Christian D. Guzman

Impact of conservation practices on runoff and soil loss in the sub-humid Ethiopian Highlands: The Debre Mawi watershed

Abstract In response to the continually increasing sediment concentrations in rivers and lakes, the Ethiopian government is leading an effort where farmers are installing soil and water conservation measures to increase infiltration and reduce erosion. This paper reports on findings from a four year study in the 95 ha Debre Mawi watershed where under the government led conservation works, mainly terraces with infiltration furrows were installed halfway in the period of observation. The results show that runoff volume decreased significantly after installation of the soil and water conservation practices but sediment concentration decreased only marginally. Sediment loads were reduced mainly because of the reduced runoff. Infiltration furrows were effective on the hillsides where rain water could infiltrate, but on the flat bottom lands that become saturated with the progress of the monsoon rain, infiltration was restricted and conservation practices became conduits for carrying excess rainfall. This caused the initiation of gullies in several occasions in the saturated bottomlands. Sediment concentration at the outlet barely decreased due to entrainment of loose soil from unstable banks of gullies in the periodically saturated bottom areas. Since most uphill drainage were already half filled up with sediments after two years, long term benefits of reducing runoff can only be sustained with continuous maintenance of uphill infiltration furrows.

A Saturation Excess Erosion Model

Abstract. Scaling-up sediment transport has been problematic because most sediment loss models (e.g., the Universal Soil Loss Equation) are developed using data from small plots where runoff is generated by infiltration excess. However, in most watersheds, runoff is produced by saturation excess processes. In this article, we improve an earlier saturation excess erosion model that was only tested on a limited basis, in which runoff and erosion originated from periodically saturated and severely degraded areas, and apply it to five watersheds over a wider geographical area. The erosion model is based on a semi-distributed hydrology model that calculates saturation excess runoff, interflow, and baseflow. In the development of the erosion model, a linear relationship between sediment concentration and velocity in surface runoff is assumed. Baseflow and interflow are sediment free. Initially during the rainy season in Ethiopia, when the fields are being plowed, the sediment concentration in the river is limited by the ability of the surface runoff to move sediment. Later in the season, the sediment concentration becomes limited by the availability of sediment. To show the general applicability of the Saturation Excess Erosion Model (SEEModel), the model was tested for watersheds located 10,000 km apart, in the U.S. and in Ethiopia. In the Ethiopian highlands, we simulated the 1.1 km 2 Anjeni watershed, the 4.8 km 2 Andit Tid watershed, the 4.0 km 2 Enkulal watershed, and the 174,000 km 2 Blue Nile basin. In the Catskill Mountains in New York State, the sediment concentrations were simulated in the 493 km 2 upper Esopus Creek watershed. Discharge and sediment concentration averaged over 1 to 10 days were well simulated over the range of scales with comparable parameter sets. The Nash-Sutcliffe efficiency (NSE) values for the validation runs for the stream discharge were between 0.77 and 0.92. Sediment concentrations had NSE values ranging from 0.56 to 0.86 using only four calibrated sediment parameters together with the subsurface and surface runoff discharges calculated by the hydrology model. The model results suggest that correctly predicting both surface runoff and subsurface flow is an important step in simulating sediment concentrations.

Spatial and Temporal Patterns of Soil Erosion in the Semi-humid Ethiopian Highlands: A Case Study of Debre Mawi Watershed

