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Fundamental Concepts of Recharge in the Desert Southwest: A Water Sci. and Appl. Groundwater Recharge in a Desert Environment: The Southwestern United States Vol. 9 FundamentalConcepts of Rechargein the Desert Southwest: A RegionalModeling Perspective Alan L. Flint, LorraineE. Flint, and JosephA. Hevesi U.S. GeologicalSurvey, Sacramento, California JoanB. Blainey U.S. GeologicalSurvey, Tucson, Arizona Rechargein arid basinsdoes not occur in all years or at all locationswithin a basin.In the desertSouthwest potential evapotranspiration exceeds precipitation on an averageannual basis and, in many basins,on an averagemonthly basis. Groundwatertravel time from the surfaceto the water table and rechargeto the water table vary temporallyand spatiallyowing to variationsin precipitation,air temperature,root zone and soil propertiesand thickness,faults and fractures,and hydrologicproperties of geologicstrata in the unsaturatedzone. To highlightthe fundamentalconcepts controlling recharge in the Southwest,and addressthe tem- poral and spatial variability of recharge, a basin characterizationmodel was developedusing a straightforwardwater balanceapproach to estimatepotential rechargeand runoff and allow for determinationof the locationof rechargewith- in a basin.It providesa meansfor interbasincomparison of the mechanismsand processesthat result in rechargeand calculatesthe potential for rechargeunder current,wetter, and drier climates.Model estimatesof rechargecompare favor- ably with othermethods estimating recharge in the Great Basin. Resultsindicate that net infiltration occursin less than 5 percentof the area of a typical south- westernbasin. Decadal-scaleclimatic cycles have substantiallydifferent influ- encesover the extentof the Great Basin, with the southernportion receiving 220 percenthigher recharge than the meanrecharge during E1 Nifio yearsin a positive phaseof the Pacific Decadal Oscillation,whereas the northernportion receives only 48 percenthigher recharge. In addition,climatic influences result in ground- water travel times that are expectedto vary on timescalesof days to centuries, making decadal-scaleclimate cycles significantfor understandingrecharge in arid lands. 1. INTRODUCTION GroundwaterRecharge in a DesertEnvironment: The Southwestern United States The purposeof this studywas to developa simplemodel Water Scienceand Application 9 for basincharacterization that allowsinterbasin comparison This paperis not subjectto U.S. copyright.Published in 2004 by of recharge mechanismsand the potential for recharge the AmericanGeophysical Union. 10.1029/009WSA 10 159 Water Sci. and Appl. Groundwater Recharge in a Desert Environment: The Southwestern United States Vol. 9 160 CONCEPTS OF REGIONAL RECHARGE under current,wetter, and drier climates,and to highlight ty data are available[Flint et al., 2000]. The BCM can also the fundamental conceptsand mechanismsthat control be usedto evaluatethe potentialfor rechargeunder current, rechargein the desertsof the SouthwesternUnited States wetter, and drier climates, and is used to evaluate the role of (Southwest).The methoddeveloped allows analysisof cli- decadal-scaleclimate cycles (El Nifio/La Nifia and the mate change,as changesin precipitationand air tempera- Pacific Decadal Oscillation) on rechargeat a pixel scale ture, to evaluatethe potential for changesin groundwater (generally30-270 meters)across the Southwest. rechargein the Great Basin and eventuallyin other areasin the Southwest.Without furtherrefinements, this modeling 1.1 Termsand Concepts approachprimarily is intended to provide a means for hydrologicallycharacterizing basins on a basin-wide or Becausemany terms relatedto infiltration and recharge regional scale on the basis of fundamentalconcepts of often have different meaningsto different researchers,the rechargeas they applyto southwesterndesert environments. termsused in this paperare definedand are consistentwith Estimatesof rechargein basinsof the Great Basin are pre- thosein mostcurrent literature. Infiltration is the entry into sentedfor the purposeof illustratingthe approach,evaluat- the soil of water made available at the ground surface ing relative proportionsof rechargeand runoff to describe [Freezeand Cherry,1979]. Net infiltrationis the quantityof the dominantmechanisms controlling recharge, and provid- waterthat movesbelow the zoneof surfaceevapotranspira- ing a comparisonwith other methodsthat have estimated tion processes[Flint et al., 2001]. Under steadystate condi- rechargein the GreatBasin. They are not relied on as accu- tions,net infiltrationis equalto rechargeunless diverted to rate enoughat this time to be used for assessmentof water an areaof flow from a springand thus lost to evapotranspi- availability. ration;even under this condition,one couldargue that some