Abstract
Abstract. This study presents unique data on the effects of antecedent soil moisture on runoff generation in a semi-arid environment, with implications for process-based modeling of runoff. The data were collected from four small watersheds measured continuously from 2002 through 2010 in an environment where evapo-transpiration approaches 100% of the infiltrated water on the hillslopes. Storm events were generally intense and of short duration, and antecedent volumetric moisture conditions were dry, with an average in the upper 5 cm soil layer over the nine year period of 8% and a standard deviation of 3%. Sensitivity analysis of the model showed an average of 0.05 mm change in runoff for each 1% change in soil moisture, indicating an approximate 0.15 mm average variation in runoff accounted for by the 3% standard deviation of measured antecedent soil moisture. This compared to a standard deviation of 4.7 mm in the runoff depths for the measured events. Thus the low variability of soil moisture in this environment accounts for a relative lack of importance of storm antecedent soil moisture for modeling the runoff. Runoff characteristics simulated with a nine year average of antecedent soil moisture were statistically identical to those simulated with measured antecedent soil moisture, indicating that long term average antecedent soil moisture could be used as a substitute for measured antecedent soil moisture for runoff modeling of these watersheds. We also found no significant correlations between measured runoff ratio and antecedent soil moisture in any of the four watersheds.
Highlights
This study was conducted using data from four small watersheds located on the Walnut Gulch Experimental Watershed near Tombstone in southeastern Arizona, USA, which is operated by the United States Department of Agriculture (USDA) Agricultural Research Service (ARS) Southwest Watershed Research Center (Moran et al, 2008)
Antecedent volumetric soil moisture immediately prior to the measured 60 rainfall events ranged from 1 % to 17 %, with an average of 8.0 % and a standard deviation of 3.0 % (Fig. 3)
This study presents unique data on the effects of antecedent soil moisture on runoff generation in a dry-land environment, along with implications for process-based modeling of runoff
Summary
Soil water content in the upper soil layer prior to a rain event can be an important factor affecting the relationship between rainfall and runoff (Yair and Klein, 1973; Abrahams et al, 1988; Karnieli and Ben-Asher, 1993; MartinezMena et al, 1998; Castillo et al, 2003; Zehe et al, 2005; James and Roulet, 2009; Brocca et al, 2009a, b; Penna et al, 2011; Tramblay et al, 2010; Kampf, 2011). Western et al (1998) analyzed relationships between watershed average soil moisture derived from point measurements and daily runoff coefficient for days with rainfall greater than 5 mm for the 10.5 ha semi-humid Tarrawarra watershed characterized by a silt loam soil type. Western et al (1998) analyzed relationships between watershed average soil moisture derived from point measurements and daily runoff coefficient for days with rainfall greater than 5 mm for the 10.5 ha semi-humid Tarrawarra watershed characterized by a silt loam soil type. Their results showed that the surface runoff was strongly controlled by soil moisture, with a threshold value of the volumetric water content varying from 41 to 46 %, below which no runoff occurred. By conducting a stochastic sensitivity analysis on the runoff response to different soil moisture scenarios using a physically based distributed model in semiarid Spain, Castillo et al (2003) drew the conclusion that “when infiltration excess overland flow is predominant, as a result of high rain intensities or less permeable soils, the runoff response. . . does not depend on initial
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