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Effective watershed planning requires an understanding of the hydrology. In the humid tropical monsoon climates and especially in volcanic highland regions such as the Ethiopian Highlands, the understanding of watershed processes is incomplete. The objective is to better understand the hydrology of the volcanic regions in the humid highlands by linking the hillslope processes with the discharge at the outlet. The Ene-Chilala watershed was selected for this study. The infiltration rate, piezometric water levels and discharge from two nested sub watersheds and at the watershed outlet were measured during a four-year period. Infiltration rates on the hillsides exceeded the rainfall intensity most of the time. The excess rain recharged a perched hillside aquifer. Water flowed through the perched aquifer as interflow to rivers and outlet. In addition, saturation excess overland flow was generated in the valley bottoms. Perched water tables heights were predicted by summing up the recharge over the travel time from the watershed divide. Travel times ranged from a few days for piezometers close to the divide to 40 days near the outlet. River discharge was simulated by adding the interflow from the upland to overland flow from the saturated valley bottom lands. Overland flow accounted only for one-fourth of the total flow. There was good agreement between predicted and observed discharge during the rain phase therefore the hillslope hydrologically processes were successfully linked with the discharge at the outlet.
Anthropogenic landscape conversion from forest to agricultural land affects baseflow. Baseflow is a source of potable water and can be used for the irrigation of high value crops. Finding ways to increase base and inter flow (i.e., groundwater flow) is, therefore, essential for the improvement of the livelihood of rural inhabitants. Therefore, the objective is to investigate the effect of landscape interventions on stream discharge and, in particular, on groundwater flow. The Tikur-Wuha experimental watershed in the upper reaches of the Blue Nile was selected because discharge data were available before and after implementation of a suite of land management practices that, among others, enhanced the percolation of water to below the rootzone. The parameter efficient distributed (PED) model was used to separate overland flow from total flow. The groundwater flow index (GWFI), defined as the quotient of the annual groundwater flow to the total stream discharge at the outlet of the watershed, was calculated. Our analysis with the PED model showed that at similar annual rainfall amounts, more baseflow and less surface runoff was generated after the landscape intervention, which promoted deep infiltration of the rainwater. The decrease in surface runoff shortly after the implementation of the land management practices is similar to observations in other watersheds in the Ethiopian highlands.
Soil degradation and associated soil erosion is a daunting socio-economic challenge for smallholder farmers that cause environmental harm in the developing regions of the world. This is especially true in the Ethiopian highlands where soil degradation and soil erosion signiﬁcantly impact the productivity of land and water. In this study, GIS-based classic RUSLE model supported with multi-criteria evaluation (MCE) analysis was used to map erosion hotspot areas in the sub-humid Ethiopian highlands. RUSLE model results indicated that annual average soil loss in the study area ranges up to 288 t ha -1. Majority (83%) of the study area has predicted soil loss between 0 and 40 t ha -1 whereas 13% of the study area has soil loss 40–100 t ha -1 and the remaining 4% is in excess 100 t ha -1. Seventy-two percent of upper Gilgel Abbay watershed was found moderately sensitive to erosion. The classic RUSLE predicted higher and lower erosion rates for the uplands and bottomlands respectively. On the other hand, the MCE analysis integrated with ﬁeld survey indicated that the potential location of gullies is the saturated bottomlands. The locations of bottomland gullies also coincided with important hydrologic parameters such as higher soil wetness and stream power index values. The implication is that in addition to RUSLE; criteria such as gullies land use, Topographic Wetness Index and stream power index values should be considered in erosion risk mapping studies in the sub-humid Ethiopian highlands, where soil saturation plays an important role in the formation and expansion of gullies.
