of 

the aquifer. This imbalance is particularly true in the central 

and southern High Plains. Such ground-water depletion has 

increased pumping costs and reduced water discharge to 

streams, among other things.

Additionally, many agricultural contaminants have been 

detected in ground water of this aquifer, including nitrate 

(Gurdak and Qi, 2006; Qi and Gurdak, 2006; Gurdak, McCray, 

and others, 2007; Gurdak, 2008) and arsenic (Fahlquist, 

2003) at concentrations that exceed current maximum contaminant levels (MCLs) for drinking water that are set by the 

U.S. Environmental Protection Agency (2008). Therefore, 

the question of sustainability of the High Plains aquifer is a 

function of changes in the quantity as well as the quality of 

ground water.

Quantity of Recharge to the High Plains Aquifer

“Recharge” refers to the amount or flux of water that

enters ground water. Water that infiltrates the land surface

and moves downward through the soil and unsaturated zone 

becomes recharge only after the water intercepts the water table. 

As used in this report, recharge is the vertical and volumetric 

flux of water across the water table of the aquifer. Upward discharge of water from underlying formations is another source of 

recharge to the High Plains aquifer (Nativ, 1992; McMahon and 

others, 2007) but is not discussed in detail in this report. Rates 

of recharge are commonly expressed as length per time (that is, 

inches per year, in./yr).

Recharge replenishes aquifers. Therefore, the rate of 

recharge (or how fast recharge occurs in a given space and 

time) affects ground-water availability and sustainability. 

Accurate knowledge of recharge is important for making 

informed decisions about ground-water management.

In many aquifers of the Western United States, including the High Plains aquifer, recharge rates vary under different land uses and with time owing to changes in climate 

that occur seasonally or during longer periods that are 

controlled by natural factors and (or) by human activities 

(Gurdak, Hanson, and others, 2007). Accurate measurements 

of recharge are very challenging to obtain in most aquifers 

because there are no easy and direct methods for observing and measuring moving water that intercepts the water 

table. In the High Plains aquifer, for example, the water table 

is commonly many tens to hundreds of feet below the land 

surface (Gutentag and others, 1984). Furthermore, the methods 

used to directly estimate recharge commonly represent the rate 

of recharge at a particular space and time and therefore may 

not adequately represent recharge at another location in the 

aquifer or under different conditions that may affect recharge 

with time.

As a result of the inherent challenges in direct methods 

of recharge estimation, indirect methods generally have been 

used in the High Plains aquifer to estimate recharge. Indirect 

methods commonly use information from other components 

of the water cycle, such as precipitation, evapotranspiration, 

streamflow, and infiltration, to infer information about

recharge. Because indirect methods do not directly measure 

moving water that intercepts the water table, recharge estimates from indirect methods are subject to uncertainty. The 

degree of uncertainty associated with any recharge estimate, 

whether from direct or indirect methods, depends upon the 

assumptions used by the investigator and the accuracy and 

precision of any measurements or calculations. Uncertainty 

associated with recharge estimates are unavoidable given 

the spatial scale of the High Plains aquifer and the historical 

and future time scales on which ground water in this aquifer 

is managed. Those uncertainty estimates that are reported 

with recharge rates are valuable information for groundwater managers.
 

Base information from U.S. Geological Survey 

digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30',

central meridian –96°

108° 106° 104° 102° 100° 98°

32°

34°

36°

40°

42°

38°

TX

KS

NM

NE

WY

CO

IA

OK

SD

EXPLANATION

Water-level change, in feet

No data or aquifer not present

More than 50

25 to 50

10 to 25

+10 to –10

10 to 25

25 to 50

50 to 100

100 to 150

More than 150

Decline

No substantial change

Rise

0 50 100 MILES

0 50 100 KILOMETERS

6 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

Previous studies of recharge to the High Plains aquifer 

indicate that the direction and rate of water movement in the 

unsaturated zone, and in turn recharge, are likely controlled 

by differences in land use and land cover (Scanlon and others, 

2005), irrigation-return flow (Scanlon and others, 2003), spatial

patterns in climate (McMahon and others, 2006), temporal 

patterns in climate (Gurdak, Hanson, and others, 2007), geomorphological features such as playas (Wood and Sanford, 

1995a,b; Scanlon and Goldsmith, 1997; Fryar and others, 

2001) and other topographic depressions (Gurdak and others, 

2008), and vegetation and soils (Keese and others, 2005). These 

and other controlling factors result in slow and fast paths for 

recharge to the High Plains aquifer (McMahon and others, 

2006). Slow paths are characterized by diffuse recharge that 

may occur after rainfall, melting snow, or irrigation-return flows

infiltrate across a uniform area of the aquifer, that percolates

relatively uniformly through the unsaturated zone, and that 

eventually intercepts the water table. Slow paths commonly 

occur in fine-grained sediments or under flat terrain (Scanlon

and Goldsmith, 1997). Conversely, focused recharge may 

result from fast paths under depressions in the land surface 

(Gurdak and others, 2008), such as playas (Wood and Sanford, 

1995a,b; Scanlon and Goldsmith, 1997; Fryar and others, 2000, 

2001) or other types of preferential-flow processes through

the unsaturated zone (Hendrickx and Flury, 2001). Focused 

recharge is characterized by water that follows rapid pathways 

to the water table; those pathways bypass a large portion of the 

areal extent of the soil and unsaturated zone.

Water movement in the unsaturated zone is fundamentally controlled by differences in energy potential (the sum of 

gravity, soil-matric potentials, and osmotic forces), which is 

conceptually similar to hydraulic head in the saturated zone 

of an aquifer. The water in the unsaturated zone moves from 

areas having a higher energy potential to areas having a lower 

energy potential. The energy potential varies with depth in an 

unsaturated zone and is generally controlled by local precipitation and evapotranspiration rates and by the hydraulic properties of unsaturated-zone materials.

Previous measurements of the energy-potential gradient at the U.S. Geological Survey High Plains UnsaturatedZone Research Network (fig. 3) in the northern High Plains

of Nebraska indicate the potential for downward water movement within the unsaturated zone, with little seasonal change 

below the root zone (McMahon and others, 2006). In contrast, 

rangeland of the southern High Plains in Texas has energypotential gradients that increase substantially with depth 

Figure 2. Change in elevation 

of the water table since before 

development to 2000 (modified 

from McGuire and others, 2003).
Base information from U.S. Geological Survey

digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30',

central meridian -96°

108° 106° 104° 102° 100° 98°

32°

34°

36°

38°

40°

42°

Loup River

Elkhorn River

Platte River

Republican River

South Platte River

North Platte River

Arkansas River

Cimarron Ri ver

Pecos River

Red River

Canadian River

Cheyenne

Scottsbluff

North

Platte

Garden

City

Wichita

Guymon

Amarillo

Lubbock

Midland

Odessa

TX

CO

KS

NM

NE

OK

WY

SD

EXPLANATION

8,000

1,000

Unsaturated-zone research site

Elevation, in feet

0 50 100 MILES

0 50 100 KILOMETERS

High Plains 

aquifer

Great Plains

physiographic

province

Background 7

Figure 3. Locations of the High 

Plains aquifer and U.S Geological 

Survey High Plains UnsaturatedZone Research Network that 

monitors recharge.

(McMahon and others, 2006). This finding indicates the potential for upward water movement from the water table to the zone 

of plant roots, which is consistent with interplaya observations 

by Scanlon and Goldsmith (1997). These findings indicate that

substantial recharge is not likely to occur in interplaya settings 

under the current climate of the southern High Plains.

The energy potential gradients in the unsaturated zone 

ultimately affect water movement and recharge rates. At 

the U.S. Geological Survey High Plains Unsaturated-Zone 

Research Network (fig. 3), estimated downward water fluxes

(or recharge rates) ranged from 0.008–4.37 in./yr(McMahon 

and others, 2006). Irrigated agricultural sites had larger fluxes

(0.67–4.37 in./yr) than rangeland sites (0.008–2.76 in./yr). 

The largest water fluxes were observed at sites in the

northern High Plains (2.76–4.37 in./yr) followed by central 

High Plains (0.20–2.13 in./yr) and southern High Plains 

(0.008–1.26 in./yr). This order is due in part to climate differences from north to south and lower evapotranspiration rates 

in the northern High Plains than in the southern High Plains. 

McMahon and others (2006) suggested that the measured 

downward water flux (0.008 in./yr) at the southern High Plains

rangeland site represents past hydrologic conditions because 

upward hydraulic gradients were observed.

The southern High Plains aquifer, where the majority of 

the playas are located, was incised by the Canadian, Pecos, and 

Red Rivers (fig. 3) and cut off from the more humid central

High Plains and its recharge sources (Seni, 1980; Gustavson, 

1986; Nativ, 1992). Nativ (1992) noted that the exact portion 

of annual precipitation that recharges the southern High Plains 

aquifer has been debated since at least the 1930s; estimates differ by more than two orders of magnitude (0–1.61 in./yr) under 

playas and diffuse-recharge settings and beneath sand dunes. 

Studies of diffuse-recharge settings in the southern High Plains 

indicate that the recharge from direct precipitation is minimal 

(Nativ, 1992). Fine-grained soil and caliche (calcium carbonate) 

and climate conditions likely limit recharge in diffuse-recharge 

settings of the southern High Plains (Broadhurst, 1942; Barnes 

and others, 1949; Ries, 1981; Knowles and others, 1984).

Quality of Recharge to the High Plains Aquifer

The relatively thick unsaturated-zone sediments of the 

High Plains aquifer contain pore-water with chloride (Cl–

) and 

nitrate (NO3

–

) concentrations from natural evapoconcentration during thousands of years of precipitation (Walvoord and 

others, 2003) and from anthropogenic nitrogen (N) primarily 
8 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

from agricultural fertilizers (McMahon and others, 2003, 

2006, 2008). Ground-water quality in the High Plains aquifer 

is potentially vulnerable to contamination from these natural and anthropogenic Cl–

 and NO3

–

 reservoirs (Gurdak and 

Qi, 2006). Ground water may be contaminated if processes 

mobilize and transport the Cl–

 and NO3

–

 reservoirs to the water 

table; such processes might be conversion of rangeland to irrigated and rain-fed cropland (McMahon and others, 2006) or 

natural climate variability (Gurdak, Hanson, and others, 2007). 

For example, McMahon and others (2006) suggested that the 

downward displacement of NO3

–

 in some unsaturated zones 

was the result of mobilization by irrigation-return flow after

rangeland was converted to irrigated cropland.

The chemical traveltimes from land surface to the water 

table are substantially different beneath fast and slow recharge 

paths; this fact has important implications for ground-water 

quality. McMahon and others (2006) suggested that, for water 

moving from land surface to the water table in the High Plains 

aquifer (fig. 4), NO3

–

 fast-path traveltimes are faster within 

irrigated cropland (months to decades) but slower under rangeland (years to centuries). NO3

–

 slow-path traveltimes are faster 

under irrigated cropland (decades to centuries) but slower 

under rangeland (millennia). Some playas likely represent fast 

paths for recharge and chemical transport whereas others may 

represent slow paths, as discussed below.

The High Plains aquifer is limited in its ability to naturally 

attenuate contaminants, such as NO3

– through denitrification,

and it has, in general, slow recharge rates—both of which 

suggest that once the aquifer is contaminated it will remain so 

for decades and even millennia (McMahon and others, 2007). 

The slow denitrification rates would require between 250 to

14,000 years to lower NO3

–

 concentrations by 1 mg/L (as N) in 

ground water of the High Plains aquifer. Additionally, because 

traveltimes through the unsaturated zone are generally long—

decades to millennia along slow flow paths—the amount of

chemical mass reaching the aquifer will most likely increase 

with time. These results highlight the importance of managing 

land use in the High Plains to minimize NO3

–

 concentrations 

in recharge. Additionally, changes in water quality with time 

may affect the ground-water resource in the High Plains aquifer 

(McMahon and others, 2007). The quality of ground water 

generally has been overlooked because the primary focus has 

been on obtaining a sufficient water supply, and it has been

broadly assumed that the High Plains aquifer contains highquality water. For the most part, findings from McMahon and

others (2007) supported that assumption. At some local scales, 

however, particularly where pumping is intense or where topographic settings are conducive to flow paths that are relatively

fast, water quality may be a limiting factor for intended uses 

such as drinking water or irrigation water.

What Are Playas?

