Arresting gully formation in the Ethiopian highlands

Over the past five decades, gullying has been widespread and has become more severe in the Ethiopian highlands. Only in very few cases, rehabilitation of gullies has been successful in Ethiopia due to the high costs. The objective of this paper is to introduce cost effective measures to arrest gully formation. The research was conducted in the Debre-Mewi watershed located at 30 km south of Bahir Dar, Ethiopia. Gullying started in the 1980s following the clearance of indigenous vegetation and intensive agricultural cultivation, leading to an increase of surface and subsurface runoff from the hillside to the valley bottoms. Gully erosion rates were 10–20 times the measured upland soil losses. Water levels, measured with piezometers, showed that in the actively eroding sections, the water table was in general above the gully bottom and below it in the stabilized sections. In order to develop effective gully stabilizing measures, we tested and then applied the BSTEM and CONCEPT models for their applicability for Ethiopian conditions where active gully formation has been occurring. We found that the model predicted the location of slips and slumps well with the observed groundwater depth and vegetation characteristics. The validated models indicated that any gully rehabilitation project should first stabilize the head cuts. This can be achieved by regrading these head cuts to slope of 40 degrees and armoring it with rock. Head cuts will otherwise move uphill in time and destroy any improvements. To stabilize side walls in areas with seeps, grass will be effective in shallow gullies, while deeper gullies require reshaping of the gullies walls, then planting the gully with grasses, eucalyptus or fruit trees that can be used for income generation. Only then there is an incentive for local farmers to maintain the structures

Evaluating hydrologic responses to soil characteristics using SWAT model in a paired-watersheds in the Upper Blue Nile Basin

Watershed responses are affected by the watershed characteristics and rainfall events. The characteristics of soil layers are among the fundamental characteristics of a watershed and they are input to hydrologic modeling similar to topography and land use/cover. Although the roles of soils have been perceived, there are limited studies that quantify the role of soil characteristics on watershed runoff responses due to the lack of field datasets. Using two adjacent watersheds (Ribb and Gumara) which have a significant different runoff response with a similar characterstics except geological settings (including soil characteristics), we studied the effects of soil characteristics on runoff and water balance. The Soil and Water Assessment Tool (SWAT) was used to simulate the surface runoff response at the outlet of the watershed and the optimal model parameters distribution was tested with a non-parametric test for similarity. Results indicated that SWAT model captured the observed flow very well with a Nash-Sutcliffe Efficiency (NSE) of greater than 0.74 and with a PBIAS of less than 10% for both calibration and validation period. The comparison of the optimal model parameter distributions of the SWAT model showed that the watershed characteristics could be uniquely defined and represented by a hydrologic model due to the differences in the soils. Using field observations and modeling experiments, this study demonstrates how sensitive watershed hydrology is to soils, emphasizing the importance of accurate soil information in hydrological modeling. We conclude that due emphasis should be given to soil information in hydrologic analysis

Performance of bias corrected MPEG rainfall estimate for rainfall-runoff simulation in the upper Blue Nile Basin, Ethiopia

In many developing countries and remote areas of important ecosystems, good quality precipitation data are neither available nor readily accessible. Satellite observations and processing algorithms are being extensively used to produce satellite rainfall products (SREs). Nevertheless, these products are prone to systematic errors and need extensive validation before to be usable for streamflow simulations. In this study, we investigated and corrected the bias of Multi-Sensor Precipitation Estimate–Geostationary (MPEG) data. The corrected MPEG dataset was used as input to a semi-distributed hydrological model Hydrologiska Byråns Vattenbalansavdelning (HBV) for simulation of discharge of the Gilgel Abay and Gumara watersheds in the Upper Blue Nile basin, Ethiopia. The result indicated that the MPEG satellite rainfall captured 81% and 78% of the gauged rainfall variability with a consistent bias of underestimating the gauged rainfall by 60%. A linear bias correction applied significantly reduced the bias while maintaining the coefficient of correlation. The simulated flow using bias corrected MPEG SRE resulted in a simulated flow comparable to the gauge rainfall for both watersheds. The study indicated the potential of MPEG SRE in water budget studies after applying a linear bias correction

Advances in water resources research in the Upper Blue Nile basin and the way forward: A review

The Upper Blue Nile basin is considered as the lifeline for ∼250 million people and contributes ∼50 Gm3/year of water to the Nile River. Poor land management practices in the Ethiopian highlands have caused a significant amount of soil erosion, thereby threatening the productivity of the Ethiopian agricultural system, degrading the health of the aquatic ecosystem, and shortening the life of downstream reservoirs. The Upper Blue Nile basin, because of limited research and availability of data, has been considered as the “great unknown.” In the recent past, however, more research has been published. Nonetheless, there is no state-of-the-art review that presents research achievements, gaps and future directions. Hence, this paper aims to bridge this gap by reviewing the advances in water resources research in the basin while highlighting research needs and future directions. We report that there have been several research projects that try to understand the biogeochemical processes by collecting information on runoff, groundwater recharge, sediment transport, and tracers. Different types of hydrological models have been applied. Most of the earlier research used simple conceptual and statistical approaches for trend analysis and water balance estimations, mainly using rainfall and evapotranspiration data. More recent research has been using advanced semi-physically/physically based distributed hydrological models using high-resolution temporal and spatial data for diverse applications. We identified several research gaps and provided recommendations to address them. While we have witnessed advances in water resources research in the basin, we also foresee opportunities for further advancement. Incorporating the research findings into policy and practice will significantly benefit the development and transformation agenda of the Ethiopian government

