Understanding the implications of water harvesting on
upstream-downstream social-ecological resilience: A case study of the
Lake Tana Basin, Ethiopia

Introduction

Droughts and dry spells are among the major factors for famine and environmental degradation in sub-Saharan Africa. Field research has shown that water harvesting practices can reduce crop failures and also improve agricultural yields. Sub-Saharan Africa has enormous potential for water harvesting. However, little research has been performed to investigate the implications of large-scale water harvesting on upstream-downstream social-ecological resilience in a meso-scale basin. This research presents the implications of intensifying water harvesting on upstream-downstream social-ecological resilience in a case study in the Lake Tana Basin, Ethiopia.

Methodology

The Soil and Water Assessment Tool (SWAT) was used for this study. Model calibration and validation at Megech River gauging station showed reasonable model performance (i.e., Nash-Sutcliff efficiency of more than 0.7). A sub-basin in Megech watershed was chosen for detailed investigation of the consequences of water harvesting on the upstream-downstream social-ecological systems. This sub-basin was further subdivided into finer scale sub-basins (of size 0.8-6 ha) to accurately represent water harvesting ponds over suitable areas. The suitable areas for water harvesting have slope of <8%, soil type of luvisols and vertisols, and agricultural land use types. Irrigation was applied to avoid crop water stress for teff (Eragrostis teff) during the rainy season, and growing a vegetable crop (onion) during the dry season. Different rates of fertilizer application were applied to explore the implications of water harvesting and nutrient application. The change in water balance, sediment, and crop yield before and after water harvesting was investigated.

Results

The impacts of irrigation and rates of fertilizer application varied across the watershed and between years. Water harvesting with baseline nutrient application increased the crop yield by 24-85% during dry years, while in wet years the change in crop yield ranged between 4.5 and 29%. Water harvesting with the recommended nutrient application in the study area increased the yield significantly (180%-250% during dry years, and 150%-185% during wet years). Onion production was up to 8 tons/ha. Intensifications of water harvesting systems reduced peak flows during the wet season  and increased stream flows during the dry season. Sediment loads at the outlet of the watershed reduced significantly. 

Conclusions

Benefits to farmers in the irrigated area will be large and overwhelmingly positive due to increased crop production, reduced soil erosion, and increased stream flows in the dry season. Downstream farmers will benefit from reduced flood damage in the wet season, and from reduced stream bank erosion, reduced sedimentation of stream channels, and increased stream flows during the dry season. However, total annual flows to downstream reservoirs would be reduced, possibly reducing electricity generation and/or irrigation from downstream reservoirs.

Please note these Proceedings have received limited or no review from supervisors and IIASA program directors, and the views and results expressed therein do not necessarily represent IIASA, its National Member Organizations, or other organizations supporting the work.

Last edited: 19 August 2015