Millions of dollars and countless lives are saved each year from timely and accurate predictions of both flood crests and flood duration by the National Weather Service. Many people depend upon this information, from dam operators to anglers, from farmers to sewage treatment plants.

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. In order to do that, there are three components one must look at: base flow, runoff, and routed flow.

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. It comes from two sources: rainfall and snow melt. 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.

When rain begins to fall, the runoff does not immediately go into the river or stream. It takes time for the runoff to flow into the river and the amount of time depends upon where in the basin the rain fell.

Routed flow is the amount of water coming from an upstream point. There are several methods of determining this flow, but all depend on the fact that all of the water that passed through an upstream measuring point must eventually pass through the downstream point, barring human intervention.

Various factors determine that the peak flow at the downstream locations never more than the upstream location, and often the peak flow downstream is less. Some of these factors are the viscosity of water, the slope of the riverbed, and the distance between measuring points.