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. Mathematical models have been developed to predict desertification, based on the premise that limited vegetation cover indicates water stress. However, local attributes may mask the smooth relationships quantified by these synthetic models, and empirical support is highly sought after in a bid to validate the soil-moisture-biomass relationship at the local scale. However, field data is limited by costly and time-consuming data acquisition. Spatially and temporally explicit realistic models may be one possible solution for studying the effect of field heterogeneity on these relationships. 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 semiarid Lehavim LTER in the Negev Desert, Israel. Within this framework, results from numerical solutions of flow equations in the shallow soil layer, near the soil surface, 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 (also known as run-on) generated in the 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, in addition to infiltration from direct rainfall, run-on contribution determines shrub patches size and vegetation cover in semiarid environments. We therefore suggest that estimations of shrub resilience in semiarid regions to water stress should consider scenarios involving surface sealing, rainfall intensity and contributing area size—each in accordance with the contribution of lateral flows to the water available to shrubs.