Essayas K. Ayana

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.

Watershed Hydrology of the (Semi) Humid Ethiopian Highlands

Understanding the basic relationships between rainfall, runoff and soil loss is vital for effective management and utilization of water resources and soil conservation planning. A study was conducted in three small watersheds in or near the Blue Nile basin in Ethiopia, with long-term records of rainfall and discharge. To better understand the water movement within the watershed, piezometers were installed and infiltration rates were measured in the 2008 rainy season. We also reanalyzed the discharge from small plots within the watersheds. Infiltration rates were generally in excess of the rainfall rates. Based on this and plot discharge measurements, we concluded that most rainfall infiltrated into the soil, especially in the upper, steep and well-drained portions of the watershed. Direct runoff is generated either from saturated areas at the lower and less steep portions of the hill slopes or from areas of exposed bedrock. Using these principles, a simple distributed watershed hydrology model was developed. The models reproduce the daily discharge pattern reasonably well for the small watershed and the 10-day discharge values for the whole Blue Nile Basin in Ethiopia. The simplicity and scalability of the model hold promise for use in un-gauged catchments.

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.

Evaluation of stream water quality data generated from MODIS images in modeling total suspended solid emission to a freshwater lake.

Modeling of suspended sediment emission into freshwater lakes is challenging due to data gaps in developing countries. Existing models simulate sediment concentration at a gauging station upstream and none of these studies had modeled total suspended solids (TSS) emissions by inflowing rivers to freshwater lakes as there are no TSS measurements at the river mouth in the upper Blue Nile basin. In this study a 10year TSS time series data generated from remotely sensed MODIS/Terra images using established empirical relationship is applied to calibrate and validate a hydrology model for Lake Tana in Upper Blue Nile Basin. The result showed that at a monthly time scale TSS at the river mouth can be replicated with Nash-Sutcliffe efficiency (NS) of 0.34 for calibration and 0.21 for validation periods. Percent bias (PBIAS) and ratio of the root-mean-square error to the standard deviation of measured data (RSR) are all within range. Given the inaccessibility and costliness to measure TSS at river mouths to a lake the results found here are considered useful for suspended sediment budget studies in water bodies of the basin.

Soil Erosion and Discharge in the Blue Nile Basin: Trends and Challenges

Future river discharge predictions seldom take into account the degrading landscape. The objective of this study was to investigate the relationship of river discharge and sediment concentrations, and the effect of changing landscape and climate on discharge and sediment transport in the Ethiopian Blue Nile basin. This study used past precipitation records and the Parameter Efficient Distributed (PED) model to examine how the relationship between precipitation, discharge, and sediment concentration changed with time. All input data to the PED model were kept constant except for a conversion of permeable hillside to degraded soil in time. The results of this study show that with a gradual increase of the degraded areas from 10 % in the 1960s to 22 % in 2000s, the observed discharge pattern and sediment concentration can be simulated well. Simulated annual runoff increased by 10 % over the 40-year periods as a result of the increase in degraded soils. Sediment loads appeared to have increased many times more, but this needs to be further validated as data availability is limited. In general, the results indicate that rehabilitating the degraded and bare areas by planting permanent vegetation can be effective in decreasing the sediment concentration in the rivers. Research should be undertaken to evaluate the effectiveness of vegetation planting.

Examining the relationship between environmental factors and conflict in pastoralist areas of East Africa.

The eastern Africa region has long been known for recurring drought, prolonged civil war and frequent pastoral conflicts. Several researchers have suggested that environmental factors can trigger conflicts among pastoralist communities, but quantitative support for this hypothesis is lacking. Here we use 29years of georeferenced precipitation and Normalized Difference Vegetation Index (NDVI) data to evaluate long term trends in scarcity of water and forage for livestock, and then ask whether these environmental stressors have any predictive power with respect to the location and timing of 11years of conflict data based on Armed Conflict Location and Event Data Project (ACLED) and Uppsala Conflict Data Program (UCDP). Results indicate that environmental stressors were only partly predictive of conflict events. To better understand the drivers behind conflict, the contribution of other potential stressors to conflict need to be systematically quantified and be taken into consideration.

Climate Change Impact on Sediment Yield in the Upper Gilgel Abay Catchment, Blue Nile Basin, Ethiopia

