Sediment yield by gully erosion in a sub catchment of the Awassa watershed, Ethiopia

Abstract

Lake Awassa is a lake in the Ethiopian Rift Valley and undergoes a water level rise since the last few decades. Local people say that this increase is caused by an increase in surface runoff due to deforestation. Another hypothesis suggests that the water level rise is due to siltation of the lake by erosion and especially gully erosion. To investigate the second hypothesis, a sub catchment of the Awassa watershed is observed in detail. In this area gully erosion is observed in 2012 by carrying out a field survey and in 1972 by stereo aerial photography. The sub catchment consists of two gully systems which contain 12 gullies in total. Gully dimension measurements were done in the field and with the aerial photographs to obtain the gully erosion rate over the last 40 years. In addition the land use was classified in 1986, 1994 and 2011 by Landsat 5 TM imagery to link the gully erosion with land use changes. After processing all the data, accurate gully measurements with the aerial photographs turned out to be impossible due to poor quality of the digitized photos. The volumes and gully erosion rates were calculated, assuming that the gullies started developing in 1972. The total volume of gully system 1 is 965000 m3 and the gully erosion rate is 23.4 t ha-1y-1, for gully system 2 the total volume is estimated to be 1778000 m3 and the gully erosion rate 20.7 t ha-1y-1. Both gully systems are active and observations in the field indicate that gully system 2 is connected with Lake Awassa and deposits its sediment in the lake. There can be concluded that gully erosion contributes to siltation of Lake Awassa.