Pitfalls of Forecasting Water-Year Type Classification in New Hydropower Licenses

Hydropower facilities typically operate under 30-year licenses from the Federal Energy Regulatory Commission. In the recent licenses that have been obtained by Pacific Gas and Electric Company (Company), a key part of the license is the setting of the minimum streamflow regime in the river reaches that are affected by project facilities such as dams and diversions. Current ecological concepts value inter- and intra-annual variation in streamflow that reflects the large year-to-year variation in water-year magnitude. Runoff forecasting based on snow resources allows the definition of water-year types as the winter progresses, and streamflows are implemented accordingly. Monthly forecasts are sensitive to a variety of error sources during the 4-5 month forecast period. Errors in water-year type designation can disrupt plans for biological studies, construction schedules, and adversely affect reproduction success of amphibian and other aquatic species. Errors in forecasting and license inflexibility can also result in implementation of streamflows that are higher than necessary and cost the Company millions of dollars. Collection of accurate data from well-designed snow course, snow sensor, and climate stations is essential to minimize forecast error.