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

^a The RETEC Group Inc., Long Beach, CA 90815
^b Dep. of Forest Resources, Univ. of Idaho, Moscow, ID 83844-1133
^c Dep. of Geography, Univ. of California Santa Barbara, Santa Barbara, CA 93106

View larger version (56K): [in a new window]   Fig. 1. Study site at the University of California Sedgwick Natural Reserve in Santa Barbara County, CA. The study site is shown both as (a) a drape of a digital orthophoto over a 30-m digital elevation model; and (b) with the individual soil profile locations displayed on a 1-m contour shade map of the hillslope.

View larger version (21K): [in a new window]   Fig. 2. Daily and cumulative values of precipitation for the hillslope study area measured using a nearby weather station.

View larger version (44K): [in a new window]   Fig. 3. Measured available soil water content (%) using time domain reflectometer probes (TDR). Weekly and biweekly values of soil water for each profile were interpolated onto an evenly spaced grid where the x and y axes are time and depth respectively. The shades of color were created to help visualize wetting and drying patterns.

View larger version (42K): [in a new window]   Fig. 4. Daily values of soil-water storage and drainage as predicted by the one-dimensional water balance model in comparison with storage values measured using time domain reflectometry (TDR) probes. Upper and lower 95% confidence intervals are constructed around the model predictions based on the error involved in estimating the a and b parameters during soil dry-down. Differences between measured and modeled values that lie within the error band are considered insignificant because of uncertainty in model parameterization. Modeled drainage is shown as negative values for when occurred.

View larger version (30K): [in a new window]   Fig. 5. Net lateral flux (gain/loss) of soil water is calculated by subtracting the measured soil-water content from the predicted values for periods when the measured values fall outside the 95% confidence interval bands. Measured values outside the band are considered significant and indicate influence of other hydrological processes that are not accounted for by the one-dimensional balance model.

View larger version (66K): [in a new window]   Fig. 6. Predicted maps of soil-water storage for the hillslope study site during the El Niño (1997-1998) and La Niña (1998-1999) water years. The shades represent volumetric soil-water content integrated over soil depth on a 2-m grid scale. Overlaid are positive (+) and negative (-) symbols indicating net gain or loss as a result of the significant differences between measured and modeled soil-water content calculated for the nine sample locations.

View larger version (58K): [in a new window]   Fig. 7. Selected maps of lateral flow distribution for the El Niño (1997-1998) and La Niña (1998-1999) water years. Blue shades depict areas of net loss. Green, yellow, and red depict a widespread range of net gain, and white shades are areas where net loss equals net gain. The delineated catchment (black polygon) defines hydrologic boundaries at the top and sides of the hillslope with a spatially distributed outlet at the downslope end.