Bertrand Creek Hydrologic Model Update and Assessment 2008; Bertrand Creek Improvement Project Grant No. G0800436

This report describes the Bertrand Creek update of the surface water quantity hydrologic model, TOPNET, which was originally developed as part of the WRIA1 Watershed Management Project. The model update has increased the spatial discretization of model elements, which has resulted in an improved representation of the spatial variability of precipitation, as well as land use and soil based parameters in both the U.S. and Canadian portions of the watershed. After updating the hydrologic model, we conducted a new calibration to the observed streamflow measurements at Bertrand Creek watershed outlet at Rathbone Road, which began in 2003. Nash-Sutcliffe efficiency statistics greater than E= 0.85 show that the updated model and calibration are a good representation of the hydrologic system, given the current drainage and groundwater pumping practices. Using a historic climate record of 60 years, the water balance presented illustrates the mean monthly relationships between precipitation, baseflow, soil storage, surface runoff, and irrigation demand. Exceedence probability flow results show that there is a low probability of sustaining both irrigation water use and instream flows without significant water management activities. The updated model developed in this work can be used to understand the potential effects of proposed management solutions with for scenario testing and sensitivity analysis in future work.