Water Balance at the Southern Limit of the Californian Mixed-Conifer Forest and Implications for Extreme-Deficit Watersheds

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Abstract
This study estimates the soil water balance at forest/chaparral, upper forest, and wet-meadow sites in the Sierra San Pedro Mártir (SSPM), Baja California, Mexico, and compares the results to previously published data for similar sites in neighboring California, USA. Changes in soil water storage (0-80cm) were determined by using a neutron probe, deep drainage (> 80cm) was measured in minilysimeters, and runoff was estimated in 1m2 plots. Average annual precipitation was 714 - 226, 674 - 137, and 551 - 112 mm at the lower forest, upper forest, and wet-meadow sites, respectively. Evapotranspiration (ET) was 390 - 122, 478 - 87, and 446 - 122 mm, or 55%, 71% and 81% of precipitation. The relationship between average precipitation and average ET for SSPM was similar to published long-term ET data for extreme-deficit watersheds in southern California. Regression analyses of streamflow and ET data in review studies revealed that, as water deficit increases, the relationship between precipitation and ET becomes increasingly stronger, while that between precipitation and streamflow becomes concomitantly weaker. The slope of the regression line gives the proportion of precipitation lost to streamflow or ET, while the intercept indicates the amount of soil water recharge required annually before streamflow can occur. Available data from long-term studies in extreme-deficit watersheds revealed a near-zero intercept, and imply that 69% of precipitation is evapotranspired from the surface soil, Also, 15% is apparently transpired by deep roots extracting water below the solum, and the remaining 16% goes to streamflow.