A climate‐induced threshold in hydrologic response in a semiarid ponderosa pine hillslope

[1] The temporal evolution of spatial hydrologic patterns in systems with strong seasonality in water availability is poorly understood. In this study, we use a distributed hydrologic model to explore the switching between local (vegetative) and nonlocal (topographic) factors in a semiarid ponderosa pine hillslope. Our modeling effort is focused on identifying threshold behavior in the hillslope response due to wetting during the North American monsoon (NAM). We calibrate the model to distributed surface soil moisture data for one summer and then test the model against a broader range of observations over multiple seasons. Model simulations are then used to identify vegetation and topographic controls on spatial patterns in soil moisture and runoff generation. Vegetation patterns primarily influence the hydrologic response during dry summer periods leading to patchiness related to the ponderosa pine stands. The spatial response switches to fine-scale terrain curvature controls during persistently wet NAM periods. Thus, a climatic threshold involving rainfall and weather conditions during the NAM is identified in the hillslope response when sufficient lateral soil moisture fluxes are activated by high rainfall amounts and the lower evapotranspiration induced by cloud cover. The spatial variability of hillslope soil moisture and runoff generation also increases due to the crossing of this threshold in the seasonal rainfall distribution. Our findings point to the influence of topography and lateral fluxes during summer periods of high wetness in forested hillslopes of the southwestern United States.

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