Impact of Climate Variability on Lake Chamo Water Balance

Abstract

One of the most significant potential concerns of climate variability is to understand changes in hydrological components and subsequent change in lakes water balance. In view of this studying the water balance components such as surface water inflow from gauged and ungauged sub watersheds, precipitation and evaporation pattern of the natural reservoir and their associated impacts vis-à-vis altering the water balance of terminal Lakes Chamo has a major concern of the present study. It ultimately focuses on evaluating the impact of climate variability on water balance of Lake Chamo, situated in the southern part of the Ethiopian Rift valley lakes basin system. Lake Chamo is the largest lake among Ethiopian Rift valley Lakes system with a total area of 328.63Km 2 and average depth of 10.1m. In order to achieve the above main objective of the study primary analysis various hydro metrological variables of the water balance components undertaken. A conceptual hydrological model that could describe the underlying input-output relationship is established for the lake catchment. Based on such established relationship net inflow to the lake surface is simulated. Inflow to the lake surface is simulated for both historical (1995 to 2004) and future climate scenario (2030s and 2090s) conditions obtained from regional climate model output. The A1B scenario Regional Climate Model (RCM) organized by International Water Management Institute (IWMI) is used for future climate condition analysis. The raw A1B scenario outputs are characterized by significant biases and hence subjected to bias correction before applying the hydrological modeling. The bias correction for A1B scenario for precipitation, maximum and minimum temperature was done by using Linear scaling approach. This analysis is based on projection of two different scenarios of future time horizons: 2030s (2031-2040) and 2090s (2091-2100). Over lake evaporation is estimated by Penman Monteith formula, over-lake precipitation is computed by area weighing method and surface inflows are simulated by using HBV model. A hydrological model, HBV, was used in order to simulate the current and future inflow to the lake. The performance of the model was assessed through calibration and validation process and resulted R 2 from 0.64 to 0.81 during calibration and from 0.63 to 0.77 during validation at the three stations. Mean annual inflow to Lake Chamo from gauged and un-gauged catchment is 4391 mmyr -1 . The estimated runoff for the period 2030s and 2090s is 4204 mmyr -1 and 3277 mmyr -1 respectively. The result shows the mean annual inflow is decreased by 4.3% and 25.4% in 2030s and 2090s respectively from the base time period. The result revealed that the maximum and minimum temperatures increase for the two scenarios in future time horizons. However, precipitation decreases in all future time horizons. The A1B scenario reveals the decreasing pattern of lake water storage due to decrease of inflows components such over lake precipitation and surface water inflow in all future time horizons. In this scenario, the over-lake evaporation shows increasing pattern for all future time horizons.