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Water Supply Information Documentation

Water Supply Forecasts based on Multiple Regression

INTRODUCTION - Water in the West almost never flows unobstructed from where it fell and melted (since it is often snow) to where it will rest in an ocean or lake. Rather it is rerouted through tunnels and ditches, diverted onto croplands and into treatment plants, and temporarily impounded in reservoirs many times along the way. To keep the science of hydrologic forecasting and the engineering of water management separate, the River Forecast Center (RFC) and its associates forecast adjusted or unimpaired runoff, not observed or regulated runoff. The relationship between observable hydrologic parameters (e.g., precipitation, snowpack, terrain, etc.) and adjusted runoff is predictable and well-defined; the same cannot be said for regulated (or managed) runoff. In the Pacific Northwest adjustments are included only if they exceed 10% of the volume at the forecasted site.

Water Supply Forecasts based on Ensemble Prediction System (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 Prediction System (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, snow pack, 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.

NWSRFS is a continual simulating model. ESP can take advantage of this constant updating to provide water supply estimation weekly and through out the year. Future, implementations will make use of the information available in the Climate Prediction Center estimation of precipitation and temperature to shift the forecasts based on our understanding of future meteorological conditions.

The ESP water supply forecast have been constructed to predict as close as possible the same adjusted volumes that are estimated in the NWRFC streamflow runoff prodcuts. These forecasts do not always represent true natural streamflow conditions. Adjustments for diversions and regulations are only partly taken into account.

ESP Natural

METHODOLOGY - Produced the same as ESP water supply, ESP natural represents natural volume forecasts. "Natural" assumes no regulation or diversions take place above the forecast point. This method takes into account all adjustments above a forecast point. In many cases, ESP natural and ESP water supply will be the same.

ESP natural does not combine the observed data with forecasted data to forecast a period which may extend into the past. It always looks into the future. ESP natural is run weekly and through out the year.

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)