The effectiveness of water management interventions is hampered by the lack of knowledge about the spatial distribution of runoff and associated soil loss. A study was conducted in the 95-ha Debre Mawi watershed in the Upper Blue Nile basin to understand where and when runoff and erosion takes place on the landscape. During the rainy phase of the 2010 and 2011 monsoons, storm runoff and sediment concentrations were measured from five sub-watersheds. In addition, perched groundwater tables, infiltration rates, and rill erosion from agricultural fields were measured. The results show that saturation excess runoff was the main runoff mechanism because the median infiltration rate was only exceeded 3 % of the time. Early during the rainy period, runoff produced from upslope shallow soils infiltrated downslope and did not reach the outlet. At the end of July, the bottomlands became saturated, and the runoff coefficient at the outlet became greater than upslope areas. Sediment concentrations were greater in the beginning of the rainy monsoon phase when the rill network had developed on the plowed land and it becomes lowest at the end of rainy phase when rill formation stopped. At all times, the sediment concentration at the outlet was greater than upslope because both runoff losses were greater in the saturated bottomlands and loose unstructured soil was available from newly forming gullies. This research indicates that watershed management interventions to control erosion should be implemented in areas which produce the most runoff such as those shallow upland soils and bottomlands near the river that become saturated by the end of the rainy phase. In addition, for proper planning and management, runoff and erosion models should capture these dynamics.

Modeling sediment concentration and discharge variations in a small Ethiopian watershed with contributions from an unpaved road

Abstract Drainage of paved and unpaved roads has been implicated as a major contributor of overland flow and erosion in mountainous landscapes. Despite this, few watershed models include or have tested for the effect roads have on discharge and sediment loads. Though having a model is an important step, its proper application and attention to distinct landscape features is even more important. This study focuses on developing a module for drainage from a road and tests it on a nested watershed (Shanko Bahir) within a larger previously studied site (Debre Mawi) that receives overland flow contributions from a highly compacted layer of soil on an unpaved road surface. Shanko Bahir experiences a sub-humid monsoonal climate and was assessed for the rainy seasons of 2010, 2011, and 2012. The model chosen is the Parameter Efficient Distributed (PED) model, previously used where saturation-excess overland flow heavily influences discharge and sediment concentration variation, though infiltration-excess occasionally occurs. Since overland flow on unpaved surfaces emulates Hortonian flow, an adjustment to the PED model (the developed module) advances possible incorporation of both flow regimes. The modification resulted in similar modeling performance as previous studies in the Blue Nile Basin on a daily basis (NSE = 0.67 for discharge and 0.71 for sediment concentrations). Furthermore, the road while occupying a small proportion of the sub-watershed (11%) contributed importantly to the early discharge and sediment transport events demonstrating the effect of roads especially on sediment concentrations. Considerations for the dynamic erodibility of the road improved sediment concentration simulation further (NSE = 0.75). The results show that this PED modeling framework can be adjusted to include unpaved compacted surfaces to give reasonable results, but more work is needed to account for contributions from gullies, which can cause high influxes of sediment.

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

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 nine 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 (PED) model as a mathematical construct to relate rainfall with discharge and sediment losses at the outlet. The PED 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 nine years well by varying mainly the portion of degraded land (and thereby representing the effectiveness of the Fanya-Juus to reduce runoff by increasing infiltration).

Improving watershed management practices in humid regions

In many parts of the world, watershed management practices have been extremely effective. However, implementation of soil and water conservation technologies in the humid African highlands, while beneficial in the short term, were remarkably unsuccessful in the long term. Insights from community knowledge perspectives have revealed that alternative methods are needed. Although conservation practices are designed to conserve water in semi-arid areas, safely draining excess water is needed in humid areas. The objective of this paper is to review current watershed management approaches used in humid regions as exemplified by those used in Ethiopian highlands and then based on these findings propose more effective practices. Although current government sponsored practices primarily protect the hillsides, direct run-off is generated from areas that become saturated on valley bottoms near rivers and on specific parts of the hillsides with degraded soils (or with highly permeable surface soils) and with perched water tables on slowly permeable horizons at shallow depths. In these areas, direct run-off is increasing with deforestation and the soil degradation, demanding additional drainage ways that evolve in the form of gullies. Therefore, watershed management interventions for erosion control should prioritize revegetation of degraded areas, increasing sustainable infiltration, and rehabilitating gullies situated at saturated bottomlands.