Under the auspices of the UN Millennium Development Goals, access to safe drinking water in the developing world, including the Ethiopian highlands, has improved greatly. However, in many cases, it is not known how safe the water is. With the intensification of agriculture and increasing applications of fertilizers, high levels of nitrate are a concern. The objective of this study is to assess the nitrate levels in drinking water supply systems. To assess nitrate levels, we sampled 213 water supply points in a 4880 km2 area in the northwest Ethiopian highlands. The results show that the average concentration was below the World Health Organization (WHO) health standard of 10 mg N-NO3/L. The average concentration in wells was 3.3 mg N-NO3/L and in springs was 1.8 mg N-NO3/L. Only in three wells, that were in agricultural cropped areas, was the WHO standard exceeded. Wells in the agricultural fields had an average nitrate concentration of 3.6 mg N-NO3/L, which was almost twice that on grazing land and four times that in upland wells. Spatially, the groundwater nitrate concentrations were greater in the moderately sloped parts of the study area where agriculture was intensive and denitrification limited. Thus, although current nitrate levels are safe, in the future, the nitrate concentration could exceed the WHO health standard when fertilizer use increases.
Soil erosion, with significant contributions from gullies, is a serious problem in the Ethiopian highlands. The objective of this paper is to examine patterns of discharge and sediment transport in the Ethiopian highlands, and to provide an initial assessment of whether soil and water conservation practices (SWCP) can reduce sediment loads in watersheds with actively eroding gullies. The study was conducted in the 414‐ha Ene‐Chilala watershed with a unimodal sub‐humid monsoon climate and actively eroding gullies in the valley bottoms. In 2013 and 2014, the local community was mandated to install upland infiltration furrows and farmers voluntarily rehabilitated six gully heads and protected 16 m of eroding stream banks. Discharge and sediment concentration were measured in two upslope watersheds and at the outlet. Since median infiltration capacity in the uplands was always greater than the rainfall intensity, saturation excess and interflow were main runoff pathways. After 175 mm cumulative rainfall, the groundwater table reached the surface in the valley bottoms, restricting infiltration, and runoff was generated as saturation excess overland flow and flowed through active gullies out of the watershed. Upland rill erosion on ploughed land early in the rain phase, and gully erosion in saturated valley bottoms thereafter, were sources of sediment in the rivers. The mandated infiltration furrows installed on the contour overtopped and damaged cropland. The off‐contour furrows increased streamflow. Gully rehabilitation of an upland gully effectively reduced stream sediment concentration in the upland weir. However, there was little benefit at the watershed outlet since the stream picked up the unconsolidated sediment from the failing banks in the downstream porting of the watershed. Therefore, soil conservation programs that in addition to installing upland practices, rehabilitate the main sediment source (gullies) appear to be the most effective approach to reducing in‐stream suspended sediment concentrations.
Study region Eastern Nile River Basin (Ethiopia, Sudan and Egypt). Study focus This study aims to understand the future water development perspective in the Eastern Nile region by considering the current water use situation and proposed reservoirs in the upper Blue Nile (Abbay) River basin in Ethiopia using a simulation approach. The study was carried out by using a monthly time step and historical ensemble time series data as representative of possible near future scenarios. Series of existing and proposed cascaded water development projects in the upper Blue Nile were considered in the study. New hydrological insights for the region The results indicated an overall energy gain in the Eastern Nile region increases by 258%. The upstream country Ethiopia can generate as much as 38200 GWh/year of Energy while the energy production in Sudan increases by 39%. The cascaded developments integrated with existing water resources systems have a performance efficiency of above 92%. This study was an indicative analysis of the potential benefit of upstream Nile development without significantly affecting existing development in the Nile Basin. Further scientific analysis in this direction would help the Nile countries to reach a water use agreement.
