[1] The hydrological response of semiarid watersheds to short but intense rainfall events is complex due to feedbacks among water fluxes, vegetation patches, topography, and soil properties. This paper seeks to quantify the combined impact of soil-surface sealing, microtopography, and vegetation patches on surface hydrologic processes on a semiarid hillslope. A modeling approach using a two-layer conceptual infiltration model and two-dimensional surface runoff model was developed to study rainfall-runoff relationships. This combined model is applicable to heterogeneous areas with spatially varying soil properties, landscape, and land-cover characteristics. Data from a semiarid site in southern Israel was used to evaluate the model and analyze fundamental hydrologic mechanisms. Our results indicate that seal layer, microtopography, and vegetation play important roles in dry land runoff processes: seal layer controls runoff generation; vegetation patches affect overland flow by enhancing local infiltration rates; microtopography has a small impact on the total amount of runoff, but shapes the spatial pattern of overland flow. The presence of vegetation patches amplifies the effect of microtopography by increasing spatial variability of infiltration and runoff. Results also show that water resource allocation favors vegetation patches through surface runoff, with maximum local cumulative infiltration one order of magnitude higher than the amount of water available from rainfall. This suggests a sophisticated system of water resource distribution in semiarid ecosystems through complex interactions among environmental factors.

Scale dependence of Hortonian rainfall‐runoff processes has received much attention in the literature but has not been fully resolved. To further explore this issue, a recently developed model was applied to simulate rainfall‐infiltration‐runoff processes at multiple spatial scales. The model consists of the coupling between a two‐dimensional runoff routing module and a two‐layer infiltration module, thus accounting for spatial variability in soil properties, soil surface sealing, topography, and partial vegetation cover. A 76 m² semiarid experimental plot with sparse cover of vegetation patches and a sealed soil surface in inter‐patch bare areas was used as a representative elementary area (REA). A series of four larger artificial plots of different areas was created based on this REA to examine the scale dependence of rainfall‐runoff relationships in the case of stationary heterogeneity. Results show that runoff depth (or runoff coefficient) decreases with increasing scale. This trend is more prominent at scales less than 10 times the REA length. Power law relationships can quantitatively describe the scaling law. The major mechanism of the scale effect is run‐on infiltration. However, rainfall intensity and soil properties can both affect the scaling trend through their interaction with run‐on. Higher intensity and less temporal variability of rainfall can both reduce the scale effect. Temporally intermittent rainfall may produce spatially oscillating infiltration rates at large scales. Vegetation patterns are another factor that may affect the scaling. Random‐vegetation patterns, compared with regular patterns with similar statistical properties, change the spatial distributions, but do not significantly change either the total amount and statistical properties of infiltration and runoff or the scale dependence of the rainfall‐runoff process.

Bed form‐driven hyporheic exchange is vital to biogeochemical processes occurring within aquatic sediment. The integrated effects of hyporheic processes in a bed form ultimately impact watershed‐scale water quality. However, much of what is understood regarding bed hyporheic exchange is based on idealized two‐dimensional bed forms despite the prevalence of potentially complex three‐dimensional (3‐D) bed forms in sandy riverbeds. We thus examined the impact of bed form three‐dimensionality on hyporheic exchange. Bed form three‐dimensionality was represented by two groups of geometric parameters: (1) crest planform curvature and (2) transverse and longitudinal wavelength and amplitude. A wide variety of synthetic bed forms was generated based on the geometric parameters. Then, surface flow over and hyporheic flow through each bed form was calculated using a 3‐D multiphysics computational fluid dynamics model implemented across a range of Reynolds Number. We found that hyporheic exchange is sensitive to both types of parameters that determine the bed form three‐dimensionality. Hyporheic exchange is dominated however by the three‐dimensionality caused by out‐of‐phase superimposed sinusoidal surfaces. The results of the complex flow models were synthesized into simple equations for predicting hyporheic flux and bulk residence time based on bed form longitudinal and transverse wavelengths, bed form height, and Reynolds Number.