Using Ensemble Streamflow Prediction for Water Supply Forecasting in British Columbia - Practicalities and Pitfalls

BC Hydro has just completed new watershed model calibrations for 20 of its reservoirs, which would enable the use of Ensemble Streamflow Prediction (ESP) techniques for preparing water supply forecasts. ESP methods have been used to forecast the seasonal water supply for BC Hydro's large interior snow-driven basins for a number of years with much success. But using the technique for smaller coastal rain- and rain/snow-driven basins has proven to be much more challenging. This paper will focus on two known issues: snowpack simulation in the model and month-to-month model calibration bias. This study showed that in coastal and interior basins, snowpack adjustments improved the forecast accuracy and slightly decreased the dispersion. The bias correction technique computed daily bias correction offsets as differences between simulated and observed flows during the calibration period. The offsets were then applied to the ensembles of the forecast. The technique produced mixed results.