ASSESSMENT OF WATER HARVESTING TECHNOLOGY EFFECT ON DOWNSTREAM WATER AVAILABILITY USING (CASE OF ALABA SPECIAL WOREDA, ETHIOPIA. ASSESSMENT OF WATER HARVESTING TECHNOLOGY EFFECT ON DOWNSTREAM WATER AVAILABILITY USING SWAT MODEL, CASE OF ALABA SPECIAL WOREDA, ETHIOPIA. ASSESSMENT OF WATER HARVESTING TECHNOLOGY EFFECT ON DOWNSTREAM WATER AVAILABILITY USING CASE OF ALABA SPECIAL WOREDA, ETHIOPIA.)

Abstract

In hydrological cycle; there is many water-related human interventions that modify the natural systems. Rainwater harvesting is one such intervention that involves harnessing of water in the upstream . Community level water harvesting technologies used in the upstream , that prevents the water runoff, it caused an impacts on downstream mainly disturbance on biodiversity and ecosystems. The main objectives of the study is assessing the effects of water harvesting technologies on downstream water availability in Alaba Special Woreda, to address the above problem. The methods used include a secondary and primary data on SWAT model, Cost Benefit Ratio and Optimal control methods to analysiz the hydrological impact , Socioeconomic impact and tradeoff of on upstream and downstream water availability of the community respectively. The downstream impacts of increasing water consumption in the upstream rain-fed areas of the Bilate and Shala Catchment are simulated using the semi-distributed SWAT model. The two land use scenarios are tested at subbasin levels (1) conventional land use which represents the current land use practice in the area (Agri-CON) and (2) in-field rainwater harvesting (IRWH), improving soil water availability through rainwater harvesting. The model was manually calibrated from January 1997 to December 2002 the Nash and Sutcliffe efficiency index of 0.74 and r 2 of 0.75. The scenarios results were compared against the baseline period 1989–2007. The simulated water balance results showed that the highest peak mean monthly direct flow was obtained on Agri-CON land use (12.71mm), followed by Agri-IRWH land use (11.5mm) and LULC 2005 (9.01mm). The Agri-IRWH scenario reduced direct flow by 10% compared to Agri-CON and more groundwater flow contributed by Agri-IRWH(19mm) than Agri-CON(12.5mm). Flow reductions are comparatively higher in the upstream parts of the basin, as a result of a relatively higher potential of developing rain-fed areas coupled with comparatively lower amount of available runoff. Although there was a visible impact of the water harvesting technique on the water yield. The overall result suggests that the water yield of Alaba Special Woreda may not be adversely affected by the Agri-IRWH land use scenario despite its surface runoff abstraction design. The study was analyzed costs-benefits of the society. The major costs include the initial construction cost of water harvesting system and the maintenance costs. The major benefits include an increase in household dispensable income, time and energy saved from collecting water. It is concluded that households with different existing water consumption pattern will benefit positively in investing domestic water harvesting systems having average Benefit Cost Ratio of 4.6 . I analyzed the tradeoff on Upstream and Downstream water use for domestic and crop production . With the optimal control approach (OCA) I identifed the economic and eco-logical conditions for water use in a regime where a social manager controls water withdrawals. Domestic is not optimal that the value of the water per demand harvested is less than the amount of water needed to domestic purpose. Storage tanks, Series of check dams, gravel filled dams are an alternative solutions for water harvesting.