Water balance (hydrology)

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The water balance is in the earth sciences , the list of the uptake and release of water in a geographic area, such as an ocean , or a landscape. In the case of landscapes, surface water , evaporation , precipitation and groundwater flows are taken into account. The water balance largely determines the ecosystems that can establish themselves in the landscape and the suitability for agricultural use.

Global water balance

Distribution of the amount of water on earth
Global freshwater water cycle. Figures in million km³, arrows with annual flow

The earth has around 1.4 billion km³ of free, i.e. available water. Despite this huge amount, water is a precious commodity because 97% is salt water and therefore only limited in its use. Only 3% is fresh water.

As a result of the sun's rays, around half a million km³ of water evaporates each year and then rains again. This global water cycle feeds all rivers and is an essential driving force behind the weather. A third of the precipitation over land comes from evaporation from sea water. This means that more water is available to the land areas than evaporates over the land. The global water cycle is not only fed by evaporation and precipitation, infiltration, surface runoff and condensation also play a role.

Above all, the large contiguous forests of the world contribute significantly to the water balance of the earth through water absorption from the soil, water storage and evaporation via the leaves.

Geographical aspects of the water balance

The amount of precipitation and the temperatures vary widely around the world. This also means that very different amounts of water evaporate in different regions. For the climatic regions, however, it is less important how large the amount of precipitation is; instead, the ratio of precipitation (N) and evaporation (V) is essential.

This then results in the following climatic differences in relation to the water balance:

Here N> V applies, areas with such a water balance are characterized by many rivers, lakes, seepage water and a sinking groundwater flow.

Here V> N applies, which means that water reserves are used up. Salt lakes, end lakes and only periodically flowing rivers are typical here. Another characteristic is the formation of salt crusts on the ground, which is emphasized by the increasing ground water flow.

  • Nival water balance

Here N → R applies, i.e. This means that the fallen precipitation is withdrawn from the water balance in the form of ice, snow or glacier in the long term and serves as a reserve (R). This type of water balance is a sub-form of the humid water balance and occurs particularly in polar regions and mountains above the snow line.

There are also mixed forms of the water balance:

In areas with a semi-humid water balance, there is a humid water balance for more than 6 months a year, which is then replaced by an arid water balance.

In contrast to the semi-humid water balance, the water balance is arid for more than 6 months and humid water balance for the rest of the time.

Water balance in water management

Water is an important natural resource for many social and economic processes. Therefore, there are laws and ordinances that regulate the handling. In Germany, these include the Federal Water Act (WHG) and the state water laws of the federal states.

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