Water catchment areas are understood as a watershed areas which discharge into a receiving body of water. They are differentiated, depending on the origin of the waters, into above ground (surface waters) and subterranean (groundwater) collecting areas. Their borders are formed by water divides, where water is directed to flow above or below, in different directions, perpendicular to morphological contours or groundwater contours. Water divides of surface catchment areas thus run along the highest morphological levels between two receiving bodies of water.
Overland flows formed by precipitation reach surface bodies of water after a certain time depending on the surface gradient and the storage capabilities of the soil. The water divides of subterranean collecting areas can be recognized from the lines of highest groundwater levels. Groundwater also flows towards bodies of water. The time required depends on the gradient and the geological structure of the groundwater aquifer. The surface and subterranean water divides of a receiving body of water do not usually coincide.
Groundwater withdrawals by waterworks and other consumers effect changes in the groundwater direction of flow, and thus the course and gradients of natural water divides. They form their own groundwater collection areas within a catchment area, and reduce groundwater amounts flowing in the natural watershed.
The total inflow of a receiving body of water corresponds to the amount of precipitation in its collecting area, minus evaporation. The portion which becomes overland flow depends on the lithological structure of near-surface strata, vegetal covering, relief intensity, and the degree of sealing and other anthropogenic influences. The remaining portion of precipitation infiltrates the soil and percolates into the groundwater table. The time required depends on the structure and thickness of the covering strata. This process is called groundwater recharge and is the critical factor in managing a water economy. Within a collecting area, only recharge amounts of water are available for waterworks and other consumers. Groundwater withdrawals larger than recharge amounts lead to the lowering of groundwater tables over a wide area, as well as ecological damages resulting from lowered water tables. Water economical considerations must be observed to avoid negative consequences. The determination that groundwater removals within a collection area exceed the amounts of groundwater recharge requires either the reduction of removals, or the increase of groundwater recharges by other measures. Recharge can be performed by the percolation of surface waters from infiltration basins made for that purpose. Increased removals of bank-filtered water by well galleries in the shore vicinity of surface waters also spare groundwater and thus aid the recharge process.