Establishing Stage–Discharge Rating Curves in Developing Countries: Lake Tana Basin, Ethiopia

A significant constraint in water resource development in developing countries is the lack of accurate river discharge data. Stage–discharge measurements are infrequent, and rating curves are not updated after major storms. Therefore, the objective is to develop accurate stage–discharge rating curves with limited measurements. The Lake Tana basin in the upper reaches of the Blue Nile in the Ethiopian Highlands is typical for the lack of reliable streamflow data in Africa. On average, one stage–discharge measurement per year is available for the 21 gaging stations over 60 years or less. To obtain accurate and unique stage–discharge curves, the discharge was expressed as a function of the water level and a time-dependent offset from zero. The offset was expressed as polynomial functions of time (up to order 4). The rating curve constants and the coefficients for the polynomial were found by minimizing the errors between observed and predicted fluxes for the available stage–discharge data. It resulted in unique rating curves with R2 > 0.85 for the four main rivers. One of the river bottoms of the alluvial channels increased in height by up to 3 m in 60 years. In the upland channels, most offsets changed by less than 50 cm. The unique rating curves that account for temporal riverbed changes can aid civil engineers in the design of reservoirs, water managers in improving reservoir management, programmers in calibration and validation of hydrology models and scientists in ecological research.

Phosphorus Export From Two Contrasting Rural Watersheds in the (Sub) Humid Ethiopian Highlands

Establishing worldwide sustainable and phosphorus efficient cropping systems is urgently needed because the supply of suitable phosphate rock is limited, and excess phosphorus in streams causes eutrophication. One of the impediments in the developing world for sustainable P practices is the lack of studies on P transport and its eventual disposition in the environment. One of these regions with few studies is the Ethiopian Highlands, with permeable volcanic soils. The objective was to establish baseline data on P watershed export in the (sub)humid highlands. Two contrasting watersheds were selected near Lake Tana. For 2 years, stream discharge and sediment, total P, dissolved P, and bioavailable particulate P concentrations were determined at the watershed outlet. The first watershed is the 57 km2 Dangishta, with lava intrusion dikes, forcing subsurface flow through faults to the surface and preventing gully formation. Subsurface flow was half of the 1745 mm annual precipitation, and surface runoff and erosion were minimal. The second watershed is the 9 km2 Robit Bata with 1,420 mm precipitation. The banks of several river banks were slumping. The upper part of the watershed generates saturation excess runoff. A hillslope aquifer in the lower part provided interflow. The average sediment concentrations of 10.5 g L−1 in the stream in Robit Bata (11 times that in Dangishta) reflected the sediments from banks slipping in the stream. The hydrology and the soil loss directly affected the phosphorus export. In Dangishta, the total P concentration averaged 0.5 mg L−1 at the outlet. In Robit Bata, the average total P concentration was 2 mg L−1. The bioavailable particulate P concentration was only twice the concentration in the runoff water. The low phosphorus content of the subsoil slipping in Robit Bata moderated biologically available particulate P at the outlet. Average dissolved P concentrations for both watersheds were around 0.1 mg L−1 in the low range found in temperate climates. It reflects the difference in length of time that phosphorus fertilizers have been applied. Our research concludes that commonly implemented practices such as strengthening river banks and stabilizing gully might not lead to improved water quality in Lake Tana.

Spatial and temporal simulation of groundwater recharge and cross-validation with point measurements in volcanic aquifers with variable topography

A physically distributed water balance model called WetSpass is applied to estimate the recharge for the semi-humid Lake Tana basin in northwest Ethiopia. Lake Tana basin is one of the growth corridors of the country, where huge waterworks infrastructure is developing. Estimating groundwater recharge at required spatial and temporal scales is a challenge in groundwater management, sustainability and pollution studies. In this study, the WetSpass model is developed at 90 m grid resolution. The spatial recharge map by WetSpass is cross-validated with water table fluctuation (WTF) and chloride mass balance (CMB) methods. The mean annual recharge, surface runoff, and evapotranspiration over the whole basin using WetSpass are estimated at 315 mm, 416 mm, and 770 mm of rainfall, respectively. The mean annual recharge ranges from 0 mm to 1085 mm (0 % to 57 % of the rainfall): 0 mm at water bodies and highest on flat, sandy loam soil and bush land cover. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strong heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. WetSpass is effective in aquifers where diffuse recharging mechanism is the predominant type and recharge is controlled by rainfall. It is less effective in the storage-controlled flat floodplain alluvial and fractured rock aquifer areas. In these areas, the point estimates by WTF and CMB are effective and can be considered as reliable values. The land use change from 1986 to 2014 brought a relatively small hydrological change in recharge although the land use has changed significantly.

Prevalence of Intestinal Parasites and Schistosoma mansoni and Associated Factors among Fishermen at Lake Tana, Northwest Ethiopia

Background. Intestinal parasites and Schistosoma mansoni infections adversely affect the health of humans in the world especially in sub-Saharan African countries including Ethiopia. Fishermen who spend most of their time in water bodies are supposed to be at high risk of schistosomiasis and other water-born parasites. However, the magnitude of these parasitic infections and their determinant factors are not well addressed. Methods. A cross-sectional study was conducted at Lake Tana among 388 fishermen from March to May 2021. Questionnaire data was collected through face to face interview. Stool sample from each participant was processed by the Kato-Katz and spontaneous tube sedimentation techniques. Data were analyzed using SPSS version 26. Results. One hundred sixty four (42.3%) and 88 (22.7%) participants were infected by intestinal parasites and Schistosoma mansoni, respectively. One hundred twenty two (31.4%) and 42 (10.8%) participants were infected by soil-transmitted helminths and intestinal protozoa, respectively. Attending only primary school ( AOR = 2.02 , p = 0.014 ) or being illiterate ( AOR = 2.54 , p = 0.004 ) and not washing hands before meal ( AOR = 2.23 , p = 0.007 ) were significantly associated with intestinal parasitic infections. Illiterate educational status ( AOR = 2.37 , p = 0.022 ), fishing by bargee ( AOR = 2.43 , p = 0.005 ), fishing ≥4 days per week ( AOR = 2.27 , p = 0.029 ), swimming habit ( AOR = 3.03 , p = 0.030 ), and participation in irrigation ( AOR = 3.09 , p ≤ 0.001 ) predispose fishermen to S. mansoni infection. Conclusion. Intestinal parasites and S. mansoni infections are highly prevalent among fishermen at Lake Tana basin. Low education level and frequent water contact predispose fishermen for intestinal parasites and S. mansoni. Therefore, health education to fishermen on intestinal parasites and S. mansoni infection and regular deworming should be advocated.