Aspect influences on soil water retention and storage

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<ul><li><p>Aspect inuences on soil w</p><p>.vSSD</p><p>tr</p><p>edatdsttely%cobthd</p><p>rid</p><p>Arid and semi-arid ecosystems are dened primarily bywaduecpeabprimeof19hotop(G20cosnowpack and soil by heterogeneous drainage and</p><p>d</p><p>The storage capacity of a soil prole depends on soilers-reis).nrereegge</p><p>@ @ K h @h 1 </p><p>HYDROLOGICAL PROCESSESHydrol. Process. 25, 38363842 (2011)Published online 11 November 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.8281evapotranspiration can result in complex spatial patternsin soil moisture (Famiglietti et al., 2008). In theseenvironments, soil water retention will determine howmuch water is discharged to the adjacent streams andunderlying groundwater and determine soil water avail-ability during the growing season. Accordingly, it isessential to understand how water inputs are retained in</p><p>@t @z @z</p><p>where is the volumetric moisture content (L3/L3), K ishydraulic conductivity (L/T), h is pressure head (L) (whichcan be negative in the vadose zone), t is time (T) and z isdistance in the vertical direction, positive upwards (L). Theshape of the SWRC, or the (h) function is a fundamentalcharacteristic of the soil (Childs, 1940). Soil properties aretypically expressed in terms of soil texture, but other*CBoE-m</p><p>Coter limitation; the growing season is often dictated by theration of water availability. In upland arid and semi-aridosystems, where precipitation is limited for extendedriods and depth to groundwater typically limits avail-ility to plants, water stored in the soil prole can be themary bio-available reservoir. Topographic indexingthods are commonly used to describe the distributionsoil moisture (Famiglietti et al., 1998; Grayson et al.,97; Western et al., 1999). In semi-arid environments,wever, seasonally dry conditions occur during whichography has little inuence on moisture redistributionrant et al., 2004, McNamara et al., 2005, Williams et al.,09). The initial variability of rainfall or snowmeltupled with complex redistribution of water in the</p><p>depth and the capacity of the soil to retain water undstresses imposed by gravitational drainage and evapotranpiration. The relationship between soil water pressu(pressure head) and volumetric moisture contentrepresented by the soil water retention curve (SWRCThe SWRC, coupled with a similar relationship betweehydraulic conductivity and moisture content or pressuhead, describe the soil hydraulic properties. Both curves aneeded to solve the Richards equation governing thtemporal variability of soil moisture content durininltration and gravitational redistribution. Considerinow in the vertical direction, the Richards equation can bexpressed as:INTRODUCTION catchments, in addition to the more commonly studiewater release mechanisms.I. J. Geroy,1 M. M. Gribb,2 H. P. Marshall,3 D. G1 United States Forest Ser</p><p>2 Civil and Environmental Engineering Department, South Dakota3 Department of Geosciences, Boise</p><p>4 Currently at: Civil and Environmental Engineering</p><p>Abs</p><p>Many catchment hydrologic and ecologic processes are impactprole thickness and water retention properties of soil. Soil wwhich in turn varies spatially in response to microclimate-inducevariables can be strongly differentiated by slope aspect. In thiscapacity for two opposing slopes in a semi-arid catchment in souaspects were subjected to a progressive drainage experiment toThe relatively large sample size (35) supported statistical anaopposing aspects. Soils on the north aspect retain as much as 25the south aspect slope. Soil porosity, soil organic matter and siltto greater soil water retention. These results, along with the oindicate that north aspect slopes can store more water fromobservation with potential implications on ecological function an</p><p>KEY WORDS storage; soil moisture; aspect; watershed; semi-a</p><p>Received 11 February 2011; Accepted 25 July 2011orrespondence to: James McNamara, Department of Geosciences,ise State University, Boise, Idaho 83725.ail: jmcnamar@boisestate.edu</p><p>pyright 2011 John Wiley &amp; Sons, Ltd.ater retention and storage</p><p>Chandler,4 S. G. Benner3 and J. P. McNamara3*ice, Durango, CO, 81301chool of Mines and Technology, Rapid City South Dakota 57701tate University, Boise, Idaho, 83725epartment, Syracuse University, Syracuse, NY, 13244</p><p>act:</p><p>by the storage capacity of soil water, which is dictated by theer retention properties are primarily controlled by soil texture,differences in insolation, wetness and temperature. All of theseudy, we compare quantitative measures of soil water retentionhwest Idaho, USA. Undisturbed soil cores from north and southstimate the soil water retention curve for each sample location.sis of slope scale differences in soil water retention betweenmore water at any given soil water pressure than samples fromntent were all greater on the north aspect, and each contributedservation that soils on north aspect slopes tend to be deeper,e wet winter months into the dry summer in this region, anlandscape evolution. Copyright 2011 JohnWiley&amp; Sons, Ltd.