Playas are small and shallow closed-basin wetlands that 

have no external drainage and commonly contain ephemeral 

lakes. About 80 percent of playas of the High Plains have surface 

areas that are smaller than 30 acres and are generally less than 

3 ft deep (Pool, 1977; Haukos and Smith, 1992; Fish and others, 

1998). Within the Great Plains region, playas are most abundant 

in the southern and central High Plains of eastern New Mexico, 

western Texas, the panhandle of Oklahoma, southeastern 

Colorado, and southwestern Kansas (Smith, 2003). An estimated 

18,679 playas are on the southern High Plains at a frequency of 

1 to 2 per square mile (Fish and others, 1998; Quillin and others, 

2005), 15,033 playas are on the central High Plains, and 27,671 

playas are on the northern High Plains (McLachlan, 2008). Playas 

also are scattered throughout parts of the central and northern 

High Plains in Nebraska and Wyoming (Smith, 2003), and an 

estimated 16,000 playas are in southwestern Nebraska. Playas 

in Nebraska are found in greatest density in the areas of the 

Southwest Playas, Rainwater Basin, Todd Valley, and Central 

Table (LaGrange, 2005) (fig. 1A). Smith (2003) described in 

detail playas in the Great Plains.

The surface area that drains into playas of the southern 

High Plains is estimated to total 30,000 mi2

 (Ward and 

Huddlestone, 1979), an area that is about 90 percent of the 

southern High Plains (Nativ, 1992). Thus, playas are important 

storage during floods and for irrigation and livestock, provide

habitat for a variety of wildlife species, and are likely an 

important source of recharge to the High Plains aquifer (Steiert 

and Meinzer, 1995; Luo and others, 1997).

Figure 4. Chemical travel times from land surface to water 

table under fast and slow flow paths in rangeland and irrigated 

cropland (modified from McMahon and others, 2006).

Water table

Chemical movement

EXPLANATION

SLOW PATH

Rangeland Chemical

application

Surface water

Years to centuries

chemical traveltime

Chemicals reach

ground water

Chemical

application

Surface water

Months to decades

chemical traveltime

Chemicals reach

ground water

Chemical

application

Millennia

chemical traveltime

Chemicals reach

ground water

Chemical

application

Decades to centuries

chemical traveltime

Chemicals reach

ground water

Unsaturated

zone

Saturated

zone

Saturated

zone

Unsaturated

zone

Not to scale

Not to scale

FAST PATH

Irrigated Cropland
Background 9

Playas hold substantial volumes of water during wet periods 

(Brian Slobe, photographer; published with permission).

The spatial distribution of playas varies across the High Plains 

(Brian Slobe, photographer; published with permission).

and morphological features such as playa area, perimeter, soil 

type, elevation, depth to playa floor, and length of shoreline.

Some National Wetland Inventory data are also available from 

the U.S. Fish and Wildlife Service, and these data are available 

digitally for the entire state of Nebraska (McLachlan, 2008).

The spatial distribution of playas in the southern High 

Plains may not be completely random (Zartman and others, 

2003). Playas tend to be more clustered north of the Canadian 

River, at the eastern edge of the Llano Estacado escarpment, 

and in the southwestern High Plains region (Quillin and 

others, 2005). Lotspeich and others (1971) noted larger but 

fewer playas in the northern half of the southern High Plains, 

which has finer soil cover than in the southern area.

Playas have three distinct physical features (fig. 5)—the

playa floor, which is the flat floor of the playa that is characteristically lined by hydric soils; the annulus, which is the 

sloped surface at the playa margin; and the interplaya region, 

which is the area between the annuli of different playas and 

includes the uplands that drain into playas (Hovorka, 1995). 

Most playas in the southern High Plains are located within 

the Blackwater Draw Formation, which consists of silty 

clay loam sediments (Holliday and others, 1996). The playa 

floor is characterized by 1 to 5 ft of hydric soils and Vertisol

clays (typically Randall clay in the southern High Plains 

and Lodgepole, Fillmore, Scott, and Massie soil series in the 

northern High Plains), which swell when wet and shrink when 

dry to form cracks as much as 3 ft deep (Hovorka, 1997). 

Clay-rich lacustrine sediments occur as much as 30 ft below 

the floor (Parry and Reeves, 1968; Claborn and others, 1985)

and are sometimes interbedded with sand units that reflect the

migration of historical sand across the playa (Hovorka, 1995). 

Additionally, numerous soil horizons buried in the subsurface, 

called paleosols, are common beneath playas and were formed 

under past climate conditions that were more stable and ideal 

for soil development (Hovorka, 1995; Bauchert, 1996). The 

water table of the High Plains aquifer is usually many tens 

of feet below the paleosols. The annulus is characterized by 

interbedded clay and loam that reflect past changes in the

size of playa lakes (Hovorka, 1995). The interplaya settings contain silty clay loam soil horizons and caliche layers 

that are usually many tens of feet thick below land surface 

(Hovorka, 1995). Caliche is a cement-like layer of deposited 

calcium-carbonate material that forms as the result of evaporative concentrations of calcium carbonate in pore waters of 

soils and sediments.

Several hundred test holes were drilled in the floors of

many playas in 1937 and 1938 and indicate caliche layers at 

various depths below many playas (White and others, 1946). 

However, many of these caliche layers contained sand and were 

relatively permeable (Nativ, 1992). Solution channels that are 

common in the caliche may provide pathways for water movement below the playa floor (Lotspeich and others, 1971; Nativ,

1992). More recent test holes drilled in playas indicate a relative 

absence of carbonates beneath playa floors and are interpreted

as evidence of recharge beneath playas (Scanlon and others, 

1994, 1995; Hovorka, 1997).

Many theories have been proposed to describe the physical, chemical, and biological development and formation of 

playas (Zartman and Fish, 1992): for example, animal activity 

(Rettman, 1981), wind erosion (Gilbert, 1895; Reeves, 1966; 

Kuzila, 1994), and dissolution of soil carbonate and piping of 

sediment into the subsurface (Wood and Osterkamp, 1984a). 

Finely and Gustavson (1981) noted a linear array of many 

playas and suggest that playa location may be controlled in 

part by underlying geologic structures. Most recent interpretations conclude that playas formed as the result of complex 

pedogenic, geomorphic, hydrochemical, climatic, and biologic 

processes (Gustavson and others, 1995; Holliday and others, 

1996; Hovorka, 1997).

Numerous studies have characterized the geomorphology 

of playas (Curtis and Beierman, 1980; Osterkamp and 

Wood, 1987; Wood and Osterkamp, 1987; Zartman and Fish, 

1992; Gustavson and others, 1995). From a spatial-analysis 

perspective, the most valuable characterizations to date have 

been the digitization of 20,577 playas across the southern 

High Plains aquifer by Fish and others (1998), Quillin and 

others (2005), and the digitization of 66,000 playas across the 

southern Great Plains by McLachlan (2008). These geographic 

information system data sets includes attributes of physical 
10 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

The capacity to hold water enables playas to support 

a diverse flora and fauna (Bolen and others, 1989; Haukos,

1991; Haukos and Smith, 1993, 1994; Hoagland and Collins, 

1997). A number of plant species are found exclusively in 

playas (Reed, 1930), and many species of birds use playas 

for winter, breeding, and migratory stopover habitats (Curtis 

and Beirman, 1980; Davis and Smith, 1998; Smith, 2003). In 

general, playas hold water and form lakes for many weeks to 

months, and thus the land within and generally immediately 

adjacent to playas is not suitable for crops. Playas may be 

suitable for pastures during the dry periods, however, when 

the lakes are dry.

Playas are important habitat in the High Plains. An American 

coot is pictured here on a playa lake surrounded by smartweed 

(Brian Slobe, photographer; published with permission).

Therefore, many early investigators concluded that recharge 

to the High Plains aquifer is predominantly beneath interplaya 

settings. More recent investigations indicate that a substantial 

portion of the water in playas may infiltrate into the subsurface

and may ultimately recharge the High Plains aquifer (Wood and 

Osterkamp, 1984b; Zartman, 1987; Wood and Sanford, 1994; 

and Wood and others, 1997; Scanlon and Goldsmith, 1997). For 

example, Wood and Osterkamp (1984b) estimated that approximately 80 percent of the water collected in a playa is recharged 

through the playa annulus. Furthermore, Allen and others (1972) 

found no minerals in the playa-floor sediments indicative of

mineral precipitation from evaporation of precipitation. The following sections of this report present information that generally 

supports the interpretation that recharge rates beneath playas 

are greater than recharge rates beneath interplaya settings of the 

southern High Plains.

Hydrology of Playas—An Overview

The hydrology of playas is characterized by cycles of 

inundation and drying out. Once inundated, the hydroperiod

of playas is variable but may last for many weeks to many 

months, although some may remain dry for years (Smith, 

2003; Melcher and Skagen, 2005a). The characteristic wet and 

dry phases of playa hydrology are a function of climate and 

the relatively thin permeable soils of the playa floor. The thick

lacustrine sediments beneath playas that have accumulated 

during many thousands of years (as described previously) 

indicate that playas have periodically flooded throughout their

geologic history (Holliday and others, 1996; Hovorka, 1997).

The large number of playas and their ability to hold large 

volumes of water in an otherwise arid to semiarid climate have 

attracted the attention of many who have studied the hydrology 

of the High Plains aquifer (Nativ, 1992). However, the hydrology of playas has been the center of conflicting hypotheses for

much of the last 60 or more years. In general, the conflicting

hypotheses differ in the relative amount of inundation water that 

is lost to evapotranspiration and the amount of water that infiltrates and becomes recharge to the High Plains aquifer. Some 

studies from the early 1900s indicate that most playa water 

is lost to evaporation and little remains for recharge, whereas 

other studies, many from the late 1900s, indicate that substantial 

volumes of water infiltrate playas and recharge the High Plains

aquifer. The following description of playa hydrology outlines 

the observations and estimates that have been used by those 

supporting the various conflicting hypotheses.

Much of the rain in the High Plains falls from spring 

through fall, which coincides with the period of highest annual 

evapotranspiration (Dvoracek, 1981; Traweek, 1981; Haukos 

and Smith, 1992, 1996). The southern High Plains has a mean 

annual precipitation of 13 to 24 in. and a mean annual potential 

evapotranspiration of 65 to 69 in. (Dugan and Zelt, 2000); 

thus most precipitation is lost to evapotranspiration. However, 

surface runoff (“run-on” in the case of closed-basin playas) 

collects in playas during moderate to intense rainstorms.

Playas of the High Plains are substantially influenced by

surrounding land use. The land surface of the southern High 

Plains generally slopes from the northwest to southeast. As a 

result, the principal drainage area for most playas is north and 

west; a smaller region of drainage lies to the southeast (Claborn 

and others, 1985). The drainage area for most playas includes 

irrigated and nonirrigated cropland and rangeland that may 

be used for livestock grazing. In the city of Lubbock, Texas, 

and other urban environments, playas are important for storm 

drainage and recreation (Hertel and Smith, 1994; West, 1998). 

The runoff into urban playas is commonly allowed to evapotranspire or infiltrate (West, 1998). Playas were frequently used

in agricultural-irrigation systems for tailwater storage and reuse 

(Fish and others, 1998). Guthery and Bryant (1982) reported 

that the number of modified playas on the southern High Plains

increased from approximately 150 in 1965 to 10,800 playas in 

1980. Modification of playas for irrigation systems has direct

effects on the biomass, floral and faunal communities, soil

erosion, and runoff, and it alters nutrient and pesticide input to 

the playas (Bolen and others, 1989).

Findings from many early investigations of playas indicate 

that much of the water that enters playas is lost to evaporatranspiration (Theis, 1937), and as little as 10 percent of water 

entering a playa infiltrates the subsurface (Schwiesow, 1965).
Saturated zone

Interplaya region

Annulus

Playa floor

Bedrock

High Plains aquifer

Water table

Unsaturated zone

Background 11

Surrounding soil texture directly influences the size of

playa lakes. Grubb and Parks (1968) qualitatively noted that 

playas in finer textured soils are larger, have a more extensive

drainage network, and have larger volumes per unit surface 

area than those playas in medium- to coarse-textured soils. 

In addition, for precipitation events of equal duration and 

intensity, surface runoff occurs more often, earlier, and for 

longer duration near playas in clayey and finer grained soils

than near those playas in more loamy soils (Gustavson and 

others, 1995). Surface-water runoff to playa lakes carries 

eroded sediment; clay particles are suspended and tend to 

settle out further toward the middle of the playa floor (Reeves,

1990; Gustavson and others, 1995). Thus, Vertisol soil is most 

common on playa floors and has characteristic vertical soil

structure built during numerous episodes of expansion and 

contraction (wet and dry cycles).

