Modeling river discharge rates in California watersheds

River discharge rates across all California's watershed have been modeled using the NASA version of the Carnegie-Ames-Stanford Approach (CASA) ecosystem model coupled with a surface hydrologic routing scheme previously called the Hydrological Routing Algorithm (HYDRA). To assess CASA-HYDRA's capability to estimate actual water flows in extreme and non-extreme precipitation years, we have organized hundreds of California river gauge records for comparison to monthly model predictions. Previously, CASA-HYDRA snowmelt algorithms were modified with equations from the USDA Snowmelt Runoff Model, which has been designed to predict daily stream flow in mountain basins where snowmelt is a major runoff factor. Based on model predictions of monthly flow rates across 336 stream gauges statewide, the multi-year model-to-measurement correlation between monthly river flow rates was R²=0.72. The model output was 15% higher across all these stream gauges than the measured monthly flow records for 1982–1990. It is plausible that the model would predict higher flow rates statewide than was measured at many gauge locations, due mainly to extensive water diversions for power generation and crop irrigation in the valley growing regions of the state. Predictions for gauges located on the state's North Coast and Sierra regions showed errors distributed fairly evenly throughout the seasons, whereas results for Central Coast and Southern regions showed higher errors mainly during the summer and fall. Future model applications for land cover and climate change in California are outlined.