Hydrological responses and scaling effects in vegetated semi-arid areas with surface sealing

Abstract:

The rainfall-runoff process is of critical importance for ecosystem sustainability in semi-arid regions. Recent studies have disclosed complex feedbacks between rainfall, vegetation patches, microtopography and soil characteristics, emphasizing the important role of soil surface sealing of the bare intershrub areas. Soil surface sealing involves the formation of a compact seal layer at the vicinity of the bare soil surface, inducing a drastic reduction of the soil infiltrability, which determines the local rainfall-runoff relationship. To quantify the interplay of surface sealing, microtopography and vegetation patches in semiarid region hydrology, a modeling approach is developed which couples a two-dimensional surface runoff model and a two-layer conceptual infiltration model, and also includes elaborate numerical treatment to study the rainfall-runoff process with the presence of a seal layer. Applying this model in a semi-arid field plot in the Lehavim LTER in Southern Israel reveals the complex interactions between these factors and the hydrological response at the hillslope scale. The seal and vegetation are major controls of the runoff generation process and runoff amount, while microtopography significantly affects the spatial pattern of overland flow and runoff routing. Also, vegetation patches can receive much more water through runon than direct rainfall could provide, which implies an important mechanism for water resources allocation in dryland ecosystems. Furthermore, the model has been applied to plots of multiple scales to examine scaling effects in the rainfall-runoff process. In agreement with observed trends, results show a decrease of runoff with increasing spatial scales, and an increase of runon effect across scales. However, this scaling effect of runoff is not linear; rather, power-law relationships are observed.