The method for the (transient) calculation of groundwater recharge rates on land areas according to the soil water balance is based on the theoretical principles set out in DVWK Bulletin 238/1996 "Determining the evaporation of land and water areas".

When calculating groundwater recharge with the soil water balance, the decisive role of the soil as a water reservoir is taken into account by including the following components:

• Potential evaporation (ET_p): Evaporation from surfaces under given meteorological conditions and unlimited availability of water. Potential evaporation is a calculated value that is determined from measured meteorological values (from: DVWK data sheet 238/1996)

• Field capacity (FC): The field capacity is the water holding capacity of the soil against gravity. The field capacity is the maximum water content that a soil can hold under natural conditions.

• Permanent wilting point (PWP): The permanent wilting point describes the plant species-dependent water content of a soil, below which there is no more water available for plants (from: Müller, Tibor: "Wörterbuch und Lexikon der Hydrogeologie"). It is therefore the lowest water content that a soil reaches under natural conditions. It depends on the type of soil and plant.

• Usable field capacity (nFK): The part of the field capacity that can be absorbed by the plants through the roots is called usable field capacity (from: Müller, Tibor: "Wörterbuch und Lexikon der Hydrogeologie"). The usable field capacity is therefore the difference between the field capacity and the permanent wilting point (nFK = FK - PWP).

• Effective root depth (W_e): The effective rooting depth is around 50 - 60 % of the maximum rooting depth. It characterises the soil volume in which the soil water balance is intensively influenced by plant water consumption.

Since the recharge calculation according to the soil water balance primarily refers to natural undeveloped areas, but a groundwater model can usually also include developed areas, these areas are still taken into account in SPRING using the recharge calculation method according to Schroeder and Wyrwich.

The recharge calculation method according to soil water balance, as it is currently implemented in SPRING, is still in a development phase. Therefore, there may well be changes to the required data and input files in the future. The latest programme description can always be found in the web help on our homepage under the menu item "Download & Support: SPRING 6 manual".

Input data in the model file