Modelling of Rainfall-Runoff Relationship in Big-Akaki Watershed, Upper Awash Basin, Ethiopia

Accurate estimation of surface runoff is a challenging task, but it is an important research topic because surface runoff plays a vital role in the study of the hydrological cycle, climate change, water resources, flood management, etc. Surface runoff reflects the amount of water that moves from the watershed into the river system and the amount that is drawn from it. The Big-Akaki watershed has suffered severe flooding due to increasing urbanization, deforestation, as well as reckless use of land and water resources that has led to the appearance of soil erosion. In our work, the SCS curve number was used to estimate runoff from the basin surface, and SWAT was used to delineate the basin and analyze the slope of the watershed, the soil and land uses. In addition, ground control points, interviews and field observation were carried out to collect data on the LULC classification. Moreover, model calibration (1991-1998) and validation (1999-2004) were performed for the monthly flow at the Akaki measuring station. The Big-Akaki watershed has a drainage area of 971,849 km2. The simulation was carried out by dividing the watershed into 33 sub-basins and assigning a hydrological response unit based on the definition of multiple HRU. The results indicate that SWAT generally works well by simulating runoff according to the result of three objectives (NSE, R2 and RSR). For surface runoff, the NSE, R2 and RSR values were 0.81, 0.82 and 0.44 during the calibration and 0.77, 0.77, and 0.48 during the validation period, respectively. Finally, the annual average precipitation and surface runoff of the Big-Akaki basin is 1,183.56 mm and 227.634 mm, respectively. In addition, the results showed a direct relationship between rainfall and surface runoff.