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Water Supply Information Documentation

Water Supply Forecasts based on Ensemble Streamflow Prediction technique (ESP)

The Northwest River Forecast Center (NWRFC) currently uses an ensemble streamflow prediction technique to make water supply forecasts for the Columbia River Basin; the coastal streams of Washington and Oregon; and the Great Basin of Oregon. Underpinning the technique is a real-time, conceptual hydrologic modeling system.

METHODOLOGY - Ensemble Streamflow Prediction (ESP) is a modeling component of the National Weather Service Community Hydrologic Forecast System (CHPS). ESP produces long-range probabilistic forecasts of hydrologic variables. ESP utilizes a conceptually based modeling system to simulate soil moisture, snowpack, regulation, and streamflow. ESP then accesses the current hydrologic model states, and uses historical meteorological data to create equally likely sequences of future hydrological conditions, each starting with the current hydrological conditions. Statistical analysis is performed on these sequences to generate probabilistic forecasts of seasonal water supply.

ESP water supply forecasts are constructed to predict, as close as possible, the same adjusted volumes that are estimated by NWRFC streamflow runoff products. For most locations, runoff is adjusted for major upstream storage changes using long-standing equations that have been coordinated with partner agencies.

ESP natural forecasts use additional modeling capabilities to provide a consistent set of natural forecasts for all locations within the network. Adjustments include upstream storage and modeled consumptive use. In addition, the natural forecasts have the benefit of a routing component that is absent from the simple adjustment scheme used in our water supply forecast.

UNITS - Seasonal water supply forecasts and runoff volumes are reported in terms of Acre-Feet (AF). An acre-foot is a volume equal to one acre of land surface covered by one foot of water (also equal to 43,560 cubic feet).

1 KAF = 1,000 Acre-Feet (K=thousands)

1 MAF = 1,000,000 Acre-Feet (M=millions)