Runoff on slopes with restoring vegetation: A case study from the Tigray highlands, Ethiopia

Daily runoff depths from 28 plots (5 m × 2 m) recorded during a 2-year period in the semi-arid to subhumid highlands of Tigray were analyzed to study the effect of vegetation restoration in exclosures and to identify other factors influencing runoff production. Plots are distributed over three study sites and located in different land use types and on different combinations of soil type, vegetation cover and slope gradient. Runoff was found to be significantly reduced when a degraded area is allowed to rehabilitate after closure. Runoff depth is significantly correlated with event variables such as rain depth, rainfall intensity, storm duration and soil moisture content. Total vegetation cover is the most important plot variable explaining about 80% of the variation in runoff coefficients through an exponential decay function. Also the runoff generating rainfall threshold has a positive correlation with total vegetation cover. Runoff was found to be negligible when the vegetation cover exceeds 65%. Other important variables affecting runoff production in the study sites are soil organic matter, soil bulk density, litter cover and slope gradient.