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Water Balance Equation

Within a watershed, all of the water falling as precipitation is either: 1) stored in the soil, 2) returned to the atmosphere, or 3) released from the watershed via runoff or subsurface flow. These processes can be summarized in the water balance equation as follows:

Q = P - ET St


  • Q = discharge of water outside of watershed via runoff or subsurface flow
  • P = precipitation
  • ET = evapotranspiration (both evaporation and evapotranspiration)
  • St = soil storage

Impervious area in an urban environment.This equation is useful because it shows that given a steady amount of precipitation, Q will increase if ET is decreased. That is, if plant cover within a watershed is decreased, evapotranspiration may also decrease, thereby possibly increasing runoff. These effects may or may not be desirable. Similarly, if more stream flow (Q) is desired, such as for recreational uses, either P must be increased via irrigation, or ET or St must somehow be decreased. This is often accomplished by increasing impervious or impenetrable surfaces such as pavement within a drainage basin.

Click here to access an interactive tool that demonstrates how watershed discharge (Q) is affected by the amount of impervious surface within the watershed.

Each of the elements of this equation are covered in more depth in the following sections. Precipitation is described in greater detail in the Climate module. Evapotranspiration, soil storage, and discharge are discussed more fully in the Soils module of this website.

Above: impervious surface decreases infiltration in this urban environment.


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