Effects of the floodplain on river discharge into Lake Tana (Ethiopia)

This paper presents a study on an extensive floodplain adjacent to Lake Tana (Ethiopia) and its lowland tributaries, to enhance our understanding of the water of the lake and to better manage the natural resources in the floodplain. Discharge measurements were made at 12 stations. The effects of the floodplain on river discharges were investigated using the upstream and downstream discharge observations of the Gumara, Rib and Megech Rivers. The total annual runoff coefficients ranged between 0.23 and 0.81 in 2012 in the basin. Discharge varied depending on drainage density (r= 0.75), lithology (r= -0.72 for percentage of Tertiary igneous rocks) and land use/land cover conditions (r= 0.61 for dominantly cultivated land with no significant other land use types). Analyses revealed that the floodplain abstracted 809. mm of water with a corresponding increase in floodplain storage of 992. mm during the beginning of the rainy season (June to July) and released stored water starting from August until the middle of September. However, the annual water balance indicated that the runoff contribution from the Rib and Megech floodplains is negligible. But the floodplain downstream of the Gumara River showed a considerable runoff contribution to the river, also in relation to the presence of springs. The floodplain acts as storage of flood water, and consequently the magnitude of peak floods was on average 71 smaller downstream than upstream in the floodplain.