Soil erosion decreases soil fertility of the uplands and causes siltation of lakes and reservoirs; the lakes and reservoirs in tropical monsoonal African highlands are especially affected by sedimentation. Efforts in reducing loads by designing management practices are hampered by lack of quantitative data on the relationship of erosion in the watersheds and sediment accumulation on flood plains, lakes and reservoirs. The objective of this study is to develop a prototype quantitative method for estimating sediment budget for tropical monsoon lakes with limited observational data. Four watersheds in the Lake Tana basin were selected for this study. The Parameter Efficient Distributed (PED) model that has shown to perform well in the Ethiopian highlands is used to overcome the data limitations and recreate the missing sediment fluxes. PED model parameters are calibrated using daily discharge data and the occasionally collected sediment concentration when establishing the sediment rating curves for the major rivers. The calibrated model parameters are then used to predict the sediment budget for the 1994-2009 period. Sediment retained in the lake is determined from two bathymetric surveys taken 20 years apart whereas the sediment leaving the lake is calculated based on measured discharge and observed sediment concentrations. Results show that annually on average 34 t/ha/year of sediment is removed from the gauged part of the Lake Tana watersheds. Depending on the up-scaling method from the gauged to the un-gauged part, 21 to 32 t/ha/year (equivalent to 24-38 Mt/year) is transported from the upland watersheds of which 46% to 65% is retained in the flood plains and 93% to 96% is trapped on the flood plains and in the lake. Thus, only 4-7% of all sediment produced in the watersheds leaves the Lake Tana Basin.
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. © 2014 Springer International Publishing Switzerland. All rights reserved.
Despite millions of dollars invested by donor agencies in soil and water conservation practices and other landscape interventions in Ethiopian highlands, and billions of hours of food-for-work farm labour, sediment concentrations in rivers are increasing. Combating these growing sediment loads requires a renewed analysis of the effectiveness of landscape interventions to reduce soil losses. This presentation combines the findings of several watershed studies in the Blue Nile basin in the Ethiopian highlands. These studies indicated that while most runoff and erosion was generated from saturated bottom lands and degraded hillsides, the majority of conservation practices is directed to the uplands where the direct runoff is minimal. Based on these studies, we recommend that in the humid areas of the Ethiopian highlands, more emphasis should be placed on erosion control in saturated bottom lands (where gullying is most extensive) and on the degraded areas in the landscape. In addition, measures need to be found to reduce interflow in bottom lands by increasing infiltration rates through the hard pan in the top 50 cm of the soil.
Soil and water conservation structures, promoted by local and international development organizations throughout rural landscapes, aim to increase recharge and prevent degradation of soil surface characteristics. This study investigates this unexamined relationship between recharge, water table depths, and soil surface characteristics (nutrients) in a small sub-watershed in the northwestern Ethiopian highlands. These highland watersheds have high infiltration rates (mean 70 mm hr⁻¹, median 33 mm hr⁻¹), recharging the shallow unconfined hillslope aquifer with water transport occurring via subsurface pathways down the slope. The perched water tables reflect the subsurface flux and are deep where this flux is rapid in the upland areas (138 cm below surface). Soil saturation and overland flow occur when the subsurface flux exceeds the transport capacity of the soil in the lower downslope areas near the ephemeral stream (19 cm below surface). Land use is directly related to the water table depth, corresponding to grazing and fallowed (saturated) land in the downslope areas and cultivated (unsaturated) land in the middle and upper parts where the water table is deeper. Kjeldahl Total Nitrogen (TN), Bray II available phosphorus (AP), and exchangeable potassium (K⁺) averages exhibit different behaviors across slope, land use transects, or saturation conditions. TN was moderate to low (0.07% ± 0.04) in various land uses and slope regions. Bray II AP had very low concentrations (0.25 mg kg⁻¹ ± 0.26) among the different slope regions with no significant differences throughout (p>0.05). The exchangeable cation (K⁺, Ca²⁺, Mg²⁺) concentrations and pH, however, were greater in non-cultivated (seasonally saturated) lands and in a downslope direction (p<0.001, p<0.005, p<0.05, and p<0.005, respectively). These results show that the perched groundwater plays an important role in influencing land use, the amount of water seasonally available for crop growth, and exchangeable cations, but have no clear effect on the concentration of the two primarily applied nutrients in fertilizers (N, P).
