Groundwater–Surface Water Interaction and Analysis of Recent Changes in Hydrologic Environment of Lake Ziway Catchment
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Date
2006-06
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Addis Ababa Universty
Abstract
The study area is northern part of the Central Main Ethiopian rift valley bounded in west and
north by Awash basin; and in east by Wabi shebele catchment. Meki and Katar are the main
rivers draining western and eastern part of the catchment respectively and both feed Lake
Ziway before outflow to Bulbula River in south west. The area has mean annual precipitation,
potential evapotranspiration and actual evapotranspiration of 887mm, 1284mm and 856mm
respectively.
Direct groundwater recharge of the catchment was estimated using empirical formula, soil
moisture budgeting and groundwater balance methods. The empirical formula gives 67mm;
while the results of soil moisture balance and groundwater balance are 57mm and 83mm
respectively.
Groundwater and surface water interaction in the area have been analyzed using groundwater
table contour, field base river discharge measurements, channel water balance and
hydrographic analysis. The analysis result shows that Meki River in volcano-lacustrine deposit
of rift floor and Katar River in Upper and Central Wonji Fault Belt are loosing reaches. In the
rest of the catchment areas, the rivers and streams are gaining reaches.
Annual and monthly lake water balances have been conducted by incorporating available and
necessary parameters. In annual basis groundwater outflow is greater than groundwater inflow.
The recession in groundwater inflow over outflow is higher in the months of July and August
due to time lag between commencement of surface moisture and contribution of groundwater
to the lake on one hand and the increase in groundwater outflow due to rising lake level on the
other hand.
Assessment of recent changes in hydrologic environment has been analyzed by trends in short
and long term precipitation, evaporation, abstraction, rivers discharge, lake level and direct
groundwater recharge. All these parameters reveal changes of hydrologic environment;
attributed both to climate change and human interference
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Keywords
Water Interaction and Analysis