The volume of water that collects in playas of the southern 

High Plains is estimated to range from 1.8 to 5.7 million acre-feet 

per year (Clyma and Lotspeich, 1966; Hauser, 1966; Hauser 

and Lotspeich, 1968; Brown and others, 1978). Zartman and 

Fish (1989) suggested that the annual volume of water in playas, 

if recharged, is equivalent to approximately 4 to 11.5 in./yr of 

recharge throughout the irrigated portion of the southern High 

Plains aquifer. However, the annual volume of water that collects 

in playas and ultimately recharges the High Plains aquifer is difficult to quantify and is not known. The annual volume of water

Figure 5. Principal physical features of playas 

include the playa floor and annulus. The interplaya 

region is the land surface that surrounds playas and 

includes the upland setting that drains into playas.
12 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

in playas is equal to a substantial percentage of the volume of 

water that has been removed from storage by pumping. In Texas 

alone, the total loss of water in storage in the High Plains aquifer 

in the interval from predevelopment to the year 2000 is estimated 

at 124 million acre-feet (McGuire and others, 2003). Dividing 

the total loss of storage in Texas by the number of years since 

predevelopment (2000 – 1957 = 43 years) equals 2.88 million 

acre-feet of water per year that was lost from storage, which is 

within the estimated range of annual water that collects in playas 

(1.8 to 5.7 million acre-feet).

Brown and others (1978) noted that the volume of water that 

collects in playas depends upon the frequency and intensity of 

precipitation and on the characteristics of the drainage area. Runoff rates are slower and runoff generally contains less suspended 

sediment in playas with greater coverage of vegetation (Brown 

and others, 1978). Playas in natural settings are commonly 

flooded for 1 to 3 months per year (Gustavson and others, 1994).

Playas in urban areas that are modified to hold stormwater may

be flooded throughout the year. James (1998) estimated that the

volume of water in five urban playas in Lubbock, Texas, ranged

from 75.8 to 264 acre-feet of water at full stage.

Once water collects in playas, the rate of water loss to 

evaporation is substantial during the summer and fall months 

(Traweek, 1981; Haukos and Smith, 1992) and may be as high 

as 0.5 in./day (Brown and others, 1978). Other estimates indicate as much as 55 to 60 percent of the available water in playas 

is lost to evaporation (Reddell, 1965; Ward and Huddlestone, 

1979). However, Nativ (1992) and Harris and others (1972) 

noted the lack of evaporite minerals within the playa-floor sediments and a lack of halophytic (salt loving) flora indicate that

the playas do not accumulate salts as a result of evaporation. 

Additionally, playa water generally has low salinity (Wells and 

others, 1970; Felty and others, 1972; Lehman, 1972).

Water that is not lost to evapotranspiration may leave the 

playa as infiltration into the subsurface. Infiltration in playas

has been reported to follow three distinct stages (I–III). In 

playas of natural setting that experience seasonal wet and dry 

periods, the infiltration rates during the stage I are relatively

high while the soil is dry. Claborn and others (1985) noted 

that numerous researchers and farmers have observed a rapid 

decline in water levels immediately following a large runoff 

event, followed by a much slower decline of water level as 

playa lakes become shallower. The rapid declines in water 

levels are hypothesized to be due to rapid infiltration through

cracks in the clay-lined floors or to rapid infiltration through

the playa annulus (Claborn and others, 1985). The amount of 

water in the soil controls the rate of infiltration during stage II

infiltration. As the soil becomes wetter, infiltration rates

slow during stage II. Stage III of infiltration occurs if the soil

becomes saturated. In stage III, the infiltration rate is constant

and determined by hydrologic properties of the soil and unsaturated zone. Playas in urban settings that are modified to hold

storm-water drainage year round are likely to have constant 

infiltration rates that indicate stage III infiltration processes.

Caliche, which occurs widely throughout the southern 

High Plains, may act as a second barrier to water flow and

chemical transport beneath the playa floor sediments (Knowles

and others, 1984). However, Stone (1984) and Wood and 

Osterkamp (1984a) observed substantially less dissolved solids 

in soil samples beneath playas than beneath interplaya areas of 

the southern High Plains, which may indicate increased flushing

by percolating water (Nativ, 1992) or that dissolved solids never 

formed beneath playas.

Recharge Rates and Chemistry 

Beneath Playas

Recharge Beneath Playas

For the purpose of synthesizing the existing knowledge 

of recharge beneath playas, the following section has been 

subdivided on the basis of four general types of studies to 

estimate water movement and recharge beneath playas. The 

four study types are water-budget studies, infiltration studies,

unsaturated-zone studies, and ground-water studies. Results 

from each study type provide information on a particular component of water movement through a playa. Additionally, results 

from each study type may represent different spatial and temporal scales of water movement, provide a range of values, and 

have inherent uncertainty that is associated with each rechargeestimation method (Scanlon and others, 2003). Therefore, the 

most reliable recharge estimates come from those studies that 

use many different approaches in an effort to help reduce the 

inherent uncertainties in estimating recharge. Such considerations are necessary when applying recharge estimates in scientific studies or management decisions. The following section

ends with a discussion of the efforts to artificially increase

recharge and the effects of sedimentation and of climate change 

and variability on playa hydrology and recharge.

Water-Budget Studies

Water budgets provide indirect (or residual) estimates 

of infiltration and recharge beneath playas (Reed, 1994; James,

1998; West, 1998). Studies that use water budgets do not 

directly measure infiltration or recharge beneath playas.

The fundamental assumption of a water-budget analysis is 

that the water entering a playa equals the water leaving a playa. 

In the case of playas, the runoff is assumed to leave the playa by 

means of either evapotranspiration or infiltration (West, 1998).

Therefore, if the total volume of water runoff to a playa and the 

total volume of water that leaves the playa because of evapotranspiration are known, then the residual value is assumed to equal 

the volume of water that infiltrates beneath the playa. Waterbudget studies rarely, if ever, use more direct methods to estimate 

infiltration or recharge for the purpose of evaluating the accuracy

and reliability of the water-budget estimates of recharge.

According to Reed (1994), water-budget analyses indicated 

that substantial volumes of water infiltrate beneath playas and

that infiltration rates substantially exceed evaporation rates from
Recharge Rates and Chemistry Beneath Playas 13

playas. James (1998) used water budgets to estimate infiltration

rates of playas in urban settings and reported that infiltration

was substantial and controlled by several factors, including the 

year-round supply of water in urban playas. West (1998) estimated average-infiltration rates beneath urban playas that hold

water year-round to range from 0.06 to 0.56 in./day. Similar 

infiltration rates were reported by James (1998) for five urban

playas; those rates ranged from 0.12 to 1.68 in./day. The estimated volume of daily infiltration beneath the five urban playas

ranged from 4,181 to 30,679 ft3/day. Interestingly, the water 

table beneath Lubbock, Texas, rose substantially during the 

1980s and 1990s, while much of southern High Plains aquifer 

experienced substantial water-table declines (Rainwater and 

Thompson, 1994; McGuire and others, 2003). Kier and others 

(1984) indicated that the rising water table beneath Lubbock 

may have been caused by recharge from the approximately 

100 urban playas in Lubbock (West, 1998) and the reduction 

in ground-water use within the city.

A water-budget study of 22 playas in the southern High 

Plains found that 30 to 50 percent of runoff into playas may 

be available to infiltrate through the playa annulus and may

ultimately become recharge (Claborn and others, 1985). 

However, Claborn and others (1985) used indirect methods 

to estimate the volume of water above the clay-lined floor

and did not collect data that could be used to verify either the 

actual volume of water in the playa or whether water actually 

infiltrated into the annulus.

Although the water-budget method has many advantages, 

including ease and flexibility of use, a number of substantial

limitations reduce the accuracy and reliability of the recharge 

estimated by this method. The accuracy of the recharge estimate 

depends upon how accurately other components in the waterbudget equation are measured, particularly when the magnitude 

of the recharge rate is small relative to that of the other variables 

(Scanlon and others, 2003). To illustrate this point, Scanlon 

and others (2003) noted that errors of 5 to 10 percent in various 

terms of the water-budget equation may result in errors in the 

recharge estimate of more than 100 percent.

Many types of unavoidable measurement errors are 

introduced during water-budget calculations that may lead 

to uncertainty in the estimates of infiltration or recharge

beneath playas. For example, investigators commonly record 

the water-surface elevation in playas to estimate the volume 

of water in a playa with time (West, 1998). This approach 

requires an accurate determination of the playa-floor and

-annulus geometry to accurately use water-surface elevation 

to determine the volume of water in the playa. Surveying 

methods that are used to determine the geometry usually 

introduce unavoidable errors that may lead to uncertainty in 

infiltration or recharge rates. Furthermore, studies that use

a water-budget method rarely collect data about subsurface 

water movement. These data could be used to determine if, in 

fact, water that infiltrates below playas actually intercepts the

water table as recharge. Evapotranspiration has the potential 

to cause subsurface water to move from depth toward the 

land surface, and such movement has been well documented 

in interplaya regions of the southern High Plains aquifer 

(Scanlon and Goldsmith, 1997; McMahon and others, 2006; 

Gurdak, Hanson, and others, 2007). However, studies that use 

only water-budget methods can not determine the potential for 

such lateral or upward water movement. 

Infiltration Studies

A number of studies have directly measured infiltration

in playas (Evans, 1990; Koenig, 1990; Zartman, Evans, and 

Ramsey, 1994, Zartman, Ramsey, and others, 1994, Zartman 

and others, 1996; Huda, 1996; Scanlon and Goldsmith, 1997). 

As Zartman, Evans, and Ramsey (1994), Zartman, Ramsey, and 

others (1994), and Zartman and others (1996) noted recharge 

beneath playas depends upon infiltration within the playa.

However, recharge rates are not typically equivalent to infiltration rates for a number of reasons, which are discussed below.

Reported infiltration rates range from 0 to 116 inches

per hour (in./hr) in playas and from 0.002 to 1.57 in./hr in 

interplaya settings (appendix 2). Infiltration rates are generally reported to be higher near the playa center than in the 

perimeter of the playa floor or in the annulus (Zartman and

others, 1996) (appendix 2). High rates of infiltration in the

playa center are attributed to preferential flow along desiccation cracks in the clay floor (Zartman and others, 1996).

Zartman, Evans, and Ramsey (1994) and Zartman, 

Ramsey, and others (1994) first observed that infiltration beneath a single playa was significantly and positively

related to clay content of the floor. This apparent contradiction

to conventional wisdom (that is, high clay content means low 

infiltration), appears to be caused by rapid water movement

down desiccation cracks in the clay floor. Immediately following ponding, large amounts of water can infiltrate though the

desiccation cracks in the playa floors. Measured infiltration

rates are generally greater during the initial flooding stage and

tend to stabilize after the underlying sediments reach saturation (Evans, 1990; Zartman, Evans, and Ramsey, 1994; and 

Zartman, Ramsey, and others, 1994; Zartman and others, 1996; 

James, 1998). For example, Parker and others (2001) reported 

the following average infiltration rate for two playas at various

times: 1-minute infiltration rates of 10.87 and 13.62 in./hr;

5-minute infiltration rates of 0.31 and 1.20 in./hr; and 60-minute

infiltration rates of 0.05 and 0.09 in./hr (appendix 2). Rapid

initial infiltration rates decrease as ponding water causes clays

to swell and thus seal desiccation cracks and close preferential 

flow paths (Zartman, Evans, and Ramsey, 1994; and Zartman,

Ramsey, and others, 1994).

Although stage I infiltration rates are high in playa soils

because of flow along desiccation cracks, infiltration rates

typically slow as the desiccation cracks seal and reach relatively low stage III infiltration rates that are based on the

saturated-hydraulic conductivity of the soil (Evans, 1990). For 

example, Scanlon and Goldsmith (1997) suggested that even 

when playa floor sediments are fully saturated, they are not

completely impermeable, and they cite a saturated-hydraulic 

conductivity of 2.8×10–5 in./hr from a playa floor as evidence.

