Modeling discharge and sediment concentrations after landscape interventions in a humid monsoon climate: The Anjeni watershed in the highlands of Ethiopia

Abstract

Increasing population and intensification of agriculture increase erosion rates and often result in severe land degradation and sedimentation of reservoirs. Finding effective management practices to counteract the increasing sediment load is becoming increasingly urgent especially in the Ethiopian highlands where the construction of the hydroelectric Grand Renaissance Dam on the Blue Nile is underway. In this paper, we examine the results of 9 years of a watershed experiment in which discharge and sediment losses were observed in the 113 ha Anjeni watershed of the Blue Nile Basin. The study period encompasses conditions before, during, and after the installation of graded Fanya‐Juu (“throw uphill” bunds) soil and water conservation practices (SWCP), which had the ultimate goal of creating terraces. We use a saturation‐excess runoff model named the parameter‐efficient distributed model as a mathematical construct to relate rainfall with discharge and sediment losses at the outlet. The parameter‐efficient distributed model is based on landscape units in which the excess rainfall becomes direct runoff or infiltrates based on topographic position or hardpan characteristics. Deviations in this rainfall–discharge–sediment loss relationship are ascribed to the changes in infiltration characteristics caused by SWCPs on the hillslopes. With this technique, we found that in the Anjeni basin, the Fanya‐Juu SWCPs are only effective in increasing the infiltration and thereby reducing the direct runoff and sediment concentrations in the first 5 years. At the end of the 9‐year observation period, the direct runoff and sediment concentrations were barely reduced compared to the levels before SWCP were installed. In addition, we found that the model structure based on landscape units was able to represent the varying runoff and erosion processes during the 9 years well by varying mainly the portion of degraded land (and thereby representing the effectiveness of the Fanya‐Juu to reduce runoff by increasing infiltration).

Determining RUSLE P‐ and C‐factors for stone bunds and trenches in rangeland and cropland, North Ethiopia

“…subfactor value for stone bunds with trenches installed in rangeland increased from 0.03 in 2010 to 0.12 in 2012 (Figure c). These results are in line with the findings of Guzman et al (), who applied the parameter‐efficient distributed model to estimate the long‐term (9 years) impact of Fany‐juu (throw uphill) bunds on sediment concentration at the outlet of a small experimental catchment in the central Ethiopian Highlands. Their results also indicate that the effectiveness of these SWC structures declines over time and is negligible after 5 years.…”

“…This forms a major bottleneck for soil erosion prediction and understanding impacts of natural resource management in Ethiopia and many other developing countries (Karamage et al, ; Mati et al, ; Nigussie et al, ; Nyssen et al, ; Sonneveld, Keyzer, & Albersen, ). A further challenge is that, although several studies show that commonly applied SWC measures such as stone bunds and trenches are highly effective (Desta et al, ; Nyssen et al, , ; Taye et al, ), their effectiveness can also strongly decline over time, due to a decline in static storage capacity (SSC; Guzman et al, ; Taye et al, ). Evaluations of the effectiveness of SWC measures should bring this temporal variability into account.…”

Assessment of Practices for Controlling Shallow Valley-Bottom Gullies in the Sub-Humid Ethiopian Highlands

“…The NPV is determined assuming that the harvested annual mass of grass from the treated gullies (Table 1) is the same over a seven-year period. This period was selected because soil and water conservation practices last approximately seven years before they become ineffective [32]. A 5% discount rate (interest rate) was used, which is equal to the current deposit rate of the Commercial Bank of Ethiopia.…”

Impact of Soil Depth and Topography on the Effectiveness of Conservation Practices on Discharge and Soil Loss in the Ethiopian Highlands

“…The infiltration furrows with bunds were built along the contour across the sloping lands to decrease runoff by reducing or stopping the overland flow and increasing the infiltration of rainwater into the soil in the furrows. Note that due to erosion and filling up of the furrows, most of these practices are only effective for approximately five years [30]. Enclosures are areas closed for free grazing and cultivation, commonly found on degraded marginal lands not suitable for crop cultivation and in valley bottoms where gullies are being formed.…”

“…Studies outside the Lake Tana basin found that infiltration furrows decreased direct runoff and sediment load but not sediment concentrations in the first three years of observation when gullies were present [23,28,29]. Another study 200 km south of Lake Tana [30] found that the long-term impact of conservation practices was minimal. None of these studies related the effectiveness of the conservation practices to watershed characteristics.…”