A nine-year study on the benefits and risks of soil and water conservation practices in the humid highlands of Ethiopia: The Debre Mawi watershed

Effect of exclosure on subsurface water level and sediment yield in the tropical highlands of Ethiopia

Grazing is a major cause of soil erosion and land degradation across many parts of Ethiopia. This study examined the effects of exclosure on subsurface water levels, soil erosion, and the relationship between daily rainfall and subsurface water levels. Piezometers were used to measure subsurface water levels in the exclosure area during 2017-2020. We found that sediment yield, runoff, and the volume of subsurface water vary greatly depending on the exclosure and temporal practices used. Exclosure of grazing land was the most effective sustainable land management practice in reducing runoff and sediment yield. In 2019 and 2020, the subsurface water level continued to rise at piezometers with exclosure, which shows that exclosure contributed to the subsurface water level rising. In addition, piezometers in grazing land and the exclosure indicate that runoff trapped by acacia decurrens trees can contribute to significant differences in subsurface water levels. Higher runoff coefficients were observed in 2017 and 2018 than in 2019 and 2020, indicating that the exclosure greatly affects runoff; therefore, its implementation is vital to reduce runoff and enhance water conservation. Sediment yields measured for 2017, 2018, 2019, and 2020 were 140.45, 133.15, 101.03, and 74.39 g L^-1 day^-1, respectively. In 2017 and 2018, sediment yield increased, while in 2019 and 2020 sediment yield decreased because of an exclosure around the gully's cross-section and communal grazing. This study shows that erosion is reduced by exclosure, possibly due to the restoration of protective vegetation cover. This study revealed that a minimum of human and livestock intervention during the study period considerably increased groundwater levels and decreased soil erosion. Generally, the results of this study indicated that exclosure has a considerable impact on runoff and sediment. Therefore, exclosure implementation is vital to reduce runoff and sediment and enhance water conservation, thus supporting the development of effective communal grazing land management measures on the study sites and other similar environmental settings.

Estimation of Soil Erosion and Evaluation of Soil and Water Conservation Benefit in Terraces under Extreme Precipitation

In recent years, soil erosion caused by water erosion has gradually increased due to the increase of extreme precipitation. In order to reduce soil erosion caused by extreme precipitation, it is necessary to monitor soil erosion and found out the factors that affect soil erosion under extreme precipitation. The objective of this study was to assess the amount of soil erosion, the damage degree of soil and water conservation measures and benefit evaluation under extreme precipitation in Henan Province. The results indicated that the ridges of terraces in two small watersheds had been damaged to varying degrees. Terraces, as one of soil and water conservation measures, can better preserve soil and water erosion under extreme precipitation. The amount of soil preserved in two small watersheds were 744.50t and 1121.01t. The amount of soil loss in two small watersheds were reduced by 67.67% and 78.63% when terraces existed. The soil conservation amount of vegetation restoration measures in two small watersheds were 2960.23t and 3320.36t. The amount of soil loss in two small watersheds were reduced by 89.27% and 90.98%, when vegetation restoration measures exist. Compared with soil and water conservation engineering measures, vegetation restoration measures can better reduce soil erosion caused by water erosion under extreme precipitation. In addition to the amount of rainfall, the soil and water conservation benefits of terraces were also affected by the width of the terrace, the slope of the terrace, and whether there were vegetation restoration measures in the terrace.

Topography Impacts Hydrology in the Sub-Humid Ethiopian Highlands

Understanding the relationship between topography, hydrological processes, and runoff source areas is essential in engineering design, such as predicting floods and implementing effective watershed management practices. This relationship is not well defined in the highlands with a monsoon climate and needs further study. The objective of this study is to relate topographic position and hydrological response in tropical highlands. The research was conducted in the Debre Mawi watershed in the northwest sub-humid Ethiopian highlands. In the monsoon rain phase of 2017 and 2018, groundwater depth, infiltration rate, and surface runoff were monitored at the upslope, midslope, and downslope positions. Surface runoff rates were measured in farmer fields through distributed V-notch weirs as estimates of positional runoff. Average water table depths were 30 cm deep in the downslope regions and 95 cm in the upslope position. The water table depth affected the steady-state infiltration rate in the rain phase. It was high upslope (350 mm h−1), low midslope (49 mm h−1), and zero downslope. In 2017, the average runoff coefficients were 0.29 for the upslope and midslope and 0.73 downslope. Thus, topographic position affects all aspects of the watershed hydrology in the humid highlands and is critical in determining runoff response.