Modelling the hydrology in the Lake Awassa catchment, Ethiopia: Influences of irrigation and the swamp effect

Abstract

During the last decades some large changes have taken place in the Main Ethiopian Rift (MER) areas regarding the hydrology. A special case of hydrological changes in one of the MER lakes is lake Awassa. The water balance of Lake Awassa and its catchment area is not well understood. The aim of this study was to quantify the water balance of the Lake Awassa catchment area. This quantification includes the impact of a large swamp (Lake Cheleleka) on the outflow from the catchment and the water withdrawals for irrigation purposes.

The current study used the Soil and Water Assessment Tool (SWAT) to quantify the water balance in the Awassa catchment. It was calibrated and validated using field and observation data from the Tikur Woha and Wosha subbasin. After calibration, model predictions of Tikur Woha streamflow were relatively close to observed values, with a NSE of 0.25 and a R2 of 0.28. The NSE value of -0.43, during validation period, indicates that the main value of the observed time series would have been a better predictor than the model. According to the PBIAS rating the model performs during both calibration and validation very good. A discharge data set without the backwater effect would very likely result into a better performance overall.

Both streamflow of the Wosha and Tikur Woha river significantly increased during the observed period from 1980 to 1996 and 1981 to 2006 respectively (p<0.05). The long-term significant increasing trend of discharge cannot be explained by precipitation changes, since the Awassa and the Wondo Genet observed time series remained constant. The discharge increase of the Wosha and Tikur Woha streams are most likely related to land use changes within the Tikur Woha subbasin, especially to the increase in agricultural land use. The increase in Tikur Woha discharge is very likely smaller than previously stated in other studies due to the backwater effect that significantly affects the discharge measurements.

The irrigation in the Awassa catchments has a medium effect on the total streamflow of the Tikur Woha River. During the dry period the irrigation decreases the streamflow at minimum of -5.8 ± 4.2% in December to a maximum of -13.8 ± 7.7% in February.

The swamp has a significant effect on the water balance in the Tikur Woha subbasin. First of all, the discharge during the wet months increases significantly with values between 100 and 150%. And secondly, all runoff waters located upstream are buffered by the swamp. When the swamp area declines the high peak discharges aren’t retained by the swamp anymore. Thus, future cultivation of the swamp area may lead to flooding of high populated downstream area.