EFFECTS OF AFFORESTATION ON RUN-OFF CHARACTERISTICS

In order to simulate the effects that changes in the vegetation cover have upon the water balance, and especially upon run-off characteristics, a continuous precipitation run-off model for small watersheds (< 1 km2) in West German highland areas, "mittelgebirge", was developed. The model consists of two parts. First, water balances are drawn up for single sites. In a second step, they are transferred to the whole of the watershed area on the basis of mapped pedohydrotopes. The calculated amounts of seepage (out of the root zone) are superimposed onto the runoff of the catchment area over a series of linear reservoirs. The model is calibrated according to measurements taken in the hydrological research area Krofdorf C (0.33 km2). Then, simulated changes in the vegetation cover (spruce, beech/oak, shrubs, grassland), encompassing almost the whole of the watershed, are computed and compared with the measured run-off, which corresponds to the actual heterogeneous vegetation cover. Afforestation of abandoned land with spruce would lead to a reduction of the total run-off of flood waters and of groundwater replenishment. Elimination of spruce tree vegetation, on the other hand, with grass as follow-up vegetation, would result above all in an increase in floods. Afforestation with beech and oak or a natural growth of shrub vegetation would entail a rainfall run-off regime that lies between these two extremes. © 1990.