SEASONAL SURFACE DRAINAGE OF SLOPING FARMLAND: A REVIEW OF ITS HYDROGEOMORPHIC IMPACTS

The combination of runoff-generating areas (saturated soils) and overland flow concentration in features such as drainage ditches makes sloping farmland vulnerable to soil erosion. The establishment of drainage ditches aims at draining the excess of water from the farmland, particularly in areas where soils are saturated in the rainy season. The hydrogeomorphic impacts on the farmland itself and on downstream areas need however also to be studied. Off site, downstream problems comprise higher peak discharges, leading to gully initiation, an increase in sediment load, and flooding problems. On-site problems such as the development of the drainage ditches into (ephemeral) gullies are less documented, although they may be important, as illustrated in the Lake Tana Basin (Ethiopia). The similarities and interactions between ephemeral gully channels and drainage ditches have to be considered to better understand all effects of drainage. Drainage ditches are a potential source of conflict between farmers with different interests and power, as well as between upstream and downstream users. A case study on drainage ditches on sloping farmlands in the Lake Tana Basin showed that nine out of ten catchments had drainage densities by ditches ranging from 53 to 510mha(-1). Drainage ditches were constructed with an average top width of 27 (+/- 9) cm. A significant correlation was found between stone bund density (physical conservation structures) and ditch drainage density (R=-072), in line with the Ethiopian government's ban on drainage ditches in farmlands where stone bunds have been constructed. Copyright (c) 2014 John Wiley & Sons, Ltd.