</p><p>; water retentionproperties such as structure, organic carbon content andbulk density exert a primary control on the SWRC and are</p></li><li><p>landscape properties and the feedbacks with hydrological</p><p>100, 200, 400 and 600 cm were used, and each pressure</p><p>3837ASPECT AND SOIL WATER RETENTIONprocesses are less understood (Broxton et al., 2009). Giventhat aspect inuences the surface energy balance, it isreasonable to expect that soils and associated ecosystemswill develop differently; a trend often noted in the soilliterature (Losche, 1970). Indeed, such aspect differenceshave been documented for hydraulic conductivity (e.g.Casanova et al., 2000) and soil depth (e.g. Khumalo, et al.,2008; Smith, 2010; Tesfa et al., 2009). Despite itspotentially profound ecological importance, there arelimited data explicitly documenting differences in soilwater retention with aspect (e.g. Leij et al., 2004; Herbstet al., 2006), likely due in part to the difculty in makingsuch measurements.In this study, we have experimentally quantied aspect</p><p>differences in soil water retention capacity in a semi-aridcatchment in southwest Idaho, USA. Using a North-Southoriented transect, we compare SWRCs and soil physicalproperties, evaluate the relationships between soil physicalproperties and water retention and discuss how these resultsaffect soil water storage and the co-evolution of geomor-phic, hydrologic and biologic systems.</p><p>SITE DESCRIPTION</p><p>This study was conducted in the 27 km2 Dry CreekExperimental Watershed, located north of Boise Idaho,USA. The upper elevations are classied using theKppen system, as moist continental climate and drysummers (Dsa), while the lower elevations are classied assteppe summer dry climate (BSa) (Henderson-Sellers andRobinson, 2001). The 650-m study transect spans acanyon; one slope is mostly north aspect (~365m), whilethe other is mostly south aspect (~285m). The averageslope angles on the south and north aspects are 25 degreesand 32 degrees, respectively.Soils in the study area are derived from Idaho Batholith</p><p>parent material, a granitic intrusion that is 80 million yearsold. Soils fall into three classications in the USDASSURGO 2.0 database (Soil Survey Staff, 2009). The soilstherefore dominant factors inuencing catchment waterretention as a whole.The spatial distribution of soil water retention relies on the</p><p>distributions of soil properties, which are commonlyunderstood to depend on Jennys (1941) ve soil formingfactors: regional climate, potential biota, topography, parentmaterial and time. In arid and semi-arid environments,differences in soil development within a common lithologyare readily observed among different microclimates. At thelocal scale, soil moisture can be strongly controlled byvegetation patterns (Madsen et al., 2008). In complexterrain, landscape scale patterns of moisture and vegetationcoincide primarily with aspect, which inuences thedistribution of incoming solar energy at the land surface.While the instantaneous impacts of aspect on snowmelt,</p><p>evapotranspiration, and other local energy balance pro-blems are well studied, the longer timescale inuences onalong the south aspect part of the transect (sites 115) are</p><p>Copyright 2011 John Wiley &amp; Sons, Ltd.step was maintained for approximately 24 h. After outowceased at the greatest applied pressure step, the soil corewas removed, weighed, dried for 24 h at 105 C, andweighed again to determine the volumetric moisturecontent at the nal pressure step. Outow volumes fromthe soil sample were then used to calculate at eachpreviously applied pressure step. The tests did not haveuniform pressure steps; therefore, the soil moisture versuspressure head data were used to estimate the vanGenuchten (1980) soil hydraulic parameters, which wereused to estimate soil moisture values at specic pressureheads for comparing moisture retention behavior betweenclassied as mesic Ultic Haploxerolls with Pachic and Lithicmodiers. Soils along the lower elevation part of the northaspect (sites 1731) are classied as frigid Ultic Haploxe-rolls, while the upper elevation sites (sites 3225) areclassied as mesic Ultic Haploxerolls with Entic and Lithicmodiers. In all cases, the vegetation on the south aspect partof the transect consists primarily of low sagebrush(Artemisia arbuscula), big sagebrush (Artemisia tridentata)and assorted forbs and grasses. Vegetation along the northaspect portion of the transect is dominated by r species(Pseudotsuga spp.) with an understory of shrubs.</p><p>METHODS</p><p>Thirty-ve sampling sites were located at approximately20m intervals (Figure 1). Sampling locations along thesouth aspect slope were centered on an aspect of 155degrees ranging between 142 degrees and 169 degrees,while those along the north aspect slope were centered onan aspect of 313 degrees ranging between 268 degrees and006 degrees. The south aspect slope ranges in elevationfrom 1361 to 1490m above mean sea level (msl), and thenorth aspect slope ranges in elevation from 1361 to 1579mabove msl.