From runoff contributor to runoff absorber: Spate irrigation on exclosures in Tigray's semi-arid environment (Ethiopia)

Despite many years of establishment of exclosures in the semi-arid environments of north Ethiopia for rehabilitation of degraded areas, its effectiveness is constrained by water deficit. There is lack of empirical data on the response of vegetation to additional water (spate irrigation) application. This study aimed to evaluate the effects of spate irrigation using storm runoff on the growth of trees in exclosures. Two exclosures (May Ba'ati and Adi Kolakol), in Dogu'a Temben district, Tigray, north Ethiopia, were used for this study. Each exclosure was divided in two blocks (irrigated and non-irrigated) to allow comparison of change of vegetation using several diversity indices as well as contrasts in tree ring widths. The amount of spate irrigation water diverted onto the exclosures (762 mm) was in the same order of magnitude as the direct rainfall on the sites (645 mm). The results show that the irrigated exclosures have larger species richness, Shannon-Weaver diversity index, tree density, abundance, evenness and basal area than the non-irrigated areas. Following low rainfall (353 mm) in 2015, these indices decreased in the non-irrigated area, whereas most indices remained much higher in the irrigated area. The findings indicate that diverting runoff to exclosures results in an average 15% increase in ring width as compared to non-irrigated trees. In addition, there is strong abstraction of storm runoff in gullies in the study area. Besides the promotion of forestry in washout semi-arid areas of existing spate irrigation systems, we suggest to apply spate irrigation to exclosures in order to improve their productivity.