Extension of the Variable Source Area Concept to Patchy Arid Landscapes

Abstract

Infiltration excess is the dominant runoff mechanism in many drylands. Arid and semiarid landscapes often feature mosaic patterns of bare soils and stone cover, interspersed with vegetated patches. In response to raindrop impact, bare surfaces are prone to the development of physical crusts with reduced infiltration rates. Surfaces shielded by plant canopy, litter, and nonembedded stones prevent crusting and maintain higher infiltration capacity. The source area for runoff in dryland watersheds is expected to vary depending on rainfall characteristics and surface conditions because bare soils exceed infiltration capacity sooner and at lower rainfall intensities compared with shielded patches. In this study, we therefore hypothesize that the variable source area (VSA) concept developed for saturation excess runoff in humid climates can be extended to drylands. We test this hypothesis at the hillslope scale for two basins in the Walnut Gulch Experimental Watershed in Arizona. By building split models, infiltration and runoff from shielded and bare surfaces within each basin were simulated separately. Infiltration parameters were estimated from 148 plot-scale rainfall simulator tests. Performance of the split modeling strategy was compared to distributed and lumped approaches. Results show that split and gridded models performed significantly better than the lumped alternative across various performance metrics. This indicates that the VSA concept can be extended to patchy arid and semiarid landscapes, whereby the source area contributing runoff to the basin outlet is at least in part modulated by soil surface condition. By providing a parsimonious alternative to conventional lumped models, the novel split approach has clear potential for improving runoff predictions in dryland settings with complex mosaic landscapes.