</p><p>Soil water retention measurement</p><p>Undisturbed soil cores with a nominal diameter and heightof 5.4 cm and 3.0 cm, respectively, were removed at eachsampling location using a hand-operated soil core extractorand subjected to a progressive drainage experiment using anautomated multistep outow apparatus described by Figuerasand Gribb (2009). Cumulative outow data and steady statesoil moisture as a function of pressure head data (i.e. (h))obtained from the experiments were used as inputs forHYDRUS 1D (Simunek et al., 2005) to estimate the SWRCsfor each sampling location.Soil cores were wetted with a solution of de-aired water,</p><p>0.30 g/L Thymol, and 0.27 g/L CaCl2 to prevent bacterialgrowth and clay dispersion (Klute and Dirksen, 1986)prior to testing. One-bar ceramic disks (part number1400B01M1-3, Soil Moisture Equipment Corporation,Santa Barbara CA) were used in the Tempe cells. Sampleswere allowed to imbibe water for at least 24 h prior totesting. Applied pressure steps of approximately 20, 40, 60,the two slopes.</p><p>Hydrol. Process. 25, 38363842 (2011)</p></li><li><p>ore</p><p>3838 I. J. GEROY ET AL.Soil physical properties</p><p>After multistep outow testing, the soil cores were driedand subjected to mechanical sieving and laser diffraction(Malvern Mastersizer 2000, Malvern Instruments Ltd.,Malvern, Worcestershire, UK) to determine the soil texturalclass fractions by weight according to the USDA method,which employs the following size standards: gravel&gt; 2mm,2mm&lt; sand&lt; 0.05mm, 0.05mm&lt; silt&lt; 0.002mm, andclay&lt; 0.002mm (Soil Survey Staff, 1999). The organiccarbon content (by weight basis) of a soil sample from each</p><p>Figure 1. Study site in the Dry Creek Experimental Watershed. Red dots shContours asampling location was determined using a Flash EA 1112Elemental Analyzer (Thermo Fisher Scientic Inc., WalthamMA). Samples were extracted from 10 cm below groundsurface using a manual coring device, placed in plastic bagsand frozen to prevent degradation prior to analysis.</p><p>Data analysis</p><p>We used box plots and the two-sample KolmogorovSmirnov (KS) test (e.g. Massey, 1951), a non-parametriccomparison of the empirical cumulative distribution func-tions of two populations, to evaluate differences betweensoil physical and hydraulic properties from 16 samplinglocations on the south aspect part of the transect (SA) andthose for the 19 sampling locations along the north aspectpart of the transect (NA). Whereas other researchers haveused correlation analysis to explore relationships betweentopographic variables and soil properties (e.g. Leij et al.,2004), the clustered nature of our sample locations aroundgenerally north facing and generally south facing aspectsprecluded use of correlation analysis in this study.</p><p>In situ soil moisture measurement</p><p>In situ measurement of the volumetric soil moisturecontent at each sampling location was performed using a</p><p>Copyright 2011 John Wiley &amp; Sons, Ltd.Campbell Scientic (Logan, UT) CR23X datalogger,TDR100 waveform generator, and a CS605 probe assembly.The CS605 probe was shortened from 30 cm to 15 cmaccording to Campbell Scientic Application Note 2S-H.The CR23X was programmed using LoggerNet version3.4.1. Each manually initiated sampling event resulted infour TDR waveforms being transmitted to the CS605probe. The apparent dielectric conductivity (Ka) that wasrecorded by the CR23X represented the average Ka of thefour waveforms. A site-specic calibration equation was</p><p>w south-facing sampling locations; blue dots are on the north-facing slope.in metersdeveloped with soil collected from a mid-elevation, southaspect location, which had a grain size distributionrepresentative of the average of all sample locations. Afourth order polynomial provided the best t of the data.On each sampling date, four individual measurements weremade within a 1m2 area at each sampling location.</p><p>RESULTS</p><p>Soil properties and aspect</p><p>Slope average physical properties of soils differ signi-cantly by aspect (a= 0.05) as shown by the boxplots inFigure 2, and the results of two-sample KS tests (Table I).The NA soil samples are generally ner grained withsomewhat less sand, more silt and more clay than SA soils(Table I), and greater saturated moisture content values, s.The slope average differences in organic carbon (1.39% forSA, 2.75% for NA) and bulk density (1.57 g/cm3 for SA,1.37 g/cm3 for NA) (or porosity (0.41 for SA, 0.50 forNA)), and s (0.41 for SA, 0.50 for NA) are notable. Sincethese properties are strongly covariate, differences in theseproperties between the two aspects were consistentlysignicant. These strong divisions in soil properties byaspect were not observed by Leij et al. (2004); however,</p><p>Hydrol. Process. 25, 38363842 (2011)</p></li><li><p>sod</p><p>3839ASPECT AND SOIL WATER RETENTIONFamiglietti et al. (1998) found strong positive correlationsbetween aspect and clay content, and strong negative</p><p>Figure 2. Boxplots of the north and south aspect values for the followingancorrelations between aspect and porosity. The increase ins is consistent with the results of Leij et al....</p></li></ul>

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