Changing river discharges:
This section presents in its first part the development of the river discharge rates of the large German rivers Rhine, Weser, Elbe and Danube. The mean discharges of all these rivers show a positive trend during the past decades, with a particular significant increase for the river Rhine since the 1970ths.
An increase of annual maximum discharge values is most significant in case Rhine after 1960, and less significant for the Danube, while the Elbe and Weser show no particular trend. Concerning trends of the low flow values, the Rhine – and to a lesser degree also the other rivers – show a positive trend, which can mainly be attributed to the low flow management in summer by using reservoirs in the mountain parts of the rivers.
The second part of this section introduces the effects of land use on the development of river floods. It is shown, that land use influences flood production the strongest (up to 20% runoff increase), if rainfall intensities are high and antecedent soil moisture is low. This situation can occur for summer convective rainstorms.
However, summer rainstorms are local meteorological phenomena which do not trigger widespread floods in large river systems, but only in small catchments. Large river floods are caused by advective, long lasting rainfall, which do hardly cause runoff processes influenced by land-use changes.