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Maintaining an Adequate Water Supply:
How DEP Forecasts Reservoir Levels

Forecasting reservoir levels is one of the most important – and most difficult – tasks DEP faces in the operation of the water supply. Reservoir levels are primarily determined by the balance between streamflow into the reservoirs, diversions (withdrawals) for water supply, and releases to maintain appropriate flows in the rivers below the dams. Streamflow is affected by rain and snowmelt, both of which are hard to predict accurately.

National Weather Service (NWS)

Given the need for rain and snowmelt forecasts to help predict streamflow and thus predict reservoir levels, DEP works closely with the National Weather Service (NWS). Data from DEP’s network of weather stations and snowpack monitoring programs, along with streamflow data from U.S. Geological Survey (USGS) gages paid for largely by DEP, is provided to NWS. The Weather Service uses this data, together with their own weather forecasts and a schedule of planned diversions and releases provided by DEP, to drive computer models that forecast reservoir levels up to two days in advance. These predictions can be seen on the Advanced Hydrologic Prediction Service website of NWS at the links provided here.

NWS also produces long-term inflow forecasts which DEP uses to roughly estimate reservoir levels one to three months in advance by balancing the forecasted inflow with expected diversions and releases.  These long-term forecasts are useful for operations that occur over long time horizons, such as releasing projected excess water slowly over a period of weeks or months. By updating these projections of reservoir levels regularly, DEP can respond to changing conditions and forecasts.

Operations Support Tool (OST)

DEP is in the early stages of testing and development of a new, cutting-edge computer system that will eventually become the primary tool guiding water supply managers making decisions about reservoir operations. This system, called the Operations Support Tool (OST), couples existing computer models of reservoir operations and water quality, ingests real-time data on streamflow, water quality, reservoir levels, diversions, and releases, and incorporates specialized streamflow forecasts.  OST then applies all the operating rules and laws governing reservoir operations, and predicts future reservoir levels and water quality. These predictions allow water supply managers to make operating decisions based on the most current data and forecasts. By running OST regularly, changing environmental conditions and streamflow forecasts will be accounted for, allowing for modification of reservoir operations if required. When complete, OST will be the most sophisticated water supply operations tool in the world, and will provide state-of-the-science guidance to DEP’s water supply managers.

Snow Pillows

The snow pillow is a device that measures the amount of water in the snowpack for a watershed area. Snowpack water is important to the water supply because melting snow drains into reservoirs. The snow pillows being used by DEP are a new design, comprised of an aluminum triangle with highly accurate scales under the corners that weigh the snow and calculate the water content several times an hour, then send the data to a DEP facility via radio transmission. DEP, the first agency in the country to use this state-of-the-art technology, has installed snow pillows on the Catskill/Delaware watersheds. The snow pillows provide continuous, real-time data on snowpack water content.

Links and Resources

Links to resources for weather forecasts, reservoir levels and more.

Reservoir Levels

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