Comparison of irrigation performance based on management and cropping types

In Awulachew, Seleshi Bekele; Loulseged, Makonnen; Yilma, Aster Denekew (Comps.). Impact of irrigation on poverty and environment in Ethiopia: draft proceedings of the symposium and exhibition, Addis Ababa, Ethiopia, 27-29 November 2007. Colombo, Sri Lanka: International Water Management Institute (IWMI).Although performance evaluation of irrigated agriculture has gained momentum since late 1980s worldwide such attempt is rarely carried out in Ethiopia. The aim of this study was to assess the performance of 7 irrigation schemes some of which are expected to contribute much to the national economy. Sugar cane is grown by three of these schemes whereas cotton is grown by three schemes and the remaining single scheme grows tobacco. With regards to management types both government agency and community managed schemes are considered. The scheme level values of water supply performance indicators show that there was no constraint of water availability and supply at scheme level. In general, schemes that grow sugar cane were found to have attained higher outputs per units of land and water used which ranges from 7794 ? 10834US$/ha and 0.24 ? 0.55 US$/m3 respectively. On the other hand, whether state farm or community managed, schemes that grow cotton have shown low output per units of land and water, i.e. 310 ? 385 US$/ha and 0.01 ? 0.05 US$/m3 respectively. Large productivity performance differences have been observed between irrigation schemes with same cropping and management types. From scheme level performance values it is not simple to identify the area where and what is going wrong which is responsible for low performance. Generally, problems casing low productivity derive both in management and deterioration of physical structures. Hence investment on improvements of physical structures, management and operation of the system at all levels will bring substantial improvement in the performances of cotton producing schemes

Dividends in flow prediction improvement using high-resolution soil database

Study region: Upper Blue Nile, Ethiopia Study focus: The recent availability of high-resolution soil data offers a better representation of spatially varying hydrologic parameters and could help in building accurate models. Despite the release of the AfSIS 250 m resolution soil data in 2015, its benefits in improving streamflow prediction accuracy is yet to be evaluated. The focus of this study is to evaluate the improvement in prediction accuracy of an uncalibrated hydrologic model built using the recently released AfSIS 250 m soil database in comparison to four other soil databases which are under use for the last decade. New hydrological insights for the region: Limited streamflow data availability has been a major impediment to assessing water resources potential of watersheds. Recently the Africa Soil Information Service (AfSIS) provided 250 m resolution datasets with soil properties for up to six soil depth layers. The objective of this study is to evaluate the flow prediction accuracy gains from the AfSIS 250 m if any, in comparison to previously availed databases using un-calibrated SWAT (Soil and Water Assessment Tool) hydrologic models. The performance of the un-calibrated models was evaluated using streamflow at two of the gauging stations. Evapotranspiration output from the respective models was also compared to remote sensing derived evapotranspiration estimates (MOD16 ET and ALEXI). Only marginal improvements in streamflow simulation were achieved by using the most detailed AfSIS soil. A satisfactory agreement was found in most of the area between SWAT simulated ET and MOD16 ET during the wet seasons whereas ALEXI ET products are seasonally more consistent and comparable to SWAT estimates over wide land cover types. Robust evaluation of these datasets in different landscapes and geographic regions is warranted before a specific application. Keywords: AfSIS soil, SWAT-grid, MOD16 ET, ALEXI, Lake Tan

Water Quality Assessment by Measuring and Using Landsat 7 ETM+ Images for the Current and Previous Trend Perspective: Lake Tana Ethiopia

Recently there are signs of water quality impairment in Lake Tana, the largest fresh water in Ethiopia. The lake is the growth corridor of the government and supports millions of livelihood around. In order to sustain the benefit and maintain the ecosystem of the lake, the lake health has to be kept safe. Therefore monitoring and evaluation of the water quality of lake is very vital. This study focuses on current and previous trends water quality of the lake through measurements and Landsat Images near entry of Gumera River. Statistical analysis of the physical (Turbidity and STD and biological (Cha-a,) and chemical (DPC) water quality parameters were done. Linear and non-linear regression models between water quality parameter and reflectance of Landsat 7 ETM+ images were fitted based on band combinations. Pervious trend in turbidity was analyzed based on the regression models. The results showed that reflectance and turbidity satisfactorily result with an R2 ranging from 0.61 - 0.68. Form 1999-2014 the turbidity of the lake has indicated an increasing trend. Delta development near the entry of Gumera River has been enlarged by 48% because of an increase sediment inflow. The sign in the decreasing water quality of the lake was attributed to the non-point source sediment and nutrient inflow to the lake with high erosion rate from the watersheds. Measures to reduce the non-point source sediment and nutrient inflow by targeting the source areas (hot spots) in the agricultural watersheds need to be priority for stakeholders working on the soil and water conservation. Moreover, reducing the recession agriculture around the lake and wetland management could be crucial for improving lake water quality

Estimating reservoir sedimentation using bathymetric differencing and hydrologic modeling in data scarce Koga watershed, Upper Blue Nile

ABSTRACT Modeling sediment accumulation in constructed reservoirs is hampered by lack of historic sediment concentration data in developing countries. Existing models simulate sediment concentration using data generated from sediment rating curves usually defined as a power function of the form S = aQb This often results in residual errors that are not identically distributed throughout the range of stream flow values adding to uncertainty in sediment modeling practices. This research measure accumulated sediment in Koga dam in the upper Blue Nile Basin and use the result to validate a Soil and Water Assessment Tool (SWAT) sediment model that uses sediment data from rating curves. Bathymetric differencing of the original and current storage digital elevation models (DEMs) indicate that the sediment was accumulating at a rate of 5 ton/ha/year while a calibrated SWAT model resulted in 8.6 ton/ha/year. Given the complicated sediment transport processes that are not fully understood and comparable rates reported in recent studies these results are satisfactory. Keywords: Reservoir sedimentation, Koga reservoir, bathymetr