Spatial patterns of catchment water redistribution: The intersection of topography and vegetation structure

Abstract:

Predicting the temporal evolution of runoff source areas represents a grand challenge in hydrology. While catchment structural characteristics, including topography and soils, are known to drive hydrologic connectivity of upland areas to the riparian and stream system, the effects of vegetation on the connectedness of water flow pathways are less well identified. The combination of lateral water redistribution by topographically driven drainage and water uptake by vegetation leads to shifting patterns of catchment storage. Hydrologic response exhibits threshold behavior with the transition from unsaturated to saturated subsurface conditions. This threshold nature in conjunction with changing storage patterns leads to effective runoff sources areas that are dynamic through time. To investigate these dynamics, we developed and applied a parsimonious but spatially distributed watershed modeling framework and used it to quantify the effects of vegetation on the temporal evolution of catchment connectivity over two years in a snow-dominated watershed in central Montana. Spatially distributed catchment water inputs were derived from two NRCS SNOTEL sites nearby with a simple temperature based snowmelt model. Evapotranspiration was measured at an eddy-covariance tower in the watershed and disaggregated based on vegetation structure. We used this modeling framework to further explore the effects of different vegetation pattern scenarios on the temporal expansion and cessation of catchment connectivity. With this work we seek to improve our understanding of how the intersection of physical (topography) and biological (vegetation) factors mediates hydrologic connectivity in headwater catchments.