Forest ecohydrological response to bimodal precipitation during contrasting winter to summer transitions

The ecohydrological response of forested landscapes in the Southwestern United States depends on the relative magnitude of precipitation during the winter and summer seasons. Prior climate studies have identified an inverse relation in the bimodal regime such that wet winter periods are followed by dry summers and vice versa. Despite this key prediction, the impact of contrasting winter to summer transitions on hillslope hydrology has not been investigated. In this study, we use the Triangulated Irregular Network-based Real-time Integrated Basin Simulator distributed hydrologic model with modifications in the snow module to generate a consistent set of high-resolution hydrologic estimates in a ponderosa pine hillslope in northern New Mexico, United States. The model is evaluated against available observations of snow depth, soil moisture and runoff over two water years yielding reliable spatial distributions during the winter to summer transitions. We find that a wet winter followed by a dry summer promotes evapotranspiration losses that gradually dry the soil and disconnect lateral fluxes in the forested hillslope, leading to soil moisture patterns resembling vegetation patches. Conversely, a dry winter prior to a wet summer results in soil moisture increases that promote lateral connectivity and soil moisture patterns with the signature of terrain curvature. An opposing temporal switch between infiltration and saturation excess runoff is also identified. These contrasting responses indicate that the inverse relation has significant consequences on hillslope water availability and its spatial distribution with implications on other ecohydrological processes including vegetation phenology, groundwater recharge and geomorphic development.