The discharge (R) is besides the Precipitation (P) and the Evaporation (E) a further number in the basic hydrological equation.

According to the common equation of the water balance R = P - E for a longer period of time results under consideration of change of storage R = P + E + (S - U) for shorter periods of time. The discharge can be devided into surface run-off (Rs) and underground run-off (Ru).

Purpose of Discharge Measurement

The discharge measurement is precondition to draw up water balances because it is one of three components of the water balance. Likewise the knowledge of the discharge is important, in order to be able to make flood forecasts due to precipitation events. Further areas of application are irrigation, drainage, generation of current by water power, water supply, water quality economy, erosion protection, salination of soil and the nature protection.
By measuring water levels at different levels (places) one receives the discharge. With rising water level also the discharge rises. In addition the discharge depends on the size of the catchment area. The more largely the catchment area, the more largely usually also the discharge. Since from the discharges alone yet the characteristic of discharge cannot be concloded, these are referred to a uniform unit area (m²). One speaks here of depth of discharge (mm). The depth of discharge can be compared with the depth of precipitation and the depth of evaporation, which are also related to a square metre.