From runoff contributor to runoff absorber: spate irrigation on exclosures in Tigray, northern Ethiopia

Exclosures have been widely implemented in north Ethiopia in the last few decades for rehabilitation of degraded areas. Despite decades of establishment, the effectiveness of exclosures is constrained by lack of adequate management such as supplementing additional water to the rain water which is characterized as short and erratic. There is no knowledge on the response of vegetation to such additional water application. This study is aimed to evaluate the effects of spate irrigation on species diversity, stocking and ring width growth of trees in exclosures. Two exclosures (May Ba’ati and Adi Kolakol), Dogu’aTemben district, north Ethiopia, were used for this study. 55. Each exclosure was divided in two blocks (irrigated and control) to allow comparison of vegetation growth. May Ba’ti was irrigated in 2005 while Adi Kolakol was irrigated from 2012 – 2016 but monitored by the researchers from 2014 to 2016. The amount of irrigation water applied is in the same order of magnitude as the direct rainfall on the site. Different species diversity indices and mean tree ring width growth were used to compare the irrigated and non-irrigated areas. The result showed that trees in irrigated block have larger species richness, Shannon Wiener diversity index, tree density per ha, abundance, evenness and basal area than in the control area. The changes of these indices were negative in years with a short rainy season in the control area, whereas most indices remain positive under irrigated area. Moreover, tree ring width growth analysis exhibits the role of additional water to vegetation growth. The result of this study indicates diverting runoff to exclosures results in 15% to 22% increase in ring width as compared to non-irrigated trees. In addition, there is strong abstraction of storm runoff in gullies