Such a saturated-hydraulic conductivity is approximately 
14 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

equivalent to a 0.25 in./yr water flux through the playa floor

(Scanlon and Goldsmith, 1997). Parker and others (2001) 

reported a similar range of saturated-hydraulic conductivities 

for Randall clay soils from floors of two playas in natural

settings. These saturated-hydraulic conductivities (1.7×10–5 

to 7.56×10–3 in./hr) are equivalent to water fluxes through the

playa floor of 0.15 and 66.23 in./yr, respectively. Because playas

are inundated for only a fraction of the year, these water fluxes

likely overestimate the actual annual water fluxes and need to be

divided by the period of inundation for a more accurate annual 

water-flux estimate.

Interestingly, Scanlon and Goldsmith (1997) concluded 

that recharge is strongly related to the volume of ponding in a 

playa and depth of infiltration. The volume of ponded surface

water is directly related to the physical characteristics of the 

individual playa, drainage pattern of the interplaya setting, and 

climate patterns. As evidence of their findings, Scanlon and

Goldsmith (1997) reported that preferential flow was greater

beneath ponding locations in playa floors where surface sediments were initially drier and cracks were more evident than 

in other locations, such as the playa center, that were more 

frequently flooded. Preferential-flow paths in playa floors may

include desiccation cracks, interpedal pores, root tubules, and 

other types of macropores (Scanlon and Goldsmith, 1997).

Unsaturated-Zone Studies 

The relatively thick unsaturated zones of the High Plains 

aquifer are ideal for application of unsaturated-zone techniques 

for estimating recharge, which are commonly used in semiarid 

and arid regions (Scanlon and others, 2003). These studies typically use physical and chemical-tracer techniques and sometimes 

numerical models to estimate recharge. Recharge estimates 

from these techniques generally represent a small spatial scale. 

Physical techniques usually include the use of infiltrometers,

which are field-based instruments that measure infiltration rates.

Chemical-tracer techniques typically include the use of applied 

tracers, such as bromide and nontoxic and visible dyes (Scanlon 

and Goldsmith, 1997), and historical or environmental tracers 

(such as tritium, 3

H) that result from human activities or natural 

evapoconcentration of salts from precipitation (chloride, Cl–

, and 

nitrate, NO3

–

) (Scanlon and Goldsmith, 1997; McMahon and 

others, 2006). Although numerical models have been used extensively in other semiarid and arid regions to estimate recharge, 

the use of numerical models may not be used as commonly 

because of complications posed by shrink-and-swell processes 

that are typical in the clay-lined floors of playas. Most techniques

used during unsaturated-zone studies provide estimates of water 

fluxes through the unsaturated zone and do not directly measure

recharge. Therefore, researchers commonly assume that water 

fluxes in the unsaturated zone (estimated below the depth influenced by evapotranspiration) represent actual recharge rates.

Unsaturated-zone studies report recharge rates that range 

from 0.11 to 4.72 in./yr in playa floors and recharge rates that

range from 0.004 to 1.26 in./yr in interplaya settings (appendix 3). 

The major findings of these studies are in general agreement that

recharge rates are higher beneath playas than beneath interplaya 

settings of the southern High Plains. For example, Scanlon and 

Goldsmith (1997), who conducted one of the most comprehensive unsaturated-zone studies of playas to date, concluded 

that playas increase recharge because of the observed results 

that water contents, water potentials, and tritium concentrations 

were much higher and chloride concentrations were much lower 

beneath playas than beneath interplaya settings.

However, the findings reported by unsaturated-zone studies

are in less agreement about which processes are most important 

in controlling recharge beneath playas. For example, Wood and 

Osterkamp (1984a,b, 1987) suggested that the playa annulus 

acts as the primary recharge zone during periods of ponding. 

Furthermore, they suggest that organic material in the playa is 

oxidized to CO2

, which dissolves in water and forms carbonic 

acid. The carbonic acid may promote dissolution of the underlying caliche, formation of solution channels, and increased 

subsurface porosity. However, the comprehensive data sets of 

Scanlon and Goldsmith (1997) generally support the conceptual 

model that infiltration occurs through playa floors and is not

necessarily restricted to the annular regions around playas.

Many studies (Wood and Osterkamp, 1984a,b, 1987; 

Claborn and others, 1985; Osterkamp and Wood, 1987) concluded that recharge is relatively higher through the annulus 

than through the playa floor; these studies cited coarser sediments in the annulus as evidence. Scanlon and Goldsmith 

(1997), however, reported only slightly coarser sediments in 

the near-surface sediments of the annulus as compared with 

sediments in the corresponding zones beneath the playa floor.

Furthermore, Scanlon and Goldsmith (1997) used total energy 

potential profiles to suggest that water drains more consistently

under playa floors than beneath playa annuli. The total energy

potential profiles beneath some playa annuli indicate higher

water fluxes than beneath corresponding playa floors, whereas

other annular potential profiles indicate lower water fluxes than

beneath playa floors (Scanlon and Goldsmith, 1997). Lower

reported chloride and carbonate concentrations in sediments 

beneath playas than in sediments beneath interplaya settings 

may be evidence of high water fluxes; they may indicate that

either chloride and carbonate never accumulated or that it was 

flushed or dissolved out of the playa profiles (Scanlon and

Goldsmith, 1997).

Scanlon and Goldsmith (1997) reported qualitative evidence of preferential (fast path) flow beneath some playa floors;

however, the preferential flow is apparently terminated at underlying layers of coarser sand. Therefore, the interbedded layers 

of sediment of different origins and hydrologic properties that 

are common in the unsaturated zone beneath playas (Gustavson, 

1996; Hovorka, 1997) may impede rapid or preferential flow

toward the water table. Assuming that most recharge occurs 

because of preferential flow through these desiccation cracks,

Wood and others (1997) estimated that recharge beneath playas 

could be as high as 5.7 to 10.1 in./yr.

Findings from most geologic studies of sediments beneath 

playas generally support the conclusion that recharge rates 

beneath playas are greater than rates beneath interplaya settings 
Recharge Rates and Chemistry Beneath Playas 15

(Holliday and others, 1996; Hovorka, 1997). Hovorka (1997) 

reported no evidence of increased salinity or permanent ponding 

beneath selected playas and offered the interpretation that typical 

playas have been recharging the underlying aquifer throughout 

their geologic history. Hovorka (1997) concluded that recharge 

has always drained playas before evaporation concentrated 

solutes, and neither carbonate nor more soluble salts have accumulated in typical playa sediments. Thus, playa water remains 

relatively fresh compared with water in the approximately 

40 saline lakes in the High Plains (Sanford and Wood, 1995), 

because recharge to the aquifer exceeds evaporation.

Other evidence of frequent ponding and rapid water flux

are the lack of chloride concentrations and calcium carbonate 

(caliche) profiles in the unsaturated zone beneath playas. Maximum chloride concentrations in interplaya-soil water exceed 

those in soil water beneath playas by as much as three orders of 

magnitude (Wood and Sanford, 1995a; Scanlon and Goldsmith, 

1997). Low chloride concentrations in sediments suggest that 

chloride never accumulated or that it was flushed out by rapid

water movement. In contrast, several thousand years of chloride 

accumulation are required to create the chloride concentrations 

found in sediment of interplaya profiles (Scanlon and Goldsmith,

1997). Gustavson and others (1995) observed that all major 

interplaya-soil series that have developed on the southern High 

Plains are calcic soils, which contain substantial secondary accumulations of calcium carbonate, primarily from evaporation and 

evapotranspiration. Low concentrations of calcium carbonate in 

playa sediments are caused by surface-water ponding; dissolution 

of calcium carbonate is facilitated by acidic precipitation, physical flushing by rapid water flux, and by limited plant growth in

playas that minimizes deposition of calcium carbonate normally 

facilitated by evapotranspiration of soil water.

The associated uncertainty from recharge rates that are 

estimated from unsaturated-zone studies may be substantial. For 

example, Scanlon and others (2003) suggested that estimates 

of water flux beneath playas that are based on chloride data are

highly uncertain because of relatively large uncertainties in the 

chloride input to the system (Scanlon and Goldsmith, 1997). 

Uncertainties in the deposition values are reported as a factor of 

–0.5 to 2, which would result in uncertainties in water fluxes of

0.12 to 0.79 in./yr (Scanlon and Goldsmith, 1997). Wood and 

Sanford (1995a,b) provide a recharge estimate (3 in./yr) with an 

error estimate (0.31 in./yr) (appendix 3).

Ground-Water Studies

The ground-water studies of playa recharge have generally used tracer-based techniques that include dating of groundwater age (tritium, 3

H) and environmental tracers (chloride, Cl–). 

Recharge estimates from ground-water studies represent recharge 

across much larger spatial scales than recharge estimates from 

unsaturated-zone studies (Scanlon and others, 2002). Therefore, 

recharge estimates based on ground-water studies in areas of 

playas may be more appropriate for ground-water resource investigations, because ground-water studies provide a more spatially 

averaged recharge rate than the point estimates obtained from 

unsaturated-zone studies (Scanlon and others, 2003). However, 

the spatially averaged recharge rates from ground-water studies 

may not provide the spatial resolution to determine effects from 

any single playa or group of playas.

Early ground-water based recharge estimates, including that of Brown and Signor (1973), reported that less than 

0.07 in./yr (appendix 3) are added to storage to the High 

Plains aquifer as recharge by infiltration from natural rainfall, whereas more than an average of 12 in./yr of water is 

being removed from storage because of pumping. Nativ 

and Smith (1987), using tritium (3

H) in ground water as a 

tracer, estimated recharge beneath playas to range from 0.5 

to 3.24 in./yr. On the basis of a comparison of estimated 

recharge rates in diffuse settings (0.01 to 0.57 in./yr) (Barnes 

and others, 1949; Klemt, 1981; Knowles and others, 1984; 

Stone and McGurk, 1985; Stone, 1990), Nativ and Smith 

(1987) suggested that the High Plains aquifer is predominantly 

recharged by focused percolation from playa lakes. Wood 

and Sanford (1995b), using chloride-mass balance approach 

from ground water (appendix 3), provided a regional estimate 

of 0.43 in./yr recharge to the northern part of the southern 

High Plains. Similarly, Fryar and others (2001) reported 

solute and isotopic data from shallow monitoring wells near 

playas receiving wastewater discharge; these data indicate a 

sequence of episodic precipitation, evaporative concentration 

of solutes, runoff, and infiltration beneath playas. The water

in these wells also indicated return flow from wastewater

and irrigation.

Using results from a ground-water flow model, Mullican

and others (1994) estimated that focused recharge beneath 

playas could be as high as 8.6 in./yr (appendix 3); however, 

these model-based estimates were determined by assuming 

that all water from a regional recharge rate of 0.236 in./yr is 

focused through playas. No data sets were collected to validate 

actual recharge rates beneath the playas.

Artificial Recharge

The large storage volume of playas has prompted many 

questions regarding the ability of playas to supplement water 

resources of the region (Aronovici and others, 1970; Aronovici 

and Schneider, 1972; Palacios, 1981), including artificial

recharge to the High Plains aquifer (Valiant, 1964). Artificial

recharge refers to any manmade modification of playas intended

to increase flow of water toward the water table of the aquifer.

Artificial recharge has been explored as an approach to stabilize

or replenish ground-water supplies from the High Plains aquifer 

(Schwiesow, 1965).

In order to increase infiltration and recharge by reducing

evaporation losses from playas, playa floors have been modified to confine water in smaller, deeper impoundments with less

surface area (Dvoracek, 1981). To increase infiltration, surface

drainage wells have been installed that use gravity to allow 

playa water to flow to the High Plains aquifer (Valiant, 1964).

These wells have been unsuccessful because the high silt content of the playa water quickly clogs the wells and the sediments 

in the aquifer (Claborn and others, 1985). Claborn and others 
16 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

(1985) reported reasonable artificial-recharge rates using water

from playa lakes in a pressure injection system at pressures of 

50 to 80 pounds per square inch. Energy costs and logistical 

considerations, however, restrict use of this approach (Claborn 

and others, 1985).

An alternative to artificial recharge is the direct use of

playa water for irrigation (Dvoracek, 1981), which eliminates 

many of the problems associated with artificial recharge. Jones

and Schneider (1972) suggested that the demand on the High 

Plains aquifer could be reduced by as much as 30 percent 

by direct pumping from playa lakes for irrigation supplies in 

combination with recycling irrigation tailwater and artificially

recharging the aquifer using playa water. However, the direct 

use of playas for irrigation has many limitations. Soil moisture 

is generally sufficient for agricultural requirements in those

seasons when playas fill with water. Therefore, playa water

needs to be stored until later in the season when irrigation water 

is required. Dvoracek (1981) proposed various playa modification schemes that may have various degrees of success in 

reducing water loss to evapotranspiration and increasing the 

efficiency of playa water storage. These modification schemes

have economic costs associated with installation, maintenance 

considerations because of sedimentation, and effects on playa 

ecosystems. Additionally, unmodified playas likely provide

the best resource and habitat for waterfowl and other species 

(Pence, 1981).

