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. 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.