Hydrologic Response to Widespread Land Cover Change Simulated from the Hillslope through the Regional Scale

Abstract:

The recent mountain pine beetle outbreak in the Rocky Mountains of Western North America provides an opportunity to probe the ecohydrologic response to land cover disturbance that is largely unprecedented in spatial extent. Understanding this response is limited by our ability to measure and model across scales. Here, we apply fully-integrated models (that span the subsurface through the land surface) across a range of different landscape scales to study the impacts of land cover change on the hydrologic cycle in important Rocky Mountain headwater systems. These models also provide a controlled method to synthesize field observations of hydrochemical properties. Analysis of hillslope outflow partitioning from groundwater and precipitation sources indicates that topography and precipitation can drive mixed signatures in groundwater inputs over prolonged periods. Watershed-scale models confirm that competition between offsetting processes at the hillslope (such as increases in evaporation and decreases in transpiration with tree death) and heterogeneous watershed properties effectively mute changes in water yield. Both hillslope and watershed models identify the importance of subsurface flow and storage changes, indicating that soil and groundwater may play an important role in the propagation of the hydrologic response to land cover change. Accordingly, regional models are used to examine changes in longer subsurface flow paths. Alterations to water sources and pathways may explain observed changes in water quality that are not expected if only the minimal detectable change in discharge is considered. This work, and its usefulness in interpreting field observations, provides additional mechanistic insight into hydrologic response from spatially and temporally variable land-cover perturbations.