Agricultural intensification to meet the food needs of the rapidly growing population in developing countries is negatively affecting the water quality. In most of these countries such as Ethiopia, information on surface and especially groundwater quality is lacking. This limits the measure that can be taken to stop pollution. We, therefore, investigated the spatial and temporal variation of groundwater quality in the upland watershed. Tikur-Wuha watershed was selected because it is located in the Lake Tana watershed, which is seeing the first signs of eutrophication. Groundwater samples were collected from July 2014 to June 2015 from 19 shallow wells located throughout the watershed. Collected water samples were analyzed both in situ and in the laboratory to determine pH, electric conductivity (EC) and total dissolved solid (TDS), concentration of chemicals (nitrate, dissolved phosphorus, calcium, magnesium, aluminum and iron) and Escherichia coli (E. coli). We found that shallow groundwater had greater chemical concentrations and E. coli level in the monsoon rain phase than in the dry phase. Wells located down slope exhibited greater concentrations than mid- and upper-slope positions, with the exception of the nitrate concentration that was less down slope, due to denitrification in the shallow groundwater. Only E. coli level was above the WHO drinking water quality standards. Further studies on groundwater quality should be carried out to understand the extent of groundwater contamination.
The Ethiopian government has been implementing a land restoration program that aimed to restore degraded ecosystems and double agricultural productivity throughout the country since 2010. However, the success of the restoration program has been limited due to the lack of integrating gully erosion control measures. Consequently, many reservoirs in Ethiopia and downstream riparian countries have lost their storage capacity due to sedimentation, and studies demonstrated that gully erosion is one of the degradation hotspots within watersheds and contribute considerable proportion of the total sediment loads from a particular watershed. This study was conducted in one of large gullies in the Debre-Mawi watershed, northwestern Ethiopia to quantify the effect of gully head treatment in reducing the amount of sediment load generated from uplands and from the gully itself. We measured discharge, and sediment load and concentration in 2013 and 2014 at the upstream (inlet) and downstream ends (outlet) of the studied gully. Before the 2014 rainy phase, a gully headcut was stabilized with gabions at the bed and the gully bank was regarded to 45o. The gully head retreated 12 m in 2013 but gully head retreat was stopped following the implementation of the treatment in 2014. The total sediment load and sediment concentration at the outlet was reduced by 42% and 30% respectively, in 2014 (i.e., after treatment) when compared to 2013 (i.e., before treatment). The result of this study support that controlling the upward retreat of gully head is effective in reducing sediment load and concentration as well as upward movement and expansion of gullies. However, maintenance of gully head control measures is the key to sustain the benefits. Keywords Bank Erosion, East Africa, Lake-Tana, Sediment, Soil and Water Conservation
For close to a century, governments have encouraged soil and water conservation measures on farmers’ fields using universal technologies independent of environmental and social context. Most of these conservation technologies-based infiltration-excess runoff mechanisms were beneficial over an extended time in the drier climates for water conservation and erosion control, but only for short times in humid areas and in neither climate for nutrient control. The objective of this presentation is to document the hydrological and management factors that should be considered in the optimum performance of soil and water conservation practices in humid regions and then use them to find more effective practices. We are using the humid and sub humid Ethiopian highlands landscape and the New York City source watersheds in the Catskill Mountains as case studies.We show that since infiltration-excess hardly occurs in these humid regions, direct runoff is generated from areas that become saturated on valley bottoms, near rivers, and on hillsides with the most severely degraded soils and perched water. Only practices that consider the saturation runoff mechanisms are effective in reducing non-point source pollution and erosion. This includes protecting areas and limiting nutrient additions in areas near streams that generate surface runoff. Keywords: Africa, Ethiopia, Nutrient, Sediment, Soil and Water Conservation practice (SWCP).
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.
