SENSITIVITY OF MOUNTAIN-BLOCK HYDROLOGY TO HETEROGENEOUS SOIL DEPTH AND RECHARGE

Many valley-fill groundwater basins in arid and semi-arid regions around the world receive a majority of recharge from adjacent mountain-block (MB) systems. Given the dependence of these basins on the mountain block, it is vital to understand MB subsurface hydrology. To date, most groundwater modeling studies of MB hydrology have represented the MB system with homogeneous soil and bedrock layers and constant recharge rates in space and time. MB systems have highly heterogeneous precipitation and geology, and the effects of these on the partitioning of flowpaths between local, intermediate, and regional flowpaths have not yet been examined. This study aims to understand how subsurface structure and spatiotemporal variations in recharge affect MB flowpaths, using a watershed in the Santa Catalina mountains north of Tucson, AZ as a case study. We use the fully integrated hydrology model ParFlow coupled with the particle-tracking code EcoSLIM to map flowpaths and hydrologic residence times to assess the sensitivity of MB flowpaths to these heterogeneities. We simulate multiple scenarios varying the depths and hydraulic conductivities of the upper layers of the subsurface to cover a range of physically realistic conditions. Results include water table configurations and transit time distributions of particles flowing through the domain (including overland flow, the vadose zone, and the saturated zone) that indicate the partitioning between intermediate and local flowpaths. By comparing these distributions between scenarios, we can determine the sensitivity of changes in flowpaths to varying degrees of heterogeneity in subsurface configurations. This work can inform and constrain future modeling efforts and guide data collection in MB systems.