QUANTIFICATION OF BLUE AND GREEN WATER RESOURCES IN LAKE HAWASSA CATCHMENT

Abstract

In most cases knowing the potential, availability and use of fresh water in a catchment would help to increase the productivity of agriculture, to improve ways and means of water management system. This Study was conducted in Lake Hawassa catchment, located in the central rift valley lakes basin of Ethiopia, to quantify the blue and green water resource using SWAT-WB, lake water balance using excel spread sheet application and water consumption demand of different water use sectors. Model sensitivity analysis shows the soil evaporation compensation factor (Esco), the soil available water content (SOL_AWC) and depth from soil surface to bottom of layer (Sol_Z) as the most sensitive parameters. The performance of the model has been evaluated through calibration and validation process. The result of SWAT-WB calibration and validation for stream flow measured with coefficient of determination ( R2 ) and Nash-Sutcliffe model efficiency (ENS) were 0.69, 0.66, 0.67 and 0.65 respectively which indicated that a good agreement between observed and simulated values. The calibrated parameters were transferred to un gauged part of the catchment to simulate hydrological components based on the hydrologic response unit (HRU) similarity. The long-term mean annual blue and green water flow and green water storage volume results obtained from model output found to be 500.24, 954.86 and 826.14 Mm3 respectively. In addition simulation of the lake water balance shown that the net blue water storage avilable in the lake was 1017.51Mm3 . The spatial and seasonal values of the result indicate that there exists a significant difference in both spatial and seasonal distributions of blue and green water flows in the catchment. The water consumption result shows that there is agricultural water consumption dominancy over other water users, accounting for 70% of the total consumption and the remaining 30% was for the other sectors. The overall consumption of water in the catchment is about 6.11% of the total Blue and Green water potential of 3.3 billion m3 per year which indicates the catchment water resource is underutilized. This study will help to prepare a future plan for the water resources development and management in the catchment, since plenty of water resource is available there. The water consumption level in the catchment is very small so further investigation is essential to use the available water resource potential of the catchment than ever so