Rapid population growth and agricultural intensification has been directly associated with land degradation and with visible deterioration in water quality in the Ethiopian lakes. Assessing the extent and the origin of pollution is cumbersome due to lack of measurements for water quality parameters. This paper reports on the dissolved phosphorus concentrations (DP) in Lake Tana and its tributaries during three years from 2010 to 2012 and the four rainy months in 2014. Concentrations in the headwaters, at the gauging station, at the river mouth, near shore (littoral) and in the open lake (pelagic) were measured for the four major rivers. Dissolved phosphorus concentration in the rivers was on the average 0.32 mg l-1 and DP in the lake was much lower with an average concentration of 0.10 mg l-1. Temporally, DP concentrations were greater in the rainy phase of the monsoon than in the dry phase from November through March. Moreover, an upward trend in dissolved phosphorus concentrations is coinciding with the area expansion of water hyacinths in the lake.
Abstract -- Increased concern for environmental sustainability has put more emphasis on predicting sediment concentrations rather than loads. Rating curves, relating sediment load to discharge, assume inherently a unique relationship of concentration and discharge and therefore although performing satisfactorily in predicting loads, it may be less applicable for predicting concentration. This is especially the case in the Blue Nile basin of Ethiopia where concentrations decrease for a given discharge during the course of the rainy monsoon phase. The objective of this paper is to improve the sediment concentration predictions throughout the monsoon period for the Ethiopian highlands. In this paper, we limit ourselves to the four main rivers in the Lake Tana basin where estimating sediment concentrations are important for the fish production and tourism industry. To improve the rating curve for sediment concentration, we assume that the sediment transport was at the transport limit early in the rainy season and then decrease linearly with effective rainfall towards source limited during the end of rainy period. The resulting concentration rating curve was more accurate in predicting sediment concentrations than deriving concentration from the existing calibrated load based rating curves. As expected, sediment load predictions were similar for both methods. The proposed rating curve after more extensive testing over a wider geographical area might offer more accurate predictions of sediment concentrations in monsoonal climates. KeyWord: Sediment concentration, rating curve, transport limit, source limit, Blue Nile Basin.
D ebre Mawi is an agricultural watershed in the upper Blue Nile Basin. The slope ranges from 1% to 30% and the altitude varies from 2,195 m near the outlet to 2,308 m in the southeast. The area receives a mean annual rainfall of 1,240 mm with most of it concentrated between June and September (Dagnew et al., 2014). Smallholder farmers produce cereals such as teff, maize, finger millet, barley, and wheat, which dominate land use in the watershed, followed by grassland and sparse vegetation. The watershed is characterized by very low vegetation cover, severe sheet, rill and inter-rill erosion, and an active gully formation. The rate of erosion in the region is far beyond the tolerable rate, reaching 36 tons/ha/year from upland erosion (Zegeye et al., 2010) and 234 tons/ha/year from gully erosion (Zegeye et al., 2014). Erosion in the watershed has resulted in a loss of crop productivity due to loss of soil nutrients and land that is severely damaged by gully erosion. Of the 528 ha covering the watershed, instruments were installed in an area of 95 ha by the Amhara Agricultural Research Institute and Bahir Dar University for erosion and hydrology studies. Traditionally, contour furrows were mainly used by farmers as soil and water conservation (SWC) measures. In 2012, the government initiated a large-scale SWC campaign, where 67 ha of the 95-ha study area was covered with SWC measures. The case study presented here discusses the effects of the government-led, large-scale SWC work on runoff and
Information on sediment concentration in rivers is important for design of reservoirs and for environmental applications. Because of the scarcity of continuous sediment data, methods have been developed to predict sediment loads based on few discontinuous measurements. Traditionally, loads are being predicted using rating curves that relate sediment load to discharge. The relationship assumes inherently a unique relationship between concentration and discharge and therefore although performing satisfactorily in predicting loads, it may be less suitable for predicting concentration. This is especially true in the Blue Nile Basin of Ethiopia where concentrations decrease for a given discharge with the progression of the rainy monsoon phase. The objective of this paper is to improve the sediment concentration predictions throughout the monsoon period for the Ethiopian highlands with a modified rating type equation. To capture the observed sediment concentration pattern, we assume that the sediment concentration was at the transport limit early in the rainy season and then decreases linearly with effective rainfall towards source-limited concentration. The modified concentration rating curve was calibrated for the four main rivers in the Lake Tana basin where sediment concentrations affect fish production and tourism. Then the scalability of the rating type equation was checked in three 100 ha watersheds for which historic data were available. The results show that for predicting sediment concentrations, the (modified) concentration rating curve was more accurate than the (standard) load rating curve as expected. In addition loads were predicted more accurately for three of the four rivers. We expect that after more extensive testing over a wider geographical area, the proposed concentration rating curve will offer improved predictions of sediment concentrations in monsoonal climates.
