Integrated model-data observations of water flow dynamics across bedrock and vegetation variations of a mountainous hillslope

Abstract

Hydrological processes in mountainous watersheds, and how soil, bedrock, and plants interact are still poorly understood. Through a dense network of soil moisture and temperature sensors, high resolution electrical resistivity tomography monitoring, and weather data we assess the above and below-ground processes driving the hydrological response of a hillslope during snowmelt and summer monsoon. The monitoring transect covers different bedrock and vegetation types, with a steep upper part characterized by shallow bedrock and covered by pine trees, and a gentle lower part underlain by colluvium and covered mostly by grass and veratrum. Coupling the monitoring data with a simplified hydrological model, we observe several important hydrological processes that show how variations in bedrock and vegetation type change subsurface flow patterns, allowing us to answer how subsurface flow pathways differ between shallow and deep bedrock units, and to assess the interactions between vegetation, bedrock types and subsurface flow dynamics. While on the steep section, characterized by thin soil and shallow bedrock, we observe mostly shallow and rapid lateral flow, on the gentle slope underlain by colluvium vertical flow is prevailing. Timelapse resistivity patterns indicate that for shallow bedrock, fractures and tree roots may provide preferential flow pathways into deeper bedrock units during snowmelt, which may provide means to mitigate summer drought conditions. Shading of the trees seems to further mitigate drought conditions by limiting evaporation of summer monsoon rainfall, leading to less drying of the shallow soil layer. In the lower, gentle part of the profile snowmelt is contributing to vertical flow recharging the aquifer, while in the summer upwelling groundwater is providing moisture to sustain plant growth. These observations show that variations in bedrock and vegetation pose a strong control on hillslope hydrology, creating spatially complex flow patterns. These results highlight the spatial heterogeneity of hydrological processes in mountainous watersheds, which need to be understood to predict how watersheds respond to disturbances.