One of the first systematic evaluations of the use of playas

to support artificial recharge to the High Plains aquifer was conducted by Brown and others (1978). This evaluation used results 

from at least six field experiments and numerous prior publications regarding the use of playas for artificial recharge (Hauser

and Lotspeich, 1968; Schneider and others, 1971; Aronovici and 

others, 1972; Brown and Signor, 1973; Reeder, 1975; Wood and 

Bassett, 1975). Brown and others (1978) suggested that under 

specific conditions, using water from playas in water-spreading

basins or injection wells may be suitable for artificial recharge

of the High Plains aquifer. Artificial recharge from playa lakes

is more likely to be successful if the water is free of suspended 

sediment and recharged in zones of the aquifer that have high 

infiltration rates and no clay or low-permeability zones in the

unsaturated zone.

Schneider and Jones (1984), who investigated infiltration

in playas that had been modified by excavation of the top layer

of soil, reported infiltration rates that were substantially greater

than in unmodified playas. Infiltration rates in these modified plays were initially high (3.28 feet/day (ft/day)), followed

by slower rates (1.42 ft/day) (Schneider and Jones, 1984). 

Dvoracek and Peterson (1970) observed similar infiltration rates

(1.54 ft/day) in modified playas. However, suspended sediments

in the water column of the modified playas were identified as

the cause of surface sealing and a reduction of infiltration rates

(Schneider and Jones, 1984). Therefore, Schneider and Jones 

(1984) concluded that in order to maintain high infiltration

rates, modified playas require periodic and costly maintenance

to remove or reduce surface seals. Urban and Claborn (1984) 

reported that geotextile materials buried beneath playas have 

had some success at filtering sediments.

Modification of playas affect the hydrology and ecology of the 

playa-wetland system (Brian Slobe, photographer; published 

with permission).

Although previous studies report greater infiltration rates

in modified playas than in unmodified playas, the findings

from such artificial recharge studies need to be considered in

light of infiltration studies (see Infiltration Studies) that report 

substantial infiltration rates in unmodified (or natural) playas.

Therefore, any cost-benefit analysis of artificial recharge needs

to evaluate the potential added benefit that playa modification

might have on increasing infiltration and recharge above the

natural rates and the potential effects of playa modification on

the ecology of the playa-wetland system.

The quality of playa water used for artificial recharge

is of concern because of its possible effects on ground-water 

quality (Felty and others, 1972). For example, Mollhagen and 

others (1993) observed detectable levels of triazine herbicides and aldicarb insecticides in playa water. Water-quality 

concerns stem from legal constraints prohibiting water-quality 

degradation in existing aquifers in Texas. The future use of 

playas for artificial recharge remains uncertain. Manmade

attempts at modifying playas for the purposes of increasing 

recharge are uncertain because of the logistical and economic 

challenges, legal considerations that differ by State, waterquality concerns for the High Plains aquifer, and the importance of playas as habitat for various flora and fauna. There is

a lack of evidence in the literature that describes the possible 

effects of playa modification for artificial recharge on chemical mobilization and water quality of the High Plains aquifer.

Sedimentation

Smith (2003) noted that “...sedimentation is likely the 

single largest immediate threat to the continued existence of 

properly functioning wetlands in the Great Plains today.” The 

accumulation of sediments from upland erosion has shortened 

the hydroperiod, decreased water volume, and increased water 

loss of playas due to evaporation (Tsai and others, 2007). 

These changes to the natural hydrology of playas may reduce 
Recharge Rates and Chemistry Beneath Playas 17

the diversity of flora and fauna habitat and increase flooding and property loss, and they may have effects on recharge 

to the High Plains aquifer (Luo and others, 1997; Smith and 

Haukos, 2002; Tsai and others, 2007).

Playas are typically surrounded by cultivated cropland 

and rangeland that may be used for grazing livestock. Although 

cultivated cropland and livestock grazing in a playa drainage area 

may contribute to a reduction in the cover of perennial vegetation and increase the potential for soil erosion and sediment 

transport to the playas, studies indicate that the sediment load 

from cultivation-dominated drainage areas is substantially larger 

than the load from rangeland-dominated drainage areas (Luo and 

others, 1997; Tsai and others, 2007). Playas in selected cropland 

settings are reported to contain 8.5 times as much sediment as 

those playas in rangeland settings (Luo and others, 1997). Average sedimentation rates (0.19 to 0.38 in./yr) of playas in cropland 

settings are substantially larger than average sedimentation rates 

(0.026 to 0.033 in./yr) reported for rangeland settings (Luo and 

others, 1997). Luo and others (1997) concluded that if sedimentation rates remain approximately constant, sediment could fill

nearly all cropland playas in less than 100 years.

As a result, Federal, State, and privately funded programs 

focus on buffering playas to protect them from sedimentation 

and contamination while simultaneously enhancing wildlife 

habitat (Melcher and Skagen, 2005a,b). Efforts such as the 

Conservation Reserve Program, which has established more 

than 1.7 million acres of perennial grass in the southern High 

Plains, have likely slowed sedimentation rates (Luo and others, 1997). Conservation practices that support native vegetation surrounding playas are likely to trap sediment and reduce 

sedimentation rates to playas (Luo and others, 1997). Given 

the large number of playas, the removal of sediment can be 

restrictively expensive. Melcher and Skagen (2005a,b) summarized wetland protection strategies and best management 

practices, including mitigation buffers that are most appropriate for reducing sedimentation and nonpoint-source contamination in playas of the High Plains region.

Sedimentation of playas has adverse effects on wetland 

structure and function (Luo and others, 1997; Smith, 2003). 

However, the effects of sedimentation on infiltration and

recharge are not clear because of a lack of supporting evidence 

from published scientific studies. Smith (2003) proposed several

factors about sedimentation that likely affect infiltration and

recharge beneath playas. Smith (2003) hypothesized that the 

coarser grained sediment that typically erodes from interplaya 

settings may mix with the clay soils of playas and fill desiccation cracks in the playa floor. Hovorka (1997) identified and

attributed subsurface silt mixed with ancient lacustrine clay 

layers as evidence of silt deposition during dry periods within 

the geologic history of the playa. However, no studies were 

identified during this literature search that provide evidence

that coarser grained materials are currently filling desiccation

cracks in clay-lined playa floors during dry periods. Furthermore, Parker and others (2001) found limited evidence of more 

permeable material deposited in desiccation cracks of playafloor soils that would provide conduits for water flow even after

the cracks had sealed as the result of wetting. As Smith (2003) 

noted, it is unknown whether this process is occurring and 

increasing infiltration, thus enhancing recharge.

Sedimentation results in shallower playas that have less 

total volume and possibly larger surface areas, and that are 

more likely to overflow and flood areas outside the playa floor

and annulus (Smith, 2003). Larger surface areas over shallower playas are subject to more rapid evaporation than playas 

that have a deeper annuli and smaller surface area (Smith, 

2003). Less water may be available for infiltration if evaporation rates increase as playas fill with sediment.

Finally, sedimentation may result in a clay-lined playa 

floor completely covered with sediment from interplaya

settings. It is unknown how such a surface layer will affect the 

playa floor’s shrink-swell properties, which result in desiccation

cracks that have been identified as important controls on rapid

infiltration and recharge. The sediment also may allow the

underlying clay to maintain a higher moisture content, thus 

preventing the formation of desiccation cracks. Future studies 

are needed to determine if the desiccation cracks form near the 

surface under layers of sediment and how infiltration characteristics may change in playa floors under sedimentation.

Climate Change and Variability

Anthropogenic climate change and natural climate variability are likely to have substantial effects on global water resources, 

including those across the Great Plains (Intergovernmental Panel 

on Climate Change, 2007). John Matthews (World Wildlife 

Fund, written commun., 2008) outlines possible effects of climate 

Agricultural lands surround many playas of the southern 

High Plains and directly affect the transport of sediment 

and contaminants to playas (Brian Slobe, photographer; 

published with permission).
18 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

change and variability on playa habitat and biodiversity. Climate 

change and variability may have important effects on infiltration and recharge beneath playas; however, no studies to date 

(2008) and to the knowledge of the authors specifically explored

how climate change and variability may alter recharge beneath 

playas. Therefore, the following section briefly outlines a few

climate projections noted by John Matthews (World Wildlife 

Fund, written commun., 2008) and possible responses of recharge 

beneath playas; these responses are based on the playa hydrology 

and recharge processes outlined in prior sections of this report.

During the next 30 to 100 years, the Great Plains may 

receive less snowfall in winter, the snow will begin falling later 

and melt earlier, and more winter precipitation will be rain rather 

than snow (Intergovernmental Panel on Climate Change, 2007). 

Under such a climate scenario of more winter rainfall, winter 

recharge beneath playas might increase because of the relative 

lack of water loss due to evapotranspiration during winter as compared with summer evapotranspiration loss. Research is needed to 

test this hypothesis.

The Intergovernmental Panel on Climate Change (2007) 

reported that annual precipitation across parts of the Great 

Plains is likely to decrease; the largest decreases are predicted 

in the southern High Plains region, especially New Mexico 

and Texas. Under such climate scenarios of less annual precipitation, the potential for recharge beneath playas may decrease 

because there is less water to run off and collect, infiltrate the

playa sediments, and ultimately recharge the aquifer. Research 

is needed to test this hypothesis.

In contrast to the southern High Plains, the northern 

High Plains, especially part of Nebraska, may have substantial 

increases in precipitation during the summer during the next 

30 to 100 years (Intergovernmental Panel on Climate Change, 

2007). Precipitation across the Great Plains region, however, is 

likely to continue to be highly random with great local variation 

in amounts and intensity (Nippert and others, 2006), which could 

result in local droughts and regional flooding (Covich and others,

1997). Under increased summer precipitation in the northern 

High Plains, recharge beneath playas may be increased. However, 

higher rates and intensity of precipitation may increase erosion 

and sedimentation rates of playas (John Matthews, World Wildlife 

Fund, written commun., 2008). Sediment may bury the playa 

floors lined with shrink-swell clay, which have been previously

identified as important conduits for infiltration and recharge.

Therefore, recharge beneath playas, as well as the wetland habitat, 

may be reduced under such climate projections. Research is 

needed to test these and other hypotheses regarding the effects of 

climate change and variability on the function of playa wetlands 

and recharge to the High Plains aquifer.

Recharge Chemistry Beneath Playas

The chemistry of recharge beneath playas is a more recent 

topic of study that has been motivated by concerns about 

ground-water quality of the High Plains aquifer and the detection of elevated concentrations of nitrate (NO3

–

 as N), dissolved 

solids, pesticides, and other chemicals in ground water that may 

be harmful to humans and animals. The biodiversity of playas 

is also at risk from nonpoint-source contamination from soil 

erosion, agricultural runoff, and direct dumping of wastes into 

playas. For example, an estimated 35 to 70 billion ft3

/year of 

irrigation tailwater, which is about 20 percent of the irrigation 

water pumped from the High Plains aquifer, flowed into playas

during the 1960s and 1970s alone (Bolen and others, 1989). 

Possible nonpoint-source contaminants in playa water may 

include nutrients, chemical fertilizers and pesticides, feedlot 

runoff, manure fertilizer, urban wastewater, organic chemicals, and trace metals (Irwin and others, 1996). The increased 

recharge rates beneath playas, as described in the Recharge 

Beneath Playas section, could be of concern if the recharge 

chemistry is of poor quality. Similar to the format of the previous section on recharge rates, the following section synthesizes 

the movement and reactions of chemicals from water in playa 

lakes and subsurface processes underlying the playa affecting 

recharge chemistry to the High Plains aquifer.

