SPATIAL AND TEMPORAL PATTERNS OF BEDROCK GROUNDWATER RECHARGE ON SOIL MANTLED HILLSLOPES

The volume and scale of mountain block groundwater circulation plays an important role in watershed hydrologic function; carbon, geochemical and nutrient budgets; and response to climate change. However, mountain block groundwater remains one of the least understood components of the hydrologic cycle. Subsurface flow on hillslopes is subject to complex gravitational and capillary forces both in the soil and underlying bedrock, the balance of which determine the amount of slope parallel soil flow and vertical bedrock recharge. In this project, full-Richard's equation modeling of variably saturated soil and bedrock groundwater flow, combined with observations of soil and bedrock hydraulic response on an upland hillslope in west-central Montana, are used to identify dominant patterns of bed-rock groundwater recharge and discharge on soil-mantled mountainous hillslopes. These tools are used to identify first order processes controlling the spatial distribution and volume of groundwater circulation on hillslopes and the partitioning between slope parallel through-flow versus bedrock recharge. Monte-Carlo simulations are used to evaluate the relative role of topography, soil characteristics, underlying lithology and antecedent moisture conditions in governing the location and volume of groundwater recharge. The basic relationships derived provide new insight into where and when deep-groundwater recharge and discharge occurs, and improved constraint on the volume of deep groundwater circulation in upland catchments. These results can be used to help hydrogeologists develop better conceptual models and estimates of bedrock groundwater circulation in upland catchments and its role in watershed hydrologic and biogeochemical function.