Restrictive soil layers commonly known as hardpans restrict water and airflow in the soil profile and impede plant root growth below the plough depth. Preventing hardpans to form or ameliorate existing hardpans will allow plants root more deeply, increase water infiltration and reduce runoff, all resulting in greater amounts of water available for the crop (i.e. green water). However, there has been a lack of research on understanding the influence of transported disturbed soil particles (colloids) from the surface to the subsurface to form restrictive soil layers, which is a common occurrence in degraded soils.

In this study, we investigated the effect of disturbed soil particles on clogging up of soil pores to form hardpans. Unsaturated sand column experiments were performed by applying 0.04 g/ml soil water solution in two sand textures. For each experiment, soil water solution infiltration process was visualized using a bright field microscope and soil particles remained in the sand column was quantified collecting and measuring leachate at the end of the experiment in the soil and water lab of Cornell University.

Preliminary results show that accumulation of significant amount of soil particles occur in between sand particles and at air water interfaces, indicating the clogging of soil pores occurs as a result of disturbed fine soil particles transported from the soil surface to the subsurface.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.

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.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.

Understanding soil hydraulic properties is crucial for planning effective soil and water management practices.

A study was conducted to evaluate the effects of different biochar and charcoal treatments on soil-hydraulic properties of agricultural soils. Biochar and charcoal treatments were applied on 54, undisturbed soil-columns, extracted from three-elevation ranges, with replications along three transects. Daily weight losses of freely draining soil-columns and soil moisture contents, at five tensions, were measured. In addition, field infiltration tests and soil analyses for particle size distribution, bulk-density and organic carbon content were conducted. Moreover, five year event precipitation data, from the watershed, was analysed and exceedance probability of rainfall intensity was computed.

Results show treatments reduced soil moisture contents, for most of the cases. However, treatment effects were significant only at lower tensions (10 and 30 kPa) and within two days after saturation (p<0.05). On the other hand, relative hydraulic conductivity (Kr) coefficients, near saturation, of amended soils were higher than the control. Acidic to moderately acidic soils with high average clay (42%) and low organic carbon contents (1.1%) were dominant. Infiltration rate ranged between 1.9 and 36 mm/h, with high variability (CV = 70%). At the same time, storms with short duration (< 15 min) and high average intensity (6.3 mm/h) contributed for 68% of annual precipitation (1616mm/year).

Dominant soil properties and rainfall characteristics suggest that infiltration could be a major problem on considerable number of fields, in the watershed. This implies, on such fields, constructing physical soil and water conservation structures alone will not reduce runoff and erosion effectively, unless soil infiltration and permeability rates are enhanced through integrated soil management approaches.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.

This study uses Tobit and Logit models to examine the impacts of selected small-scale irrigation schemes in the Lake Tana basin of Ethiopia on household income and the likelihood of poverty, respectively.

Data for these analyses were collected from a sample of 180 households. Households using any of the four irrigation systems had statistically significantly higher mean total gross household income than households not using irrigation. The marginal impact of small-scale irrigation on gross household income indicated that each small scale-irrigation user increased mean annual household income by ETB 3353 per year, a 27% increase over income for non-irrigating households.

A Logit regression model indicated that access to irrigation significantly reduced the odds that a household would be in the lowest quartile of household income, the poverty threshold used in this study. Households using concrete canal river diversion had higher mean cropping income per household than those using other irrigation types.

Key challenges to further enhancing the benefits of irrigation in the region include water seepage, equity of water distribution, availability of irrigation equipment, marketing of irrigated crops and crop diseases facilitated by irrigation practices.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.

Although Ethiopia has a large potential to develop irrigation, only 5% of the 3.5 million hectares of land potentially available has been developed. To examine the underlying causes, this study evaluates the suitability of surface water irrigation for the Lake Tana Basin development corridor.

Surface water availability and land potentially suitable for medium and large-scale irrigation development (200 ha and larger) was considered. Surface water potential was examined by considering river discharges. Land suitable for irrigation was determined with a GIS-based multi-criteria evaluation (MCE), which considers the interaction of various factors, such as climate, river proximity, soil type, land cover, topography/slope and market outlets.

The result indicates that nearly 11% of the Lake Tana Basin is suitable for surface irrigation. However, by analysing 27 years of river discharge, less than 3% of the potential irrigable area (or less than 0.25% of the basin area) could be irrigated consistently by run-of-the river-systems. Thus, the irrigation potential in the Lake Tana Basin can only be met by increasing dry season flows (if proven feasible) and by supplying water from existing or future reservoirs or by using water directly from Lake Tana.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.

With the construction of the new Renaissance Dam at the Ethiopian Sudan border, reducing sediment load in the Blue Nile is becoming increasingly important. Past attempts of decreasing sediment concentrations have been only partially successful. In this paper, we examine the temporal distribution of sediment generation within small watersheds and systematically compare this with the observed sediment concentration at various watershed scales using the Parameter Efficient Distributed (PED) model.

The model is based on the concept that runoff and erosion are generated mainly from areas that become saturated during the rain storm. These runoff source areas consist of shallow soils over a dense hardpan or areas where the water table is close to surface. Saturated areas are also prone to gullying. Simulation of watershed evaluations indicate that most erosion occurs from degraded areas, from temporarily saturated agricultural land and from gullies in the saturated bottomlands near the river.

In addition, we found that the annual runoff and sediment concentrations increased significantly in the Blue Nile basin at the border with Sudan. The model results would indicate that rehabilitating the degraded and bare areas by planting permanent vegetation and preventing further incision by gullies would be extremely effective in decreasing the sediment concentrations. Reduced tillage would likely result in less sediment transport but would increase use of pesticides and the cattle cannot graze freely anymore. Tentatively, we conclude that decreasing upland erosion might decrease sediment concentration downstream, since there is relatively little sediment storage in the main rivers of the Blue Nile basin.

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This paper was first presented at the Nile Basin Development Challenge Science meeting. The NBDC Science meeting was held on 9 and 10 July 2013 at the ILRI-Ethiopia campus, with the objectives to exchange experiences and research results across NBDC scientists involved in the NBDC projects and to discuss challenges and possible solutions.