Water Quality of Playa Lakes

More than 25 studies have collected various types of waterquality data from playas of the southern High Plains aquifer 

(Casula, 1995). The objectives of most studies are synoptic in 

nature and include data collection of playa-water quality at a 

particular time and place (Sublette and Sublette, 1967; Rekers 

and others, 1970; Bureau of Reclamation, 1982; Nelson and 

others, 1983; Buck, 1989; Huang, 1992). The objectives of other 

water-quality studies vary and include the evaluation of playas 

for mosquito habitat (Ward, 1964); the suitability of playas as a 

source of water for irrigation (Lotspeich and others, 1969); the 

effects of agricultural-wastewater runoff and land-use effects 

on playa-water quality (Felty and others, 1972; Mollhagen and 

others, 1993; Pezzolesi, 1994; Irwin and others, 1996; Thurman 

and others, 2000; Purdy, Straus, Harp, and others, 2001; Purdy, 

Straus, Parker, and others, 2001; Hudak, 2002); the suitability 

of playas as storage reservoirs (Reeves, 1970); the potential 

effects on ground-water quality from artificial or natural recharge

beneath playas (Wells and others, 1970; Wood and Osterkamp, 

1984a,b; Ramsey and others,1988, 1994); the effects on wetland 

habitat (Horne, 1974; Parks, 1975; Becerra-Munoz, 2007); and 

the presence of waterborne-bacterial pathogens (Westerfield,

1996; Warren, 1998; Hamilton, 2002). 

The water quality of playa lakes has been reported to 

differ greatly in space and time because of physical characteristics of the playa floor and annulus, soil and land-use characteristics of the interplaya settings, and variability in the annual 

and interannual cycles of precipitation, evaporation, and 

infiltration that affect erosion and runoff chemistry (Curtis and

Beierman, 1980; Casula, 1995; Hall and others, 1995; Willig 

and others, 1995; Fish and others, 1998). Runoff and material 

transported into playas is proportional to drainage area. 

Casula (1995) reported that many water-quality constituents 

show moderate positive correlations between playa drainage 

area; those constituents include total-dissolved solids (TDS), 

chloride, sulfate, alkalinity, pesticides, and pH. Lake area is 
Recharge Rates and Chemistry Beneath Playas 19

reported to inversely correlate with TDS, specific conductivity,

chloride, sulfate, pH, and many pesticides and may directly 

correlate with dilution of chemical constituents (Casula, 

1995). Casula (1995) did not obtain strong statistical relations 

between playa characteristics and water quality and attributes 

those findings to a lack of land-use variables in the statistical

models. Fish and others (1998) suggested that temporal variability that is likely caused by changes in climate may present 

a substantial challenge to understanding the effects of land use 

on spatial variability of playa-water quality.

Playa lakes commonly contain water with less than 

200 mg/L dissolved solids and 400 to 500 mg/L suspended 

solids (Wood and Osterkamp, 1987; Zartman and others, 2001), 

which is characteristic of freshwater lakes and different from the 

approximately 40 saline lakes present in the region (Wood and 

Osterkamp, 1987). The lack of saline playa water, lack of salt 

accumulation in the playa sediments, and presence of freshwater flora in playas indicates that evaporation that produces

salts is not a dominant process affecting water quality. Many 

researchers suggest that if playa water is lost solely from evaporation, salts and minerals would be concentrated in the water 

and sediment and more halophytic (salt-loving) flora would be

present (Smith, 2003).

One of the more recent and spatially extensive surveys 

of water-quality conditions in 99 playa lakes throughout the 

southern High Plains reported elevated concentrations of nitrate 

(1.64 to 4.23 mg/L as N) and arsenic (5.10 to 67.0 μg/L), 

and numerous pesticide compounds (Mollhagen and others, 

1993). Although the range of nitrate concentrations exceeds the 

background concentration of 4 mg/L (as N) in ground water of 

the High Plains aquifer (Gurdak and Qi, 2006), these concentrations do not exceed the MCL for drinking water (10 mg/L as N) 

(U.S. Environmental Protection Agency, 2008). However, 59 

playas from this survey contained arsenic at concentrations that 

exceed the MCL for drinking water (10 μg/L) (Mollhagen and 

others, 1993; U.S. Environmental Protection Agency, 2008). 

Arsenic concentrations in playa lakes sampled by Mollhagen 

and others (1993) range from 5.10 to 67.0 μg/L and have an 

average concentration of 13.1 μg/L.

Playa water is not used for direct human consumption; 

even so, elevated arsenic in recharge water could pose a health 

concern. Fahlquist (2003) detected elevated arsenic concentrations in ground water of domestic-supply wells at concentrations ranging from 1.7 to 107 μg/L, and 14 (of 48) samples 

exceeded the MCL. Arsenic concentrations in ground water of 

the High Plains aquifer have been suggested to originate from 

organic-rich shale, volcanic ash, discharge from saline lakes, 

or oilfield brines (Fahlquist, 2003). Others suggest that historical use of arsenic-based pesticides and defoliants elevated the 

background concentrations of arsenic in the soil surrounding 

the playa (Mollhagen and others, 1993). Furthermore, Thurman and others (2000) reported the detection of a number of 

the major cotton and corn herbicides and many of their metabolites (daughter products) in playa water. However, Thurman 

and others (2000) did not collect ground-water-quality data 

to determine if the herbicides or metabolites have reached the 

ground water beneath playas.

The southern High Plains is well known for its large 

confined-beef-cattle feeding operations (Parker and others,

2001). An estimated 7 million cattle are fed in these operations 

(Southwest Public Service, 1999). Approximately one-half of 

the confined-animal feeding operations in the High Plains use

playas as collection basins for feedyard runoff and as storage 

basins until solid manure can be dredged for use as agricultural fertilizer (McReynolds, 1994). Playas in such operations are often modified to include primary storage ponds of

sedimentation basins between the feedyard and playa (Purdy, 

Straus, Harp, and others, 2001). These modifications help to

catch storm-water runoff that may contain manure, sediment, 

and other chemicals.

The effects of confined-animal feeding operations on

water quality of the playas has not been extensively studied 

(Purdy, Straus, Harp, and others, 2001). Those studies of 

water quality of playas in such operations reported elevated 

concentrations of nutrients, salts, and pathogens, and elevated 

biochemical oxygen demand (Sweeten, 1994); those playas 

generally have lower quality water than natural playas (Parker 

and others, 2001).

Purdy, Straus, Parker, and others (2001) studied the 

effects of feedyards on endotoxin concentrations, fecal coliform count, and other water-quality conditions during winter 

and summer in playas that are located in confined-animal

feeding operations. Although Purdy, Straus, Parker, and others 

(2001) found that such activities reduce playa water quality, 

including endotoxin concentrations, general water quality, and 

fecal coliform counts, they suggested that such deteriorated 

playa-water quality likely does not pose a threat to human or 

animal health or the environment if the water remains in the 

playas. The authors based their conclusions “on the premise 

that feedyard playas play a minor role in recharging ground 

water” (Purdy, Straus, Parker, and others, 2001). However, 

Purdy, Straus, Parker, and others (2001) also stated that there 

is an urgent need to examine ground-water recharge from 

playas because it is unknown what role playas used in these 

feeding operations play in recharging the perched aquifers and 

Playas are often modified to hold stormwater runoff from feedyards 

(Brian Slobe, photographer; published with permission).
20 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

the deeper High Plains aquifer. Purdy, Straus, Harp, and others 

(2001) and Purdy, Straus, Parker, and others (2001) concluded 

that livestock should not be allowed to access playas that 

receive runoff from confined-animal feeding operations and

that water removed from playas in these feeding operations 

may have serious effects on the health of cattle and humans.

Although recharge is not well characterized beneath most 

playas in such feeding operations, conditions in feedyard playas 

may help to minimize chemical mobilization. For example, it 

has been hypothesized that animal wastes and a certain bacterium create elastic slime that may help to seal the playa floors

(Lehman and Clark, 1975; Stewart and others, 1994; Purdy, 

Straus, Harp, and others, 2001; Purdy, Straus, Parker, and 

others, 2001). Additionally, reports of ground-water quality near 

selected beef cattle feedyards indicated no substantial effects on 

ground-water quality from the feeding operations (Sweeten and 

others, 1995).

Subsurface Processes Affecting 

Recharge Chemistry

Previous studies provide evidence of direct relations 

between the water quality of playa lakes, subsurface processes, 

and resulting chemistry of recharge to ground water of the High 

Plains aquifer. The biogeochemical processes in saturated or 

inundated playa sediments can have substantial effects on the 

chemistry of recharge (Pezzolesi and others, 2000). The inundation of and biological activity in playas affects dissolved oxygen 

in playa waters, which influences the movement of nutrients,

trace metals, and organic chemicals and, in turn, the decomposition of organic matter (Pezzolesi and others, 1995, 2000).

Because of the dominance of cotton production throughout the southern High Plains and the historical use of arsenicbased and organochlorine pesticides on this crop, greater 

concentrations of arsenic and other trace metals have been 

hypothesized to occur in the soils of playas surrounded by 

cotton crops (Irwin and others, 1996; Venne and others, 2006). 

Although some studies have shown arsenic concentrations 

in playa sediments that are generally 6 to 7 times as great as 

worldwide soil-background levels, results generally indicate 

no substantial differences in trace-metal concentrations found 

in soils from playas in cropland and rangeland settings (Irwin 

and others, 1996; Venne and others, 2006, 2008). Moreover, 

Venne and others (2006) found that trace-metal concentrations 

in sediments were at least 5 times as high as concentrations 

in amphibian tissue, which indicates that bioaccumulation 

of metals did not occur. This study concluded that no apparent relation exists between land use (cropland and natural 

grassland), trace-metal concentrations in playa sediments, and 

trace-metal concentrations in amphibians. Trace-metal concentrations may be ubiquitously distributed in playa sediments of 

the southern High Plains (Venne and others, 2006).

Evidence supports substantial differences in soil chemistry between playas receiving wastewater from confined animal

feeding operations and those playas in natural settings. Stewart 

and others (1994) reported total soil N ranging from 3,000 to 

4,000 milligrams per kilogram (mg/kg) and soil phosphorus 

(P) to be 2,000 mg/kg from playas receiving wastewater from 

beef and dairy lots. By comparison, soils of playas not receiving feedlot wastewater had approximately 168 mg/kg total N 

and 28 mg/kg total P (Haukos and Smith, 1996). However, 

other studies have shown that playa wetlands are effective at 

filtering nutrients (N and P) through biomass uptake (Pezzolesi

and others, 1998).

Numerous studies have shown that NO3

–

 is attenuated in 

soils of playas receiving runoff from confined-animal feeding

operations and in treated sewage and industrial waste (Fryar 

and others, 2000, 2001). Fryar and others (2000) observed that 

ponded surface water impeded oxygen diffusion and caused 

anaerobic conditions in the near surface of playa-floor sediments, thus promoting denitrification of nitrate within playas.

Additionally, chloride concentrations in sediments beneath 

playas receiving feedyard wastewater have been observed 

to increase with time and depth (Clark and others, 1975). 

However, nitrate concentrations did not increase with depth or 

time below playas receiving feedyard wastewater, which likely 

indicates that denitrification is removing nitrate (Clark and

others, 1975). As a result, smaller concentrations of nitrate are 

likely present in recharge beneath some playas.

However, the previously mentioned studies focused on 

playas that were continuously flooded. Fryar and others (2000)

noted that the removal of nitrate by denitrification is likely to

be more temporally and spatially variable in playas that are not 

continuously flooded, such as those found in natural settings.

Fryar and others (2000) reported elevated nitrate in ground 

water in the vicinity of one playa that received wastewater. 

Playas that have short and frequent episodes of flooding and

drying are more likely to have desiccation cracks that promote aerobic conditions in the soil as well as the potential for 

rapid macropore flow. Denitrification in the playa subsurface

limits but does not preclude ground-water contamination 

resulting from wastewater discharge to playas or from other 

playas that focus recharge (Fryar and others, 2000). Furthermore, geochemical conditions that promote denitrification

may promote mobilization and delivery of trace metals and 

some organic compounds in recharge. For example, Thurman 

and others (2000) speculated that metabolites from cotton 

herbicides may have the ability to leach from subsurface 

sediment beneath playas into the ground water. However, 

very few studies have installed monitoring wells immediately 

downgradient from playas to evaluate recharge chemistry from 

playas. Consequently, and as first noted by Fryar and others

(2000), additional monitoring of ground-water quality near 

playas—especially those that receive feedlot wastewater—

is warranted.

Additionally, no research examined the effects of artificial recharge on the fate and mobilization of contaminants

in the modified playas. The practice of removing clay-lined

floors and dredging playa sediments may reduce the natural

attenuation capacity of the playa-wetland system and possibly increase the mobilization of some contaminants moving 

toward the water table.
Conclusion Regarding Conceptual Model of Recharge Beneath Playas 21

Conclusion Regarding Conceptual 

Model of Recharge Beneath Playas

Three prominent conceptual models of recharge beneath 

playa have emerged from the literature synthesis:

1. Playas are evaporation pans (for example, Lehman, 1972; 

Claborn and others, 1985).

