Impact of Spatial Variability in Root‐Zone Storage Deficit on Runoff Generation

Abstract

The root zone stores water for plants and is a source of drainage that drives runoff. Previous work demonstrated that significant drainage to drive runoff is predicated on the replenishment of the root‐zone moisture deficit. A method for quantifying the root‐zone storage deficit (hereafter, “the deficit”) based on a running balance of water entering the root zone via rainfall and snowmelt and water leaving the root zone via evapotranspiration has been gaining popularity over the past decade. Distributed deficit data have been shown to be useful for improving runoff predictions at the basin scale, but there is opportunity to further explore variability in deficit and runoff dynamics throughout basins and how this heterogeneity plays a role in catchment behavior. Here we calculate deficits at 4 km resolution in a set of basins across the Sierra Nevada in California. We demonstrate that deficits can vary substantially across a catchment, with deficit growth and replenishment occurring at different times and with different magnitudes in different locations. More importantly, heterogeneity in pixel deficits results in different timing and amount of inferred runoff from different parts of the catchment, differences that persist when pixel calculations are aggregated to the catchment scale. Our findings reveal that, even using this simple mass balance approach, it is important to account for spatially variable behavior in order to accurately capture the magnitude of runoff at the catchment scale and that deficit tracking approaches may contribute to quantifying runoff in ungauged basins.