According to Intergovernmental Panel on Climate Change (IPCC) future projections, precipitation and temperature will increase over eastern Africa in the coming century. This chapter presents basin-level impact of climate change on sediment yield in Upper Gilgel Abay catchment, Blue Nile Basin, Ethiopia, by downscaling HadCM3 global climate model using Statistical Downscaling Model (SDSM). IPCC-recommended baseline period (1961–1990) was used for baseline scenario analysis. Future scenario analysis was performed for the 2020s, 2050s, and 2080s. Globally, HadCM3 model is widely applied for climate change studies and it consists of A2 (medium high emission) and B2 (medium low emission) scenarios. Impact assessment on sediment yield was done by Soil and Water Assessment Tool (SWAT) hydrological model. SWAT model performance in simulating daily sediment yield for the study area was satisfactory with Nash–Sutcliffe Efficiency (NSE) of 0.58 and 0.51 for calibration and validation periods, respectively. Mean annual changes of precipitation and temperature (maximum and minimum) were applied to quantify these impacts. The result of downscaled precipitation and temperature reveals a systematic increase in all future time periods for both A2 and B2 scenarios. These increases in climate variables are expected to result in increase in mean annual sediment yield of 11.3, 16.3, and 21.3 % for A2 scenario and by 11.0, 14.3, and 11.3 % for B2 scenario for the 2020s, 2050s, and 2080s, respectively. This increase in sediment yield is double the increase in stream flow due to climate change for all time periods. Future work need to consider also impact of land use change on the catchment for future sustainable development plan.

Climate Change Projections in the Upper Gilgel Abay River Catchment, Blue Nile Basin Ethiopia

According to future projections, precipitation and temperature will increase over Eastern Africa in the coming century. This chapter presents basin-level impact of climate change over the Upper Gilgel Abay River catchment, Blue Nile basin, Ethiopia, by downscaling the Hadley Centre Coupled Model, version 3 (HadCM3) global climate model using the statistical downscaling model (SDSM). The baseline period (1961–1990) recommended by the Intergovernmental Panel on Climate Change (IPCC) was considered for analysis of the baseline scenario. For future scenario analysis, the time periods of the 2020s, 2050s, and 2080s were applied. Mean annual rainfall will be expected to increase by 2.21, 2.23, and 1.89 % for A2 scenario and by 2.06, 1.85, and 0.36 % for B2 scenario by the 2020s, 2050s, and 2080s, respectively. The projected average temperature increases by 0.43, 1.05, and 1.92 °C for A2 scenario and by 0.47, 0.87, and 1.38 °C for B2 scenario in the three time periods. In the study area, the minimum temperature increases by 0.55, 1.06, and 1.83 °C for A2 scenario and 0.50, 0.87, and 1.29 °C for B2 scenario in the 2020s, 2050s and 2080s, respectively.

Watershed Storage Dynamics in the Upper Blue Nile Basin: The Anjeni Experimental Watershed, Ethiopia

Understanding functions of a watershed is important for implementing appropriate soil and water conservation measures and for planning and development of sustainable water resources use. Watershed storage is a significant part of a catchment water budget and its quantification provides a clue to understand the fundamental catchment hydrological processes. This study is aimed to investigate the dynamics of watershed storage of the Anjeni experimental watershed in the Upper Blue Nile basin for which a long series of rainfall and runoff data is available for this study. A daily water balance equation was used to quantify the watershed storage over the distinct rainy season. On average, 86 % of the annual rainfall occurs during distinct rainy season. The study showed that the watershed storage increases with the increase of cumulative rainfall till the watershed stores its maximum capacity. After this maximum capacity, the watershed storage remains constant, even if rainfall continuous. The Anjeni watershed stores an average of 380 mm of water after a cumulative effective rainfall of 625 mm. Before the maximum storage was reached, about 60 % of the effective rainfall is used to wet up the watershed. Then, the remainder becomes surface runoff and interflow, during which about 40 % of the flow appeared at the outlet.

Climate Change Impact on Stream Flow in the Upper Gilgel Abay Catchment, Blue Nile basin, Ethiopia

According to Intergovernmental Panel on Climate Change (IPCC ) future projections , precipitation and temperature will increase over eastern Africa in the coming century. This chapter presents basin-level impact of climate change on stream flow in Upper Gilgel Abay catchment , Blue Nile basin, Ethiopia , by downscaling HadCM3 global climate model (GCM) using statistical downscaling model (SDSM). IPCC recommended baseline period (1961–1990) was used for analysis of baseline scenario. For future scenario analysis time periods of the 2020s, 2050s and 2080s were used. Globally, HadCM3 model is widely applied for climate change studies and it contains A2 (medium–high emission) and B2 (medium–low emission) scenarios. The impact assessment on stream flow was done using the soil and water assessment tool (SWAT ) hydrological model. The performance of SWAT model in simulating the stream flow was shown with a Nash–Sutcliffe Efficiency (NSE) of 0.76 and 0.78 for calibration and validation periods, respectively. Mean annual changes of precipitation and temperature (maximum and minimum) were applied to quantify these impacts. The result of downscaled precipitation and temperature reveals a systematic increase in all future time periods for both A2 and B2 scenarios. These increases in climate variables are expected to increase mean annual stream flow by 7.1, 9.7, and 10.1 % for A2 scenario and by 6.8, 7.9, and 6.4 % for B2 scenario for 2020s, 2050s, and 2080s, respectively. Future work need to consider impact of land use change on the catchment for future sustainable development plan.