Lateral Flow and Contributing Area Control Vegetation Cover in a Semiarid Environment

Semiarid ecosystems attract attention, because they support the life of large human populations, while functioning under growing threats of degradation due to global climate change that may lead to reduced productivity and possibly irreversible desertification. Empirical support is consequently highly sought after to validate the soil-moisture-biomass relationship at the local scale in these ecosystems. Combining physically based models and a database of 32 years of field and remotely sensed data, we examined the effect of soil-moisture content (SMC) on vegetation cover and patch size distribution in the heterogenous semiarid Lehavim LTER in the Negev Desert, Israel. Results from numerical solutions of flow equations in the shallow soil layer allowed predictions of distributions of SMC as the outcome of vertical and lateral water fluxes. The association of vertical fluxes with patch size distribution and vegetation cover did not support the notion that direct rainfall is sufficient for the shrubs' survival in drought conditions. However, re-infiltration of downslope runoff, generated in sealed bare intershrub soil area, significantly contributed to the water available to supporting shrub patches. Accordingly, our approach determines potential vegetation cover in semiarid areas, due to the dependency on contributing area and rainfall-infiltration-runoff processes. This underscores the fact that, additionally to infiltration from direct rainfall, run-on contribution controls vegetation cover in semiarid environments. We therefore suggest that estimations of shrub resilience to water stress should consider scenarios involving surface sealing, rainfall duration and intensity, and contributing area size-each in accordance with the contribution of lateral flows to the water available to shrubs.