Intensifying agriculture in Africa is degrading the water quality of rivers and lakes, thereby, threatening the sustainable use of water resources. In Lake Tana in the Ethiopian highlands, the first signs of eutrophication were recently observed. Since relatively little is known about the non-point source pollution in sub-Saharan Africa, the objective was to examine non-point sources phosphorus contribution of an agricultural watershed near Lake Tana. The 7-km2 Awramba watershed was selected. Dissolved phosphorus concentration (DPC) in groundwater and soil-available phosphorus was measured at three landscape positions and discharge, sediment, and DPC at the outlet. The results indicated that DPC in groundwater was most elevated in the periodically saturated valley bottoms while available P was the greatest in the cropped fields at mid-slope. The DPC increased with discharge. During base flow the concentration in the stream was similar to that in ground water. The results were consistent with other hydrological watershed studies in which the major source of runoff, erosion, and phosphorus originated from valley bottom lands. Hence, converting the periodically saturated areas in the stream corridor to buffer zones with reduced P input can be an important tool in reducing P from rural watersheds in the Ethiopian highlands.
Soil erosion and nutrient loss reduces food production and hampers poverty alleviation efforts in the highlands of Eastern Africa. Although intensive efforts have been underway to halt land degradation since the 1980's, erosion continued unabated and gullies are swallowing up productive cropland. Some of the lost soil fills up reservoirs in Sudan and will be a treat for new and planed reservoirs (such as the Grand Ethiopian Renaissance Dam-GERD) in Ethiopia. Soil nutrients carried by the water and eroded soil are causing eutrophication on natural lakes such as Lake Tana. Current measures to reduce soil loss are ineffective and new approaches that both consider the hydrology of the whole landscape (instead of the current plot based erosion research) and use traditional farmers knowledge for locating erosion control practices are required. The goal of this project is therefore to propose more effective soil and water conservation practices by identifying those parts in the landscape that contribute to most of sediment and nutrient at the outlet.
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 inﬁltrateddownslope and did not reach the outlet. At the end of July, the bottom lands 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 bottom lands 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 bottom lands 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.
