Challenges in Alpine Watershed Modeling Associated with Q Similarity to the P – Q – ET Residual

Monday, 15 December 2014
Steven M Jepsen1, Thomas C Harmon1,2, Carolyn T Hunsaker3, Matthew W Meadows1 and Yuning Shi4, (1)University of California Merced, Sierra Nevada Research Institute, Merced, CA, United States, (2)University of California Merced, School of Engineering, Merced, CA, United States, (3)USDA Forest Service, Pacific Southwest Research Station, Fresno, CA, United States, (4)The Pennsylvania State University, Earth and Environmental Systems Institute, University Park, PA, United States
There is a need to accurately account for the different fluxes of water (streamflow, ET, groundwater flow) exiting alpine catchments at the present time and into the future, as this will inform efforts to plan and adapt to future changes in the environment. This is particularly true for the Sierra Nevada Mountains of California, which supply over half of California’s developed water to over 38 million people and where the maritime snowpack volumes are predicted to shrink substantially with 21st century climate warming. With this need in mind, we are modeling streamflow from a 1 km2 catchment (Providence P301) in the Southern Sierra Critical Zone Observatory during the period of 2009 – 2012, using the Penn State Integrated Hydrologic Model (PIHM). Our objective is to examine possible future changes in the water budget of the catchment with reduced snowpacks in a warmer climate, and to formalize procedures for model calibration and sensitivity analysis. During the 4-year period of examination, the observed mean residual in the water budget components, P – Q – ET, is comparable in magnitude to the stream discharge, Q (22 cm versus 51 cm, respectively). The comparable magnitude of these quantities leads to unsatisfactory error statistics in modeled runoff (typical Nash-Sutcliffe values ~ 0). Substantially better statistics are obtained after normalizing modeled discharge to observed annual totals (Nash-Sutcliffe values ~ 0.6 – 0.7), suggesting that much of the interannual variability in timing of runoff is being captured. Possible causes for the large observed residual in P – Q – ET are explored, including measurement errors in the individual water budget components, and modeled changes in subsurface storage. Based on observed changes in precipitation over the last decade, change in subsurface storage likely accounts for no more than one-fourth of the observed residual in P – Q – ET, with measurement error accounting for the remainder.