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ABSTRACTA water resources development that is carried out on the basis of water budget information definitely will minimize and/or avoid the water resources that are going to be exposed to risk and mismanagement, and hence led to a proper utilization of this precious resource. The main purpose of this study was to conduct the water balance of the May Nugus catchment, located in Tigray Regional State, Northern Regional State of Ethiopia, having an area of about 14.73 km2. The catchment includes a small dam called May Nugus that has an effective watershed area of 13.05km2, a reservoir capacity of 23.82 x 106 m3 and command area of about 1.24 sq. km. The hydrology of the area was characterized based on land use, soil, slope, rainfall, temperature, evaporation, evapotranspiration, and runoff. In order to determine the basic hydrologic parameters, meteorological data was collected from the nearby stations. The rainfall coefficient method was used to determine the monthly distribution of rainfall and then to distinguish between rainy months and dry months. The Penman method was used to calculate the evaporation from the reservoir. The Thornthwaite method and Thornthwaite water balance model were used for determination of the potential evapotranspiration and actual evapotranspiration, respectively. For runoff estimation, the method employed was runoff coefficient. The catchment is characterized by one rainy season and two dry seasons during the year. The rainy season has five months whereas the dry seasons have seven months. The mean annual rainfall of the catchment is 737.57 mm. Rains in the rainy season accounts for 81.24 % of the mean annual rainfall whereas the amount of rainfall that occurs during the seven months of the dry seasons in total accounts for 18.76 % of the average annual rainfall. The total annual water loss by evaporation from the reservoir is 1404.4 mm. The mean annual actual evapotranspiration of the upper catchment and the command area is 613.33 mm and 569.84 mm, respectively. The mean annual runoff generated from the upper catchment and the command area is estimated to be 1.54 million cubic meters and 0.146 million cubic meters, respectively. The amount of water that percolates into the ground in the upper catchment and command area as a groundwater accretion is 0.06 million cubic meter and 0.22 million cubic meter, respectively, whereas the amount of water that percolates into the ground at the reservoir site as a groundwater accretion is 1.24 million cubic meter. The total amount of water which is actually available to recharge the groundwater circulation within the catchment is 1.52 million cubic meter. In general, the command area of the May Negus catchment has very good groundwater potential, if it is developed and utilized properly, that can be used conjunctively with dam water for multi purposes.
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