Surface Runoff, Percolation, Total Runoff and Evaporation from Precipitation 2005

Introduction

The prerequisite for a water-management planning process and a management system for water resources oriented toward the principles of sustainability is that the knowledge of surface runoff and percolation, and of new groundwater formation, be as precise as possible. For this, an accounting of the components of the water balance is of special importance, especially in the Berlin area, which has only limited water resources, compared with other urban areas, and where the number of its inhabitants and their drinking and industrial water needs, and the associated sewage output, result in a structural water-management deficit.

Moreover:

• It is important for the prevention of water pollution to be able to assess the amount of surface water flowing into the local bodies of water, since the precipitation water carries a considerable pollutant load with it into those bodies of water;
• It is important for the protection of groundwater to have knowledge of the percolation capacity of the soils, since the transportation of substances from contaminated soils occurs largely via percolation water;
• It is important for conservation and landscape management to assess the water availability for vegetation from new groundwater formation and capillary water rising from the groundwater surface.

The water supplied by precipitation to an area is broken down into the various components of the water balance, depending on climatological conditions and other local characteristics. These components are evaporation, surface runoff, sub-surface runoff (percolation or new groundwater formation) and water-inventory change. The parameter which must initially be ascertained is total runoff, the sum of surface and sub-surface runoff.

According to the general water-balance equation, total runoff equals the difference between precipitation and real evaporation. In this calculation, evaporation is the decisive quantum which, under natural conditions, is determined largely by vegetation, climatic conditions and soil conditions.

In urban areas however, real evaporation is considerably different from that of the surrounding countryside. Buildings and impervious areas in cities cause evaporation to be considerably lower than in areas covered with vegetation. While the plants continually perspire through their foliage, the only water to evaporate from the surfaces of buildings and impervious areas after rainfall is that small amount which has remained on their surfaces. Thus, total runoff is considerably higher in urban areas than in vegetation-rich areas.

Fig. 1: Water balance in vegetation areas and impervious areas

Total runoff best characterizes the hydrologic conditions of catchment areas and segments. For closed catchment areas, the sum of the runoff of all segments equals the total surface and sub-surface runoff of the area, i.e., the water supply.

In urban areas with impervious surfaces, part of the total runoff flows directly into the watercourses via the appropriate inflow points, or indirectly via the sewage treatment plants – regardless of the degree of connection of these areas to the sewage system. The rest of the runoff infiltrates the ground at the edge of the impervious areas or within the partially impervious areas, into the strata below the evaporation-affected zone, and thus recharges the groundwater. Given knowledge of the status of the expansion of the rainwater sewage system, the percolation, or new groundwater formation, for these areas can therefore be ascertained by subtracting the entry of rainwater into the sewage system from the total runoff amount.

The values on percolation and surface runoff thus ascertained are primarily of importance for water-management issues, and are also important characteristic quantities for the water balance of urban areas.

Moreover, in the context of the assessment of the efficiency of the soils for precautionary soil protection or for intervention assessment under the Conservation Law, the determination of percolation on pervious soil surfaces is of special interest. On the one hand, the differing percolation capacity of soils can be derived from this value. On the other, the effect that any planned future impervious coverage would have on the percolation capacity of a project area can be assessed in the context of the planning process.

These statements cannot be made on the basis of the values of Map 02.13.2, since the respective reference surfaces shown here are given with mean average values of segments containing both impervious and pervious, and both sewer-system-connected and non-connected portions.

For these reasons, in addition to Map 02.13.2, the ascertainment and representation of percolation on pervious areas has been carried out for Map 02.13.4. It shows percolation of precipitation on pervious surfaces. The values shown refer only to the pervious portions of the blocks or segments.