2. Playas are not exclusively evaporation pans, and recharge is 

restricted to the annulus of playa (for example, Osterkamp 

and Wood, 1987; Wood and Osterkamp, 1987).

3. Playas are not exclusively evaporation pans, and recharge 

is focused through clay soils of the playa floor (for example, Broadhurst, 1942; White and others, 1946; Wood and 

Sanford, 1995a,b; Scanlon and Goldsmith, 1997; Wood 

and others, 1997).

The synthesis provided in this report demonstrates that 

playa focus recharge is possible at substantially (1 to 2 orders 

of magnitude) higher rates than in interplaya areas of the 

southern High Plains aquifer (fig. 6A, appendix 3); it thus provides evidence against interpreting playas as strictly evaporative 

pans (conceptual model 1). Higher recharge rates beneath playas 

are supported by high water flux, contents, and potentials; by

low chloride and high tritium concentrations in the pore water; 

and by low caliche content in the sediments. Additionally, 

infiltration rates are significantly and positively related to clay

content of the floor. This apparent contradiction of conventional

wisdom is caused by rapid infiltration down desiccation cracks.

A

B

C

D

E

B

High Plains aquifer

Water table

RECHARGE ESTIMATES, INCHES PER YEAR

A

B

C

D

E

Regional

Interplaya

Playa

RECHARGE SETTING

A

Nonspecific

Nonspecific

Irrigated

cropland

Nonirrigated

cropland

Rangeland

Annulus

Floor

0.001 0.01 0.1 1 10 100

Figure 6. A, The recharge estimates listed in appendix 3 are 

summarized for each recharge setting (A–E); they generally 

indicate larger recharge rates beneath playas than beneath 

interplaya settings (note: x-axis is logarithmic). The size of 

arrows in (B) shows relative magnitude of recharge rates, which 

are approximations based on reported values and locations. 

Letters A–E refer to recharge settings lettered similarly in figure 

parts A and B.
22 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

The rapid infiltration rate decreases as ponded water causes

expansion of the soil matrix and sealing of desiccation cracks. 

Studies report strong correlations between ponding and depth of 

infiltration and recharge, evidence of water movement beneath

clay-lined playas, and limited movement through the playa 

annulus; these studies do not support the interpretation that 

recharge is restricted to the playa annulus (conceptual model 2).

Many questions remain regarding factors that control 

recharge beneath playas; however, conceptual model 3 is best 

supported by most published findings for recharge to the High

Plains aquifer (fig. 6A). Reported recharge rates beneath playa 

floors range from about 0.01 to more than 10 in./yr, whereas

most interplaya settings in croplands and rangelands have 

recharge rates reported to range from about 0.01 to 1 in./yr 

(fig. 6A). Although reported recharge estimates through the 

playa annulus range from about 0.5 to 5 in./yr and are generally higher than most estimates reported for interplaya 

settings, a number of recharge estimates through the playa 

floor are higher than the reported rates through the annulus

(fig. 6A). The reported recharge rates for nonspecific regional

recharge settings (fig. 6A) range from about 0.05 to almost 

10 in./yr, which is similar to the range reported for recharge 

beneath playa floors. However, the nature of regional-recharge

methods makes it very difficult to distinguish focusedrecharge processes from diffuse-recharge processes. The 

nonspecific regional recharge estimates may reflect an average or integration of recharge beneath playas and recharge 

beneath interplaya settings (fig. 6A). Therefore, without 

recharge contributions from playas, regional recharge to the 

southern High Plains aquifer could possibly be 1 to 2 orders 

of magnitude smaller. Properly functioning playa wetlands, 

which have shrink-swell soils that produce desiccation cracks 

and rapid infiltration rates, are thus important for the overall

recharge contribution to the southern High Plains aquifer. The 

literature synthesis described in this report did not evaluate 

recharge beneath playas of the northern High Plains because 

no published studies were identified. Therefore, until future

research on playas of the northern High Plains is published, it 

is unknown if recharge processes beneath playas in that region 

are similar to those in the southern High Plains.

Needs for Future Research

The synthesis of previous studies that is outlined in this 

report indicates that a number of gaps remain in understanding 

and predicting recharge rates and chemistry beneath playas of 

the High Plains aquifer. The conditions in and around playas 

that control recharge rates and chemistry have a direct effect 

on the diversity of flora and fauna in the playa, land-use

characteristics for farmers and ranchers, and the future sustainability of ground water in the High Plains aquifer. Therefore, 

a number of important research needs remain. These needs are 

posed as the following questions to help address existing gaps 

in the current state of knowledge about recharge and chemical 

transport beneath playas of the region.

What is still unknown about recharge and chemical 

transport beneath playas to the High Plains aquifer?

Additional data are needed to support an understanding 

of the subsurface rate of water movement and fate of chemicals after infiltration in the annulus or through the playa floor.

For example, the potential for lateral movement of water from 

the annulus to interplaya sediments and subsequent loss to 

evapotranspiration is unknown. More important, a relatively 

small number of recharge estimates have used unsaturatedzone or ground-water studies, which provide much more 

meaningful and detailed estimates of recharge than waterbudget or infiltration studies. Additionally, current research

does not clearly describe how playa modification for artificial

recharge affects the fate of contaminants in playas and mobilization toward the water table. A number of specific knowledge

gaps remain and include the effects of sedimentation on 

infiltration rates in playa floors and the shrink-swell characteristics of Vertisol soils, transport of organic chemicals and trace 

metals under anaerobic subsurface conditions in playa sediments and under playa modifications for artificial recharge,

and the effects of climate variability and climate change on 

the hydrology and recharge potential of playas. Future studies 

that develop predictive models of recharge rates and chemistry 

beneath playas will likely provide valuable tools for playa and 

ground-water management and conservation. Are innovative 

and wetland-friendly approaches for artificial recharge beneath

playas possible? How important are playas for recharge to the

northern High Plains aquifer, for which comparatively little 

research has been reported?

What scientific approaches should be considered before 

implementation of best management practices?

As Melcher and Skagen (2005a) suggested, interdisciplinary and collaborative scientific studies are needed. Collaborative studies between geologists, hydrologists, ecologists, 

biologists, agronomists, and land-conservation scientists will 

likely result in knowledge that best fills the remaining gaps in

information about playas.

Future studies concerned with the role of playas in recharging the High Plains aquifer will likely refine conceptual model

3 of recharge by using a systematic approach on various spatial 

and temporal scales and by using a wide range of hydrologic, 

biogeochemical, and isotopic methods. As described by Nativ 

(1992), a systematic approach would include data collection of 

the amount of precipitation and runoff to a playa, the volume 

of water stored and variations with time, the evapotranspiration 

of water from the playa, and changes with water contents and 

total-potential gradients beneath playas and corresponding interplaya areas with time. Additional information may be gained 

by using biogeochemical and isotopic indicators that trace 

water and chemical directions and rates of movement within 

the subsurface.
References Cited 23

Acknowledgments

The authors thank the Playa Lakes Joint Venture (PLJV) 

for its support of this study. The authors thank Bill Johnson 

(Texas Parks and Wildlife Department), Ted LaGrange 

(Nebraska Game and Parks Commission), Suzanne Paschke 

(USGS), Mike Carter (PLJV), Jeff Ver Steeg (PLJV), 

Christopher Rustay (PLJV), Megan McLachlan (PLJV), and 

Debbie Slobe (PLJV) for providing constructive reviews of 

earlier versions of this report.

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30 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

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Appendixes

Appendix 1 33

A

acre feet of water A unit of volume equal 

to 1 acre of surface area to a depth of 1 foot 

that approximately equals 43,500 cubic feet or 

325,851.4 gallons of water.

anaerobic As used in this report, lack of 

oxygen in the soil, unsaturated zone, or water.

annulus The sloped surface at the margin of 

a playa that separates the playa floor from the

interplaya region.

aquifer A geologic formation, group of formations, or part of a formation that contains 

sufficient saturated permeable material to

yield useable quantities of water to springs 

and wells.

artificial recharge Recharge at a rate greater 

than natural, resulting from deliberate or incidental human activities.

C

caliche A white and cement-like layer of 

calcium carbonate (CaCO3

) that is deposited 

in the shallow subsurface. Caliche is common in semiarid and arid regions; it forms by 

evaporative concentration of calcium carbonate contained in pore water that originated as 

precipitation.

central High Plains See High Plains aquifer.

central High Plains aquifer The part of the 

High Plains aquifer system that underlies the 

central High Plains.

conceptual model A mental model or idea 

of the specific workings of a particular physical, chemical, or biological process. Conceptual models are commonly developed by 

scientists to test specific hypotheses and often

represent the initial step in developing more 

quantitative models of the process of interest.

D

diffuse recharge A type of recharge in 

which precipitation or melting snow infiltrates throughout a uniform area of an aquifer, 

Appendix 1. Glossary

Note: Words in bold and italics throughout the report are described in this glossary.

percolates relatively uniformly through the 

unsaturated zone, and eventually intercepts 

the water table. Diffuse recharge often occurs 

along slow flow paths.

E

endotoxin A potentially toxic natural compound found inside pathogens such as bacteria 

and released when bacteria die.

ephemeral Lakes, wetlands, streams, or 

other bodies of water that are intermittently 

wet and dry.

evaporation The transformation of a liquid 

to a vapor.

evapotranspiration — The loss of water from 

a given area by evaporation from the land 

combined with transpiration (loss of water to 

the atmosphere) from plants.

F

focused recharge A type of recharge characterized by rapid movement of water through 

the soil and unsaturated zone that bypasses a 

large portion of the soil and unsaturated-zone 

matrix. Focused recharge often occurs along 

fast flow paths.

G

ground-water availability The amount 

of ground water that is available to support 

current uses of a particular aquifer or groundwater resource.

ground-water sustainability The amount 

of ground water that will be available to 

support future uses of a particular aquifer 

or ground-water resource. Alley and others 

(2002) state that ground-water sustainability is the development and use of ground 

water in such a manner that can be maintained for an indefinite time without causing

unacceptable environmental, economic, or 

social consequences.
34 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

H

hydric soil A characteristic soil of many 

wetlands that are saturated or flooded for

prolonged periods of time, which produces 

anaerobic conditions.

hydroperiod The number of consecutive 

days that a wetland is inundated with surface 

water.

High Plains aquifer The High Plains aquifer 

(174,000 square miles) underlies parts of eight 

States (Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and 

Wyoming) and can be divided into northern, 

central, and southern subregions of the High 

Plains (fig. 1B). The High Plains aquifer, 

which is commonly known as the Ogallala 

aquifer, contains six primary hydrogeologic 

units (fig. 1C), of which the Ogallala Formation is the largest.

I

infiltration The process by which precipitation or melting snow enters soil or rock across 

its interface with the atmosphere (Wilson 

and Moore, 1998). Infiltrated water may be

consumed by evapotranspiration or become 

deeper percolation and recharge. Infiltration may follow three distinct stages (I–III). 

The infiltration rates during the stage I are

relatively high while the soil is dry. The 

amount of water in the soil controls the rate 

of infiltration during stage II infiltration. As

the soil becomes wetter, infiltration rates slow

during stage II. Stage III of infiltration occurs

if the soil becomes saturated. In stage III, the 

infiltration rate is constant and determined by

hydrologic properties of the soil and unsaturated zone.

interplaya The land surface areas that surround playas and include upland settings that 

drain into playas.

L

Llano Estacado A term that is used synonymously with the southern High Plains region. 

The Llano Estacado is bounded by the Canadian 

River to the north, the caprock escarpment to 

the east, Edwards Plateau to the south, and 

the Pecos River Valley to the west. The Llano 

Estacado is estimated to be 98 percent internally drained by an estimated 20,500 playas 

(Osterkamp and Wood, 1984). Discharge from 

the Llano Estacado is found in an estimated 30 

saline lake basins and along the eastern caprock 

escarpment.

M

milligrams per liter (mg/L) A unit expressing 

the concentration of chemical constituents in 

solution as weight (milligrams) of solute per 

unit volume (liter) of water; equivalent to one 

part per million in most surface and ground 

water. One thousand micrograms per liter 

equals 1 mg/L.