A basincharacterization model (BCM) was developedfor rechargeoccurs, even if only to a small local or perched this studyto determinethe spatialand temporalvariability aquifer.Percolation (or drainage)is the processby which of net infiltration (all terms are definedbelow), which is watermoves downward through the unsaturatedzone [Flint assumedto be equalto rechargebecause the model assumes et al., 2001]. Rechargeis the entryinto the saturatedzone of steadystate conditions and no lateral subsurfaceflow. The water made availableat the water table surface[Freeze and BCM uses a mathematical deterministic water-balance Cherry, 1979]. Dischargeis the removal of water from the approachthat includesthe distributionof precipitationand saturatedzone acrossthe water table surface[Freeze and the estimationof potentialevapotranspiration, along with Cherry, 1979]. soil water storageand bedrock permeability. The BCM was Travel time in the unsaturated zone is the time it takes for usedwith availableGIS data(digital elevationmodel, geol- water that hasbecome net infiltrationto rechargethe water ogy, soils, vegetation,precipitation, and air temperature table (hoursto millennia);it is controlledby net infiltration, maps),and GIS datathat was developedfor this study. the thickness of the unsaturatedzone, and the effective The BCM can be usedto identify locationsand climatic porosityof subsurfaceflow paths [Flint et al., 2000]. As conditionsthat allow for excesswater, quantifyingthe climatechanges, the traveltime of infiltratingwater through amountof water availableeither as runoff or as in-place the unsaturatedzone may vary; the spatialdistribution of rechargeon a monthly basis, and allows inter-basincom- rechargealso may vary. Rechargethat occurstoday is spa- parisonof rechargemechanisms. The model doesnot dis- tially variableowing to the thicknessesof soil andalluvium, tinguish between mountain front and stream channel the thicknessof the unsaturatedzone, and to the layering recharge,which are referredto in this paperas runoff, nor andproperties of geologicand sedimentstrata. Recharge is does it explicitly define the percentageof runoff that temporallyvariable owing to changesin processescontrol- becomes recharge. Because the accurate estimates of ling net infiltration (primarily climate) for timescalesof rechargecannot be calculatedwithout furtherrefinement to yearsto centuries. the BCM to estimatethe partitioningof runoff,it calculates Rechargeis often discussedas dominantwithin oneof the potential in-placerecharge and potential runoff, and pro- following basin locations:mountain block, diffuse,moun- vides the distribution of both in a basin. These values can be tain front, streamchannel, and playa lake. Mountainblock combinedusing assumptionsof the amountof runoff that rechargeoccurs directly into the underlyingbedrock with- resultsin rechargeto estimatetotal potentialrecharge. outrunoff and is widely distributedin areasof highermoun- A simplecalculation of traveltime throughthe unsaturat- tainous terrain particularly where there is permeable ed zone can be estimatedif steady state conditionsare bedrock.Diffuse rechargeis areally distributedin alluvial assumedand if unsaturatedzone thickness and permeabili- valleys but away from the stream channels (similar to Water Sci. and Appl. Groundwater Recharge in a Desert Environment: The Southwestern United States Vol. 9 FLINT ET AL. 161 mountain block recharge).Mountain block rechargeand USGS for the Southwest(Plate 1). There are areasthat are diffuserecharge occur in direct responseto the infiltration calculatedto be hyper-arid on a grid cell basis, such as of rainfall and snowmeltand will be referredto in this paper DeathValley, that do not appearwhen averagedfor an entire as in-place recharge.In-place rechargealso can occur in basin. responseto the local-scalelateral redistributionof rainfall The Great Basin representsan arid environmentand is and snowmelt following runoff and subsequentoverland centrallylocated within the Southwest.It was selectedfor a flow that doesnot reach the larger streamchannels. Water preliminaryanalysis to determinethe feasibilityof applying that doesnot rechargein placeis referredto asrunoff in this a simple basin characterizationmodel for estimating paper.Runoff may becomemountain front recharge,which rechargebecause of the ability to compareit to previous occursat boundariesbetween mountain blocks and deeper analysesof rechargein the Great Basin. The Great Basin alluvial valleys, or beneath ephemeral streams as the studyarea is 374,218 kin2and contains a total of 258 hydro- streamstransition from uplandareas with thin soilsto allu- logic units(hydrographic areas and subareas),which will be vial valleys and basins with thick soils. Stream-channel referredto as basinsin this paper(Figure
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