Components of Water Balance

Water Balance - the Hydrological Cycle

According to DIN 4049, ground water is a permanent consequence of the changing of water in its state and location in form of precipitation, run-off and evaporation.

The air-transported humidity comes as precipitation onto the ground. A part evaporates (transpiration, interception, evaporation), a part flows on the surface into rivers, another part flows underground (ground water). Ground water can come to the surface as springs.

The exact knowledge of hydrological basic data (precipitation, evaporation, surface run-off) as well as hydrogeological basic data (subterranean run-off, use, reserve) is needed for the clarification and evaluation of ground water processes.

The hydrological basic equation describes the hydrological cycle in a quantitative way:

P = R + E 

P = Precipitation; from aerosphere retiraled water
E = Evaporation from soil and free expanses of water
R = Run-off

This equation is only valid for long periods of time. It is used to calculate mean values for long periods of time.
For shorter periods of time the equation is:

P = Rs + Ru + E
or rather
P = Rs + (R - U) + E

Rs = surface run-off (brook, river etc.)
Ru = underground run-off; proportion of run-off, which moves into subsoil and ground water.

Ru = S - U

S = Storage; increase of water deposit of an area for defined period of time (= increase of ground water deposit)
U = Usage; decrease of water deposit of an area for defined period of time (= decrease of ground water deposit)

The modified basic equation contains the yearly weather-related fluctuations of precipitation, evaporation and run-off.
Reserves und depletion play an important role in the water balance cycle.

Precipitation

Precipitation (drizzle, dew, fog, rain, snow) plays an important role in the water balance process because ground water mainly consists of trickled precipitation. Snow is rare in semi-arid areas, so although it can recharge the ground water in lower areas, it does not have much significance in African water management.

Run-off

Another important hydrological factor is the run-off.

Evaporation

The common equation of water balance (P = R + E) for a long period of time is amended for shorter periods as follows: E = P - R - (S - U). A considerable part of the precipitation.

Evaporation can be subdivided into:

  1. Evaporation = Evaporation from soil or water surfaces
    A special form of evaporation is interception. It absorbs a part of the precipitation by plants. The precipitation evaporates directly from the plant without reaching the soil.
  2. Evapotranspiration (ET) = Sum of evaporation and transpiration of the vegetation
    The evaporation depends only on physical factors, especially on solar energy, air temperature, humidity and the topography and state of subsoil.

The type of vegetation plays an important role in evapotranspiration. We further differentiate between the potential (maximum possible amount of evaporation) and the actual amount of evaporation. Read more about it here.

The modified basic equation contains the yearly weather-related fluctuations of precipitation, evaporation and run-off.
Reserves und depletion play an important role in the water balance cycle.

Precipitation

Precipitation (drizzle, dew, fog, rain, snow) plays an important role in the water balance process because ground water mainly consists of trickled precipitation. Snow is rare in semi-arid areas, so although it can recharge the ground water in lower areas, it does not have much significance in African water management.

Run-off

Another important hydrological factor is the run-off.

Evaporation

The common equation of water balance (P = R + E) for a long period of time is amended for shorter periods as follows: E = P - R - (S - U). A considerable part of the precipitation.

Evaporation can be subdivided into:

  1. Evaporation = Evaporation from soil or water surfaces
    A special form of evaporation is interception. It absorbs a part of the precipitation by plants. The precipitation evaporates directly from the plant without reaching the soil.
  2. Evapotranspiration (ET) = Sum of evaporation and transpiration of the vegetation
    The evaporation depends only on physical factors, especially on solar energy, air temperature, humidity and the topography and state of subsoil.

The type of vegetation plays an important role in evapotranspiration. We further differentiate between the potential (maximum possible amount of evaporation) and the actual amount of evaporation. Read more about it here.