Abstract:In Ethiopia the population is rapidly expanding. As a consequence the landscape is rapidly changing. Eucalyptus plantations are increasing and irrigation projects are implemented. The hydrological effects of the changing landscape on river (low) flows have not been well documented and therefore the amount of water available in the future might be over optimistic. The objective of this paper is to establish how low flows have been impacted by new developments in irrigation and by landscape change. For this paper, we choose the Gilgel Abay in the headwaters of the upper Blue Nile basin, since it has both good quality discharge data and it is located in the Tana Beles growth corridor. Numerical and statistical means were used to analyze the 25 years of available low flow data. We found a statistically significant decreasing trend (P<0.00001) of low flow in the Gilgel Abay. From 1980’s to 1990’s the low flow decreased by 25% and from 1990’s to 2000’s the low flow was reduced by 46%. The deterministic analysis with the Parameter Efficient Distributed (PED) model supported the statistical findings and indicated that in the middle of the nineteen nineties, after irrigation projects and eucalyptus plantations increased greatly, the low flows decreased more rapidly. Key words:Eucalyptus; Blue Nile; Gilgel Abay; low flow; irrigation potential; PED model; watershed
Experimental research in the Ethiopian highlands found that saturation excess induced runoff and erosion are common in the sub-humid conditions. Since most erosion simulation models applied in the highlands are based on infiltration excess we, as an alternative, developed the Parameter Efficient Distributed (PED) model which can simulate water and sediment fluxes in landscapes with saturation excess runoff. The PED model has previously only been tested at the outlet of a watershed and not for distributed runoff and sediment concentration within the watershed. In this study, we compare the distributed storm runoff and sediment concentration of the PED model against collected data in the 95 ha Debre Mawi watershed and three of its nested sub-watersheds for the 2010 and 2011 rain seasons. In the PED model framework the hydrology of the watershed is divided between infiltrating and runoff zones, with erosion only taking place from two surface runoff zones. Daily storm runoff and sediment concentration values, ranging from 0.5 mm to over 30 mm and from 0.1 g L−1 to 35 g L−1 respectively, were well simulated. The Nash Sutcliffe efficiency values for the daily storm runoff for outlet and sub-watersheds ranged from 0.66 to 0.82 and the NSE for daily sediment concentrations were greater than 0.78. Furthermore, the model uses realistic fractional areas for surface and subsurface flow contributions, for example between saturated areas (15%), degraded areas (30%), and permeable areas (55%) at the main outlet, while close similarity was found for the remaining hydrology and erosion parameter values. One exception occurred for the distinctly greater transport limited parameter at the actively gullying lower part of the watershed. The results suggest that the model based on saturation excess provides a good representation of the observed spatially distributed runoff and sediment concentrations within a watershed by modeling the bottom lands (as opposed to the uplands) as the dominant contributor of the runoff and sediment load. This article is protected by copyright. All rights reserved.
There is a scientific knowledge gap in locating runoff producing areas and their spatial variation in the landscape of the Ethiopian highlands and the upper Blue Nile basin. Identification of these spatial variations and runoff generation mechanisms is needed for optimum implementation of conservation measures. In many cases poor performance of soil and water conservation practices for erosion control can be traced back to non-optimum placement. A 95 ha watershed in the headwaters of the Blue Nile basin was used to investigate the runoff processes using the 2010 and 2011 monsoon, precipitation and runoff measurements made at sub-watersheds and watershed outlets. Perched water table heights and infiltration rates were also recorded. With the median infiltration rate exceeded by the rainfall intensity only 3% of the time the dominant runoff generation mechanism is found to be a saturation excess runoff process. Using the saturation excess interpretation of the Natural Resources Conservation Service Curve Number method, the seven day cumulative runoff fits better to the input as 7-day cumulative effective rainfall amount than to the rainfall intensity. In a monsoon climate where the watershed is dry at the onset of the rainfall, the saturated areas and the runoff coefficients varied in the landscape (upslope locations vs downslope locations) as the rainy season progresses and available water storage adjusts.
The water quality of Lake Tana has recently showed degradation with occurrence of first signs of eutrophication. Hence monitoring water quality of Lake Tana is paramount. This study presents current and historical water quality of lake using measurements and Landsat Images near entry of Gumera river. Statistical analysis of the measured water quality parameters (Turbidity, Cha-a, STD, DPC) were carried out. Regression models between water quality parameter and reflectance from Landsat 7 images were developed based on band combinations. Historical trend in turbidity has also been analyzed based on the regression models. The results of indicated reflectance and turbidity satisfactorily relationship (R 2 = 0.61-0.68). The trend in turbidity of the lake showed increasing trend. Delta development of the Gumera river near the shore grown 48 % due to an increase sediment inflow. Lake water quality degradation is attributed to the non-point source sediment and nutrient inflow to the lake with increasing erosion rate from the watersheds. Measures to minimize the non-point source sediment and nutrient inflow by targeting the high source areas in the agricultural watersheds should be placed. Moreover reducing the recession agriculture around the lake and wetland management would be crucial for sustaining lake water quality management.