Impact of Water Abstraction on the Water Balance of Lake Ziway, Ethiopia

The available water volume of Lake Ziway in Ethiopia is declining at an alarming rate which hinders the lake services for a wide variety of sectors. However, there is a lack of systematic study to evaluate the contribution of water withdrawal by various sectors to the decline in the lake actual storage. In this study, we conducted a Water Abstraction Survey (WAS) to estimate actual water withdrawal from the lake. We applied a water balance model to evaluate the isolated impact of water withdrawal on the lake water volume and level. Three development pathways were simulated and the likely impact of these pathways on the lake water volume and level were assessed. Results indicate that water withdrawal for irrigation and domestic has significantly contributed to the change in the actual storage of Lake Ziway. Under full development pathways, irrigation water withdrawal from the lake is estimated to be 95.3 MCM. This will cause the mean annual water level of the lake to drop by 0.94 m, which yield with 38 km2 reduction of the surface area of the lake. Overall, this will result in a reduction of mean annual lake volume by 405 MCM, which accounts for 26% of the average lake volume. Hence, our result indicates that the current impact of water abstraction on Lake Ziway storage is substantially large and will exacerbate in the future. This calls serious action on the management of water abstraction from the lake.


Introduction
 Most of them focused on the water budget of the lake under the natural conditions.  However, there is no study that quantitatively estimated actual water abstraction from the lake and its impact on the water level based on WAS.
 The exact magnitude of water withdrawal and its impact on the water level from available studies are uncertain.
 The aim of this study is to estimate water withdrawal from the lake and its impact on lake water level using water balance modelling approach

Materials and Methods
 This study is based on the assessment of existing and satellite hydrometeorological dataset, water abstraction survey, and review of published studies.
 We applied the hydrological model output coupling with water balance modelling and field surveys.
 The methodologies followed in this study are: 1.
We estimated water abstraction for all abstraction points based on Water Abstraction Survey (WAS).

2.
We evaluated the water balance components of the lake under the natural condition on a monthly time steps. 3.
TThe implication of existing and future water abstraction on the water level was evaluated using a water balance modelling approach.
 First, the amount of water abstracted was measured using a bucket with a known size. Then, the inflow and outflow water balance components of the lake were determined at monthly time steps including water abstraction.
 After all water balance terms estimated a spread sheet water balance model is developed to simulate lake volume.
 The lake volume was then converted to lake level using the bathymetric relationships.
 Three development scenarios were built: existing development (ED), likely future development (LD) and full development potential (FD).

Impact of water abstraction
 The BS is the natural simulated water level from 1986-2000 without water abstraction.
 ED, LD and FD scenarios 2000 , 3100 and 5000 ha of irrigate land withdrawing water from the lake.
 this, first we assessed the lake water balance without and with water abstraction components.
 Then, we simulated the lake volume (corresponding lake level) for the baseline natural condition and scenario development.
 The isolated impact of water withdrawal from the lake on the water level will be estimated from the net difference between the simulated for natural and scenario condition

Results and Discussion
Water Abstraction  The amount of annual water withdrawal from the lake for irrigation and domestic purpose revealed 38 Mm 3 volume of water to irrigate 2000 ha of agricultural lands for three seasons per each year.
Water abstraction for development scenario  Estimate of Monthly water abstraction 12 Lake Water Balance  The monthly average simulated water balance components of Lake Ziway from 1986 to 2000 result revealed that rainfall, river inflow and evaporation constitute 33, 60 and 83 % of the annual water balance of the lake, respectively.
 This indicates that the river inflow contributes the major Lake inflow and evaporation over the lake surface accounts the major lake water loss. Lake water level simulation for development scenario condition  The simulated monthly water level for the three scenarios was not reasonably followed the baseline lake level over the simulation period. Concluding Remarks  This study indicated that an accurate estimate of actual water withdrawal and its impact on the water level can be estimated using WAS and water balancing approach, respectively.
 As a result of 37 Mm 3 annual water withdrawals from the lake for irrigation, the mean water level and volume of Lake Ziway drop by 0.36 m and 162 Mm 3 , respectively.
 If full planned development pathway to be implemented the water level and surface area of the lake drop by an additional 0.37 m and 11 km 2 , respectively. This consequently will yield to a significant reduction in the volume of the lake by 405 MCM, which accounts for 26% reduction of average lake volume.
 Our study indicates that water abstraction directly from the lake has a significant impact on the water level, volume and surface area of the lake. The water level change also further reflected in the temporal variation on the climate change trend.
 Further studies should incorporate future climate change impact assessment.