Berlin Environmental Atlas 01.12 Soil Functions (Edition 2002)

01.12.4 Regulatory Function for the Water Balance

Description

The regulatory function for the water balance is determined by the storage or retention capacity of the soils. It has an effect on the groundwater and surface-water runoff. The exchange frequency of the groundwater is used as a criterion for this soil function (cf. Map 01.11.4). A low exchange frequency means that the dwell time of the water is long and the water quantity retained in the soil is high. Thus, a low exchange frequency must be seen as positive for the landscape water balance. Longer dwell times in addition permit a better decomposition of immitted substances and therefore have a positive effect on the seepage water quality. However, with high storage capacity and low exchange frequency, the new groundwater entry rate is low, since the precipitation water largely remains in the soil, and is taken up by the plants.

Methodology

The regulatory function for the water balance is derived directly from the evaluation of the exchange frequency of the groundwater (cf. 01.11.4). The evaluation is carried out according to the three layers "low", "medium" and "high", where a very low exchange frequency is considered "high", according to Table 1, a low to medium exchange frequency is considered "medium", and a high to very high exchange frequency is considered "low".

Seepage (without consideration for sealing) (cf. Map 02.13.4) was used for the calculation of the exchange frequency of the groundwater. The level of seepage is in turn not influenced only by precipitation and soil conditions, but substantially, too, by the level of evaporation which is dependent on vegetation and hence on use. In interpreting the map, it must therefore be taken into account that sections with the same soil associations could be evaluated differently, depending on the vegetation influenced by the seepage.

Map Description

Numerous near-natural soil associations receive a high evaluation of regulatory function, with an exchange frequency of groundwater of less than once per annum. These include all groundwater-influenced soil associations with bogs and gleys which are supplied with sufficient water in the topmost meter of soil throughout the year. Due to the high evaporation levels of the vegetation, the seepage from precipitation is very low here - to some extent, groundwater discharge even occurs - so that the exchange frequencies are also very low. The soils of the plateaus of boulder clay/ glacial till constitute another group. They have great storage space, and can retain the precipitation water which occurs very well, due to their low permeability. The dune sites with fine sand as their main soil types also have a great storage space, e.g., clay soils, and should also be assigned to this class.
Near-natural groundwater-remote sites with a groundwater exchange frequency of once to twice per annum achieve a medium evaluation level. These are primarily rusty earths on end and push-moraines, sandy-wedge braunerde on the glacial till plateaus with sand inlays, and gley braunerde-rusty soil associations in the valley-sand sections. In addition, there are soils of dumped and moved natural substrate, like sands and loams, from which regosol-para-rendzina-hortisol soil associations have developed. Soils with a low evaluation, i.e., an exchange frequency of the groundwater of 3 - 4 times per annum, are concentrated in the inner-city area, industrial areas and railway yards. Coarse dumped materials such as construction rubble and track gravel provide high soil permeability, so that the precipitation water percolates quickly.