Evaluation of surface runoff and sedimentation into the Lake Ziway from selected land cover types, central rift valley of Ethiopia

Abstract

Study regionLake Ziway watershed, Central rift valley of Ethiopia Study focusLake Ziway is currently under huge threat of sediment deposition and water consumption from the upper stream areas of the watershed. The objective of this study was to quantify the sediment and runoff contribution of different land cover types in the Lake Ziway watershed. The study has selected three sites systematically in the lower part of the watershed. Within a site, four land cover types that share similar biophysical characteristics were selected for installing runoff plots with the size of 43.3 m2. The rainfall, runoff and sediment samples collected twice a day from each runoff plot. The composite samples filtered by applying 10 % HCL and the extracted sediment dried in oven at 105°C for 24 h. The dried sediment weighed and computed for each land cover types. New hydrological insights for the regionThe surface runoff from cultivated land is significantly higher than the remaining three land cover types. The total surface runoff in the first and second year from cultivated land is 1033.5 mm and 685.5 mm respectively. The sediment contributed from grassland is significantly lower than the remaining three land cover types. The maximum daily sediment loss per hectare from cultivated land is 84.05 kg and 22.67 kg in the first year and second year respectively. The average annual sediment loss from each land cover types are 5.1, 1.1, 0.9 & 0.5 ton/ha for cultivated land, degraded land, grassland and forestland respectively in the first year. There is moderate linear relationship between surface runoff and rainfall across each land cover. The relationship between the surface runoff and sediment loss is positive and linear. The major source of sediment to the Lake Ziway is from the cultivated land, whereas the least sediment source is from forestland of the watershed. The sediment contribution from cultivated land to the lake should be the priority site for conservation planning of the lake and the entire watershed.