The roles of soil surface sealing, microtopography and vegetation in rainfall-runoff processes in semi-arid areas

Water resources are vital for arid and semiarid land ecosystems. Temporal and spatial distribution of water resources may significantly impact the development and sustainability of these ecosystems. One of the major elements in the water budget is the rainfall-runoff relationship. The response of semiarid regions to short but intense rainfall events has not been fully understood yet due to complex feedbacks between water, vegetation patches, topography and soil characteristics. The widely observed soil surface sealing is an important complicating factor in this system. It complicates impact of topography and vegetation on hydrological processes. Soil surface sealing involves the formation of a compact seal layer at the vicinity of the soil surface, inducing a drastic reduction of the soil infiltrability, thus affecting the local rainfall-runoff relationship. This study aims to quantify the interplay of surface sealing, microtopography and vegetation in semiarid region hydrology. A modeling approach is developed including a two-dimensional surface runoff model and a two-layer conceptual infiltration model as well as elaborate numerical treatment to study the rainfall-runoff process with the presence of a seal layer. The combination of the two models can simulate the rainfall-infiltration-runoff process on a heterogeneous surface with spatially varying soil and landscape properties. The model has been validated in the semiarid field plot of Lehavim LTER, Israel. The modeling results show that seal layer, microtopography and vegetation play important roles in the dryland runoff processes. 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. Water allocation can favor vegetation patches through surface runoff. This implies that more water resources may be available for plants when a seal layer develops at the soil surface.