N

northern High Plains See High Plains aquifer.

northern High Plains aquifer The part of the 

High Plains aquifer system that underlies the 

northern High Plains.

O

Ogallala aquifer See High Plains aquifer.

P

pedogenic Processes related to soil 

development.

percolation The process of water moving vertically or laterally through the soil or 

unsaturated zone (Wilson and Moore, 1998).

pesticide A chemical applied to crops, 

rights-of-way, lawns, or residences to control 

weeds, insects, fungi, nematodes, rodents, and 

other “pests.”

potential evapotranspiration The theoretical 

combined loss of water from a given area by 

evaporation from the land and transpiration 

(loss of water to the atmosphere) from plants.

playa As described by Smith (2003), playas 

are “...shallow, depressional recharge wetlands occurring in the Great Plains region that 

are formed through a combination of wind, 

wave, and dissolution processes with each 

wetland existing in its own watershed. As 

the words depressional and recharge imply, 

Great Plains playas only receive water from 

precipitation and runoff. Naturally, water is 

only lost through evaporation, transpiration, 

and recharge.” Although by definition playas

are wetlands, the water that collects in playas 

often forms lakes and as a result are often 

referred to as “playa lakes or lakes.”
Appendix 1 35

predevelopment As used in this report, the 

“predevelopment” period is the period before 

extensive ground-water pumping; conditions 

in the aquifer in the “predevelopment” period 

represent its undisturbed state. McGuire and 

others (2003) indicated that the median measurement year in the predevelopment period 

was 1957.

R

rangeland As used in this report, rangeland 

is native short- and mid-grass prairie that was 

never cultivated but may or may not be used 

for cattle grazing.

recharge The flux of water to ground water.

As used in this report, recharge is the vertical, volumetric flux of water across the water

table or saturated zone of an aquifer. Rates of 

recharge are often expressed a length per time 

(for example, in./yr).

S

southern High Plains See High Plains 

aquifer.

southern High Plains aquifer The part of the 

High Plains aquifer system that underlies the 

southern High Plains.

U

unsaturated zone The subsurface zone 

between land surface and the water table, 

characterized by pore water under pressure 

less than atmospheric pressure. The matrix of 

the unsaturated zone is not completely filled

with water, and thus gases exist in the pore 

spaces of the unsaturated zone.

V

vadose zone The unsaturated zone.

Vertisol A type of soil that, when dry, is 

characterized by wide vertical cracks in the 

soil profile that swell shut when the soil is

hydrated. Vertisol soils are common to the 

floors of playas particularly in the southern

High Plains.

W

water cycle A cycle (also called the hydrologic cycle) that describes the existence and 

movement of water on, in, and above the Earth.

wetland Areas inundated or saturated by surface water or ground water at a frequency and 

duration sufficient to support a prevalence of

vegetation adapted for life in saturated soil conditions; such areas generally include swamps, 

marshes, bogs, and playas (U.S. Army Corps of 

Engineers, 1987; LaGrange, 2005).
36 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

Appendix 2. Infiltration Estimates Beneath Playas of the Southern High Plains.—Continued

[Infiltration rates compiled from studies that have directly measured infiltration from playas in the southern High Plains. hr, hour; in., inch; min., minute;

s, second; stage I–III infiltration is defined in the Glossary under the term infiltration]

Study Infiltration rate Approach Setting Notes

Lehman and Clark (1975)—1 playa studied

0.04 in./hr

0.002 in./hr

0.0008 in./hr

(day 0)

(day 1)

(day 10)

Constant head permeameter Playa floor Randall clay; feedyard 

runoff

1.57 in./hr

0.04 in./hr

0.002 in./hr

(day 0)

(day 8)

(day 45)

Constant head permeameter Interplaya Permeable buried soil; 

feedyard runoff

Evans (1990)—3 playas studied

0.39 in./min.

1.81 in./min.

26 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage I infiltration

0.87 in./min.

3.1 in./min.

41 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage I infiltration

0.47 in./min.

6.4 in./min.

98 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage I infiltration

0 in./min.

1.3 in./min.

1.5 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage III infiltration

0 in./min.

0.47 in./min.

2.5 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage III infiltration

0 in./min.

0.59 in./min.

1.5 in./min.

(minimum)

(average)

(maximum)

Double-ring infiltrometer Playa floor Stage III infiltration

Zartman, Evans, and Ramsey (1994); Zartman and others (1996)—1 playa studied

116 in./hr 

24 in./hr

(min. 1)

(min. 5)

5-in. diameter cylinder infiltrometers Playa floor (center) (10 s fill time)

60 in./hr

20 in./hr

(min. 1)

(min. 5)

5-in. diameter cylinder infiltrometers Playa floor (outerbasin) (10 s fill time)

88 in./hr

22 in./hr

(min. 1)

(min. 5)

5-in. diameter cylinder infiltrometers Annulus (10 s fill time)

10.6 in./hr

2 in./hr

(min. 1)

(min. 130)

80-in. diameter basin infiltrometer Playa floor (center) (~1 hr fill time)

5.5 in./hr

1 in./hr

(min. 1)

(min. 130)

80-in. diameter basin infiltrometer Playa floor (outerbasin) (~1 hr fill time)

3.2 in./hr

1 in./hr

(min. 1)

(min. 130)

80-in. diameter basin infiltrometer Annulus (~1 hr fill time)

2.1 in./hr

0.37 in./hr

(min. 1)

(day 1)

350-in. diameter basin infiltrometer Playa floor (center) (~1 hr fill time)
Appendix 2 37

Appendix 2. Infiltration Estimates Beneath Playas of the Southern High Plains.—Continued

[Infiltration rates compiled from studies that have directly measured infiltration from playas in the southern High Plains. hr, hour; in., inch; min., minute;

s, second; stage I–III infiltration is defined in the Glossary under the term infiltration]

Study Infiltration rate Approach Setting Notes

Wood and others (1997)—2 playas studied

45 in./yr

76 in./yr

107 in./yr

(minimum)

(average)

(maximum)

Water budget Playa floor

30 in./yr

47 in./yr

64 in./yr

(minimum)

(average)

(maximum)

Water budget Playa floor

Parker and others (2001) — 2 playas studied

4.41 in./hr

0.004 in./hr

0.004 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 1 (minimum) 15 samples

10.87 in./hr

0.31 in./hr

0.05 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 1

(average)

15 samples

19.92 in./hr

0.99 in./hr

0.13 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 1 (maximum) 15 samples

7.60 in./hr

0.004 in./hr

0.004 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 2 (minimum) 11 samples

13.62 in./hr

1.20 in./hr

0.09 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 2

(average)

11 samples

19.76 in./hr

4.49 in./hr

0.24 in./hr

(min. 1)

(min. 5)

(min. 60)

Flexible-wall permeameter Playa floor 2 (maximum) 11 samples
38 Recharge Rates and Chemistry Beneath Playas of the High Plains Aquifer

Appendix 3. Recharge Estimates for the Southern High Plains.—Continued

[Recharge estimates compiled from water-budget, unsaturated-zone, and ground-water studies in the southern High Plains]

Study type

and publication

Recharge

(in./yr) Approach Setting Notes

Water budget 

Johnson (1901) 3.0–4.0 Observation Regional

Gould (1906) 6.0 Observation Regional

Theis (1937) 0.1–0.7 Darcy’s law Regional

White and others (1946) 0.06 Water budget Regional

Barnes and others (1949) 0.098 Water budget Interplaya—nonspecific

Cronin (1961) 0.5 Darcy’s law Regional

Havens (1966) 0.82 Water budget Regional

Rayner and others (1973) 0.175 Water budget Regional

Lansford and others (1974) 0.39 Ground-water modeling Regional

Brutsaert and others (1975) 0.183 Water budget Regional

Morton (1980) 0.2–2.2 Ground-water modeling Regional

Texas Department of Water 

Resources (1981)

 0.5–1.0 Ground-water modeling Regional

Bureau of Reclamation (1982) 0.9 Water budget Regional

Bureau of Reclamation (1982) 1 Water budget Playa floor

Wood and Osterkamp (1984a) 0.1 Literature Regional

Wood and Osterkamp (1984b) 1.6 Literature Playa annulus

Wood and Osterkamp (1984b) 2.36 From Luckey and others (1986) Playa annulus Evapotranspiration not 

considered.

Luckey and others (1986) 0.086–1.03 Ground-water modeling Regional

Wood and Osterkamp (1987) 1.97 From Luckey and others (1986) Regional

Wood and Osterkamp (1987) 1.57 From Luckey and others (1986) Playa annulus

Nativ and Riggio (1989) 0.01–1.6 Water budget Playa floor

Dugan and others (1994) 0.5–1.5 Water budget Regional

Mullican and others (1997) 0.4 Ground-water modeling Interplaya—nonspecific Ogallala outcrop area.

Luckey and Becker (1999) 0.06–0.08 Ground-water modeling Interplaya—nonspecific Low permeability soils.

Luckey and Becker (1999) 0.6–0.9 Ground-water modeling Interplaya—rangeland Sand dune setting.

Dugan and Zelt (2000) 0.1–1.5 Percentage of irrigation water Interplaya—irrigated 

cropland.

Dutton and others (2000) 0.1–1.7 Ground-water modeling Regional

Stovall and others (2000) 0.6–5.5 Ground-water modeling Regional

Unsaturated zone 

Klemt (1981) 0.2 Neutron probe logging Regional

Klemt (1981) 0.1–0.2 Neutron probe logging Interplaya—nonirrigated 

cropland.

Knowles and others (1984) 0.06–0.57 Neutron probe logging Interplaya—nonspecific

Knowles and others (1984) 0.83 Neutron probe logging Interplaya—rangeland Sand dune setting.

Stone (1984) 0.007 Chloride mass balance Interplaya—irrigated 

cropland.

Stone (1984) 0.009 Chloride mass balance Interplaya—rangeland Dryland pastures.

Stone (1984) 0.05 Chloride mass balance Interplaya—rangeland Sand dune setting.

Stone (1984) 0.11 Chloride mass balance Playa floor

Stone and McGurk (1985) 0.05 Chloride mass balance Interplaya—rangeland Sand dune setting.

Stone and McGurk (1985) 0.48 Chloride mass balance Playa floor

Wood and Sanford (1995a) 3 (±0.31) Unsaturated-zone tritium Playa annulus

Wood and others (1997) 3 (±0.31) Unsaturated-zone tritium Playa floor

Scanlon and Goldsmith (1997) 0.004–0.16 Chloride mass balance Interplaya—nonspecific

Scanlon and Goldsmith (1997) 0.24–0.39 Chloride mass balance Playa floor Runoff to playas not 

factored in.

Scanlon and Goldsmith (1997) 2.4–3.9 Chloride mass balance Playa floor Runoff to playas factored in.

Scanlon and Goldsmith (1997) 4.72 Unsaturated-zone tritium Playa floor

Wood and others (1997) 1.06–1.22 Unsaturated-zone tritium Playa floor Assumes matrix flow only.

McMahon and others (2006) 0.67–1.26 Unsaturated-zone tritium Interplaya—irrigated 

cropland.

McMahon and others (2006) 0.008 Chloride mass balance Interplaya—rangeland
Appendix 3 39

Appendix 3. Recharge Estimates for the Southern High Plains.—Continued

[Recharge estimates compiled from water-budget, unsaturated-zone, and ground-water studies in the southern High Plains]

Study type

and publication

Recharge

(in./yr) Approach Setting Notes

Ground water

Brown and Signor (1973) 0.02–0.07 Ground-water budget Regional

Mullican and others (1994) 0.2 Ground-water modeling Interplaya—nonspecific

Mullican and others (1994) 8.1 Ground-water modeling Playa floor

Mullican and others (1994) 8.6 Ground-water modeling Playa floor

Nativ and Smith (1987), 

Nativ (1988).

0.5–3.24 Tritium Playa floor

Wood and Sanford (1994) 0.35 Chloride mass balance Playa annulus

Wood and Sanford (1995a) 0.43 (±0.078) Chloride mass balance Regional

Scanlon and Goldsmith (1997) 7.87–24 Ground-water chemistry Playa floor Playa receives long-term 

wastewater.

Wood and others (1997) 0.87–1.73 Chloride mass balance Playa floor Assumes matrix flow only.

Wood and others (1997) 5.7–10.1 Chloride mass balance Playa floor Assumes flow in desiccation

cracks only.

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Manuscript approved for publication, January 16, 2009

Edited by Mary-Margaret Coates

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