THE INCREASED FLOOD FREQUENCY OF THE RIVER RHINE: THE RESULT OF CLIMATE OR LAND-USE CHANGE?

A number of severe floods along the River Rhine in the past few decades has caused much concern in Germany. Other rivers, such as the Danube in Southern Germany and the Oder in the Eastern part of the country, show a similar picture. It seems that the frequency of such floods has increased. It is generally assumed that climatic changes are responsible for the changed flood behavior of the Rhine. To examine whether such assumptions are justified, we analyzed flood data of the Rhine, collected in Cologne in the period 1900-2000. Our analysis indicates that the discharge behavior of the Rhine at Cologne has changed significantly in the course of the past century. Both the flood frequency and the mean flood peak have increased in the course of time. At least two factors seem to be responsible for the changed discharge behavior. The precipitation-runoff relation of the Rhine catchment as well as the frequency of extreme rainfall events seem to have changed. In our opinion changes in land use in postwar Germany (after 1950), especially in agriculture, can be made responsible for the changed precipitation-runoff behavior. They include the decrease of the meadowland area, the increase of the drained agricultural land area, the growth of the land area used for the cultivation of runoff-enhancing crops, and the large-scale physical degradation of arable soils. Additionally, the urban area with sealed surfaces was almost doubled. To reduce the threat of river floods, we recommend that conservation tillage in Germany is practized more widely.