Hydrological losses from precipitation

Hydrological losses are the parts of precipitation that do not become direct (surface) runoff. They are:

1. Evaporation: Water changes to vapour from open water, wet soil and wet surfaces. It depends on temperature, humidity, wind and radiation.
2. Transpiration: Water taken from the soil by plant roots and released through leaves. Together with evaporation it is called evapotranspiration, the largest loss from land.
3. Interception: Rain held on leaves and branches and evaporated later.
4. Depression storage: Water held in small depressions on the ground, lost by evaporation and infiltration.
5. Infiltration: Water entering the soil. It is the biggest loss in a storm; it recharges soil moisture and groundwater.
6. Watershed leakage: Water leaving the catchment through deep percolation or seepage to another basin.

The sum of interception, depression storage and early infiltration is called the initial loss (abstraction). The remaining part of rain is rainfall excess (effective rainfall) that forms direct runoff:

$$P_{eff} = P - \text{losses}$$

Evaporation and transpiration continue in all periods (long-term losses), while the others occur mainly during storms.