Water introduced into an area by precipitation enters various water balance processes, such as evaporation, surface and underground drainage, and changes in the water supplies. These processes and quantities depend upon the area’s characteristics and climatological conditions. Runoff formation is derived as an average temporal value from the difference between precipitation and evaporation in area segments of a water catchment area. Changes in the water supplies, such as soil moisture, groundwater, and bodies of water, are neglected. Runoff formation best characterizes the hydrological conditions of area segments and catchment areas. Areas covered with vegetation have large amounts of evaporation, particularly areas with near-surface groundwater and/or bodies of water with their large surfaces. These surfaces do not produce much runoff. Sealed surfaces hinder evaporation and produce high quantities of runoff. Area segments are fed by surface or underground inflows from runoff formed by higher area segments, in addition to its own runoff. These water quantities formed by runoff and inflow then flow to areas located below. The water supply of a closed catchment area is thus the sum of runoff from all area segments and total surface and underground inflows. The quantity of exploitable water is generally much smaller and is determined in consideration of such factors as hydrology (temporal availability), hydrogeology (exploitability), hydrochemical (properties), and influences of use-structures (degree of development, nature preserves).
Meaningfulness of Parameter
Knowledge of regional distribution of runoff are the basis for:
- evaluating the area’s water resources as well as areas to be supplied and areas to be depleted;
- identification of water resources by comparing water supplies and water uses, and determination of measures to secure water supply sustainability;
- to issue resource-sparing water use permits;
- to determine optimal locations for water use facilities in regard to availability and allocation.
Runoff is a medium of material transport and thus of pollutant transport. The maximum available amount of runoff is the value required to calculate the transport of substances, and thus for estimating the degree of vulnerability of water resources to pollution.
Berlin-Brandenburg is part of the north German lowland, with relatively thick unconsolidated stone deposits. Among these deposits are the ground moraine flat upland areas formed during the ice ages. The groundwater aquifer is covered by a mantle of 10 to 30 m thick and poorly-permeable boulder marl complexes.
End moraine areas are to be considered as complex groundwater aquifers not covered by a mantle because of their heterogeneous geologic composition and the fracture zones created by glacial dynamics.
Individual groundwater storeys in ground and end moraines are recharged entirely by runoff. Hydrological investigations and observations of outflows into watercourses are required to clarify what quantities of alimentation (feed) amounts in the upper groundwater story can be used locally; and what quantities flow underground to watercourses in the flat upland areas, or flow through deeper groundwater layers into the "Urstromtal" glacial valley watercourse. These calculations require a knowledge of the underground catchment area (balance area).
The ice-age melt-water deposits of the outwash plains and glacial valleys, in contrast, form large areas of groundwater aquifers without a covering mantle, i.e. they are not covered by non-permeable strata. In the glacial valley and outwash plain areas infiltrated precipitation percolates to the groundwater table, after losses due to evaporation. The time required for groundwater recharge here corresponds to runoff formation (cf. Map 01.05, SenStadtUm, in preparation).
Particular hydrological problems occur in urban areas with sealed surfaces (building roofs, streets, squares, etc.). Runoff formation here is clearly increased due to reduced capabilities of water storage compared to unsealed areas. Some precipitation is discharged directly into watercourses, depending on the type of sewer system (cf. Map 02.09, SenStadtUm 1992b). The remainder of runoff infiltrates at the edge of sealed surfaces, or within partially sealed areas, reaches deeper layers underneath the evaporative zone, and recharges groundwater. Groundwater recharge for these areas can be determined by comparing runoff quantities with quantities led off by the rainwater drainage system, if the structural conditions of the rainwater drainage system are known. Such statements are relevant for the water and substance balances of inner city watercourses and groundwater.