Evaluation of Surface and Groundwater Availability Under Changing Climate in Abaya-Chamo Lakes Basin, Ethiopia
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2018-05-05
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Abstract
The study employed steady-state integration approach through a parameter which involves the coupling of climate model outputs (CMIP5); a distributed surface water balance model (WetSpass) and a groundwater flow model (ModFlow). The main focus is to quantify the present and future water resource availability under Changing Climate until 2099, and in a way to understand the hydrologic system in Abaya-Chamo lakes basin, Ethiopia. The lakes basin is part of Main Ethiopia Rift system characterized by complex rift margin, escarpment, and highland.
Though the record of river flow data plays an essential role in water resource management and planning, there is high limitation and irregularity in data and many of hydrologic basins are ungauged. Understanding base flow processes in particular as a groundwater contribution of river flows can be used as an indicative tool to describe hydrologic systems. In this regard, a multivariate analysis using BFI in controlling basin hydrology suggests, morphometric variables such as slope, Elevation, hypsometric integral with associated climate appear to be a dominant factor than land cover and lithological factor.
Spatially distributed water balance simulation at annual time step shows, 74.6% of the precipitation is lost through actual evapotranspiration, 15.7% becomes surface runoff and only 9.7% recharges the groundwater system. Besides, the model indicated the spatial variation of components with different land cover and soil texture combinations. Surface runoff and groundwater recharge are more sensitive to soil textural classes while actual evapotranspiration varies well with land use type.
The future climate (precipitation and temperature) is projected using RCP4.5 and RCP8.5 scenarios of the latest CMIP5 Multi-model outputs (available 24/26 GCMs). The projected annual temperature shows a rise by +0.84, +1.58, and +1.950C under the rcp4.5 in 2020s, 2050s and 2080s respectively whereas the change increased by +0.94, +2.13 and +3.570C under rcp8.5 in the study area. Similarly, the projected annual precipitation change is increased by 4%, 5.9%, and 8.9% under the rcp4.5; whereas, 1%, 9%, and 19.54% under rcp8.5 in the respective period. A significant decrease of wet season precipitation is also anticipated in 2050s. iv
Future water balance components were then simulated using the WetSpass model. The results show that the average recharge continuously increasing for all scenarios; the increases are 5.8%, 19.7% and 43.1% in 2020s, 2050s, and 2080s respectively. The simulated mean actual evapotranspiration increase is about 2.1%, 5.7% to 9.8% respectively with small range uncertainties. The mean annual surface Runoff is increased by 4%, 6% and 9% for corresponding future periods. The changes of groundwater level simulated in main aquifers are correlated with an increase of precipitation and/or recharge, thus average annual groundwater level could rise by 0.15%, 0.4%, and 2.03%. These, in turn may increase in groundwater discharge.
Seasonally, during the dry period, significant increases in all simulated mean water balance components have estimated with relatively smaller ranges of uncertainty as compared to the wet season. But, an increase of seasonal temperature and a decrease of precipitation in 2050s during the wet months could lead to a decrease in surface runoff. A clear linear correlation is observed in between precipitation and surface runoff in all months from the near to the far future under both low-intermediate and high-end scenarios.
The uncertainty range of projected precipitation in 2080s is about -1 to 69.9% as compared to the base period and the simulated corresponding impact as a change would be about (-0.5 to 35.6%) AET, (-1 to 72.2%) ROF and (-4.9 to 329.1%) R.
Generally, there is a remarkable change in the percentage fraction of future water balance particularly in actual evapotranspiration and recharge; while surface runoff will keep constant. The mean projected percentage fraction of actual evapotranspiration is showing a decline by 1.3, 2.1 and 3.5% (is about 73.7%, 72.5% and 71% of annual projected precipitation in 2020s, 2050s, and 2080s respectively). While recharge about 11.8%, 12.4%, and 13.6%, will increase by 2.1, 2.7 and 4.1% respectively as compared to present day water balance. Thus, these average values indicate significant changes in the water balance and groundwater system. It is therefore important for water resource developers to take the changes into account for future sustainable water resource management, planning, and implementation strategies.
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Base Flow Index, Multivariate, Water Balance, Surface and Groundwater, RCP, CMIPS, Projected, Future Periods, Major-River Basin, Lakes Basin, Abaya-Chamo