Supported by thirteen River Forecast Centers , local Weather Service Forecast Offices are responsible for issuing forecasts and warnings of floods in the United States. Determining the flow of water is done in various ways. River forecasting considers base flow, runoff, and routed flow. Gathering information on river stages and various data distribution systems , the forecaster uses a rainfall-runoff model to assist in plotting the flow and stage for a given forecast point. River Forecasting River forecasters must be able to forecast the amount of water flowing through a certain point along a river.
Base flow is the amount of water coming from ground water.
It is never a constant flow and will increase immediately after rainfall, and then decrease until the next rainfall. Runoff is the amount of water coming from surface runoff. Forecasters estimate an amount of runoff for each type based on certain characteristics of the weather and the river basin. Snow melt is based on the air temperature and the amount of sunshine. Rainfall runoff is estimated based on the slope of the land, the amount of urbanization, the soil types, the amount of the last rainfall, the time since the last rainfall, and the amount of evaporation occurring over the river basin.