Investigation of Water Supply Sources Sustainability the Case of Harar Town, Ethiopia”
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Date
2024-11
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Addis Ababa Univesrity
Abstract
In the present era sustainable water system is a concept that consuming the provided water sufficiently for a given need by preserving for future needs. In this research the water supply sources sustainability of Harar town was investigated in a more comprehensive approach. using the main and evaluation criteria that determine the sustainability of water supply sources. The sustainability analysis was performed from the experts’ preferences in the field of water sectors by distributing questioners to respond on the preferences of main criteria, evaluation criteria and alternative scenarios. The average aggregation of the experts’ preferences was used to attain final decision of influential evaluation criteria. The influential evaluation criteria used to examine the relative importance of the alternative water supply sources for the town.
The water supply sources sustainability were affected by various driving factors such as population increase, climate change and LULC change and it is important to quantify these effects for better understanding of their impacts on the water resources. The water supply sources effects due increasing population in the future is determined by analyzing the demand and supply data until the end of the water supply sources identification period of 2070. On the other hand, the climate change effects also have a considerable effect on the sustainability of water supply sources and that needs special attention to determine the extent of the effects. Similarly, LULC change effects on water supply sources sustainability was quantified. In this research climate change impacts on the water supply sources from historical (1979-2014) to future (2024-207) periods were identified using hydrological model under two emission scenarios. The climate models (CORDEX RCMS) were used in this research to generate the climate variables used for hydrological model simulation. The LULC change effects on the water supply sources were examined under LULC data for period (2001 to 2020). The LULC map that used for this research was developed from Landsat image by employing supervised image classification under ArcGIS software. The quantification of climate change and LULC change process were taken place by SWAT model that requires various spatial and temporal data. During SWAT model setup process the study area watershed of the upper Erer subbasin (466 km2) were produced using the watershed delineation process. Nevertheless, hydrometeorological required for SWAT model are scarce and no continuous data. Despite the inadequacy of hydrometeorological data especially the rainfall distribution is variable both in amount and existence. The occurrence of rainfall on the stations with in a time gap probably affecting the areal rainfall estimation. Hence, in this research the spatiotemporal rainfall distribution implications on areal rainfall estimation characteristics were examined using more comprehensive method that involves the spatiotemporal rainfall distribution and mutual rainfall occurrence for the stations. Moreover, stream flow data were inadequate and continuous data are unavailable in the region. To fill the data gap, the streamflow data was measured on site for the year 2020 to 2021 using the installed staff gauge. The discharge was estimated from stream flow records using the rating curve.
The water supply sources sustainability investigation output indicates that most influential evaluation criteria were capital cost (12.33 %), political (7.48 %), seasonality of sources (6.16 %), and availability (5.99 %). The alternative scenario analysis showed that the advanced potable water supply sources scenario was the best, followed by potable water supply and the business as usual is the least preference. The population estimation of the town using the United Nations Medium Variant Population Forecast indicates that the current population of the town was 127,854 and it becomes 929,418 by the year 2070. The daily water demand was higher than supply each year. In the region the increase of population along with other driving factors will deteriorate the future condition of the water supply sources. In this study autocalibration for SWAT model was used to identify sensitive parameters in the study area and manual calibration used to calibrate and validate. The SWAT model performance measures was within satisfactory range of PBIAS ≤ (±25%), R2 > 0.8 and NSE > 0.5. The calibrated and validated SWAT model output indicates that monthly mean temperature varies from 0.04 to 6.250C under RCP-4.5, and varies from 0.03 to 6.59oC under RCP-8.5. The monthly mean precipitation to be decline by 90.71 mm and rise by 211. 22 mm under RCP-4.5 and decline by 84.97 mm and rise by 235.62 mm under RCP-8.5. The LULC change and their effect examination indicates that agricultural lands (3.97%) forest lands (3.99 %), and water bodies (0.60 %) were decreased and settlements (2.30%), shrub land (3.07%) and bare land (3.19) were increased. The LULC change impacts on hydrological response such as surface runoff, ground water flow, and water yields were increased by 6.03, 8.22, and 10.39 %, respectively. While, evapotranspiration was decreased by 3.34%. In this research the climate change and LULC change have significantly affect the hydrological component of the sub basin.
In the study area the average annual rainfall amount for Dire Dawa, Harar and Haramaya, Girawa, and Gursum stations are found to be 647, 816, 801, 958, and 840 mm with coefficients of variation of 23, 20, 20, 19 and 31%, respectively. The rainfall distribution at the gauging stations varies significantly both temporally and spatially. The joint probability of rain days estimation approach analysis output indicates that the monthly time step has better performance than daily, decadal and seasonal. The joint probability approach is used along with rainfall amount under monthly rainfall for areal rainfall estimation assessment in rainfall-runoff modelling. The efficiency of the hydrological model simulation highly dependent on the accuracy of data used during calibration and validation periods. The rating curve model performance measure values were in acceptable ranges of coefficient of determination of (0.98), root mean square error (4.49) and Nash–Sutcliffe Efficiency (0.97). This indicates that rating curve model was reasonably used to estimate the discharge in the upper Erer subbasin. The overall discoveries from this research can be used as valuable information for regional water authorities and decision-makers including planners in sustainable water resources management.
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Keywords
Sustainable, Water supply sources, Climate change, LULC change, Water balance components, Rainfall distribution, Streamflow measurement, Upper Erer subbasin, Rating curve, Harar town