Spatially Explicit Treatment of Soil-Water Dynamics along a Semiarid Catena

for model calibration. Thus, less effort has been expended in developing comprehensive statistical methVolumetric soil-water depth profiles at nine sample locations on ods to characterize the spatial variability of hydrological a 2-ha hillslope were monitored throughout the 1997-1998 El Niño and the 1998-1999 La Niña cycles. A hydrological model integrating and ecological processes, especially at scales appropriate the soil-water measurements with digital terrain analysis and a onefor hillslope analysis. dimensional water balance model was developed to map dominant Statistical correlations among soil properties and terhydrological patterns of soil-water storage and lateral flow redistriburain attributes generated from a DEM have greatly ention. Statistical correlations between hydrologic behavior and the comhanced the quantitative investigation of hydrological pound topographic index (CTI) generated from a digital elevation processes in soils (Beven and Kirkby, 1979; O’Loughlin, model (DEM) were used to generate spatially distributed input param1986; Moore et al., 1991). Similarly, soil-landscape modeters of initial water storage and soil-controlled evapotranspiration els developed by integrating digital terrain analysis and utilized in the model. The results suggest that differences in water statistical modeling have enhanced prediction and mapstorage and availability are highly modified by climatic conditions ping of soil properties (Moore et al., 1993; Gessler et and local topography. Nearly three times higher than normal rainfall al., 1995, 2000; McKenzie et al., 2000). Integration of in the El Niño year caused deeper infiltration of water and led to GIS-based digital terrain analysis with monitoring studsignificant subsurface water redistribution into the concave hillslope positions which remained moist throughout the 1998 summer. Water ies can provide simultaneous feedback to predictive infiltration and distribution was diminished considerably in the drier models and facilitate quantitative and dynamic modelthan normal La Niña, and led to a complete dry-down in 1999. Actual ing of distributed ecological processes. For example, the evapotranspiration was 87% of total precipitation during the El Niño, effect of water distribution and the resulting patterns compared with 100% in the subsequent La Niña. The good correlation of available soil water on net primary productivity, soil between modeled and measured water storage shows that even a organic C, and soil respiration deduced from monitoring simple one-dimensional model combined with the appropriate input sites, can be evaluated in the context of predicted soilparameters is a suitable tool for estimating changes in soil-water landscape relationships. Using this approach, we can content on hillslopes where lateral flow is a significant functional explore, capture, map, and interpret spatial variability of component of the soil hydrology. hydrological patterns and related ecological responses based on their appropriate spatial and temporal resolution. A of water to terrestrial ecosystems is a In a previous study, we utilized terrain analysis, statismajor determinant of ecosystem production and tical modeling, and field sampling to model the distribuhealth because it supports plant growth directly and tion of soil properties on a 2-ha hillslope catena in Caliredistributes nutrients within landscapes (Aber and fornia (Gessler et al., 2000). Here, we extend this Mellilo, 1991; Schuur et al., 2001). It follows that the analysis to visualize the spatial variability of hydrologirole of topography in determining water movement in cal processes on the same catena. Using data from soillocal landscapes must be well characterized to provide water profile monitoring stations (point location), we an accurate picture of ecosystem function. On hillslopes, test the hypothesis that water in excess of evapotranspiknowledge of the topographical influence on water flow ration follows clearly defined patterns as it moves downis essential in understanding soil differentiation (Milne, slope through the soil and along the soil–rock interface. 1935; 1936), and hence the patterns of ecosystem process We place point-location soil-water profile measuresuch as evapotranspiration, C storage, and net primary ments in a topographic context by developing statistical productivity (Band et al., 1993; Gessler et al., 2000). For relationships with terrain attributes calculated from a this reason, some hydroecological models have incorpoDEM. Finally by linking water-balance model paramerated topographical analysis in their model structures ters with terrain attributes, we portray the water dynameither by deriving a set of flow planes (TOPMODEL: ics over two distinctly different hydrological years. The Beven and Kirkby, 1979; TOPOG: Vertessey and Elsenfirst was the wettest on record driven by an unusually beer, 1999), or dividing watersheds into smaller landstrong El Niño weather pattern, whereas the second scape units (FOREST-BGC: Running and Coughlan, was drier than normal, driven by a La Niña pattern. 1988). In either case, limited point-data obtained from ground observation stations and gauges are typically MATERIALS AND METHODS used to define the input parameters and are often used Study Site and Topographic Characterization The catena study was established on a 2-ha zero-order waF. Chamran, The RETEC Group Inc., Long Beach, CA 90815; P.E. tershed within the San Ynez Basin in Santa Barbara County, Gessler, Dep. of Forest Resources, Univ. of Idaho, Moscow, ID 838441133; and O.A. Chadwick, Dep. of Geography, Univ. of California Abbreviations: DEM, digital elevation model; GIS, geographic inforSanta Barbara, Santa Barbara, CA 93106. Received 20 Dec. 2000. mation system; CTI, compound topographic index; TDR, time domain *Corresponding author ([email protected]). reflectometer; Et, actual evapotranspiration; Et,w, potential evapotranspiration; Et,s, soil controlled evapotranspiration. Published in Soil Sci. Soc. Am. J. 66:1571–1583 (2002).