Excess sediment and nutrient losses from intensifying agriculture degrade water quality and boost plant growth. The relationship between circulation patterns, spatial water quality degradation, and water hyacinth infestation is not adequately studied. The objective of this study is, therefore, to investigate the effect of lake circulation patterns on sediment and nutrient distribution and its implication on the spread of water hyacinth in a tropical lake. This study was carried out in Lake Tana, the largest freshwater lake in Ethiopia, where sediment and nutrient concentrations are increasing, and water hyacinths have become a challenge since 2011. The lake circulation pattern was simulated by the Delft3D model based on a bathymetry survey, discharge, and meteorological forcings. To predict the transport path of sediments and dissolved nutrients, an inert tracer was released in the four main river inlets of the lake. Observed lake water level measurements were used to validate the model. Our results show that the lake circulation pattern could explain the transport path of sediment and nutrients and the location of the water hyacinths found in the northeast of the lake. Sediments and nutrients from the largest river, Gilgel Abay, in the southeast of Lake Tana, flow through the two outlets nearby with little sediment deposition due to the relatively short retention time. The phosphorus-rich sediments of the 24 h at 105 °C remaining three main rivers joining the lake at the north and east are transported to the northeast. Thus, the management and control of water hyacinths should focus on the northern and eastern catchment areas of Lake Tana.

The Lake Tana Basin, comprising the largest natural lake in Ethiopia, is the source and the uppermost part of the Upper Blue Nile Basin. In this review paper, research papers, mainly on the rainfall-runoff modeling and lake water balance, and on the hydrogeology, have been reviewed. The earlier water balance estimation attempts used simple conceptual and statistical approaches and calculate on a monthly timescale. More recent research has been using advanced semi-physically or physically based distributed hydrological models. Accordingly, mean annual precipitation over the lake was estimated in the range 36.1–53.1%; lake evaporation at 45.3–57.5%; river inflow (all gauged and estimated ungauged) at 43.6–63.9%; and river (lake) water outflow at 0–9.2%. With the few isotope studies, groundwater inflow and outflow are found insignificant. Different studies had estimated groundwater recharge, ranging from 57 mm to 850 mm. The basin has a heterogenous aquifer system consisting of different volcanic rocks and alluvio-lacustrine sediments. Generally, groundwater with low TDS, Ca–Mg–HCO₃ type, isotopically relatively enriched, and high TDS, Na–HCO₃ type, isotopically relatively depleted, water types have been identified. In this paper, major research gaps such as aquifer hydraulic characterization, surface-groundwater interaction, groundwater flow and groundwater balance have been identified. Hence, future research shall focus on the groundwater resources, so that existing surface water studies are updated and future water usage options are explored.

In this study, analyses of future scenarios of water demand and supply as well as an assessment of trade-offs for water allocation across the different water use sectors in the upper Blue Nile basin are made. Different periods of regulation of Lake Tana (Ethiopia) and subsequent impacts on the lake level hydrological regime are investigated. A decision support system is developed and available water supply for normal- and low-flow hydrological conditions are determined based on recorded flow data and a simple rainfall-runoff model. Different scenarios have been triggered and simulations are conducted to understand the implications of planned water resource developments in the area. We found that the annual inflows to Lake Tana under an average hydrological condition are about 5.7 × 10⁹ m³ and are estimated to reduce by about 27% when all planned water resources development projects are implemented in the catchment. These projects aim at the generation of 460 MW hydroelectric power and about a billion m³ per annum supply of water to the large-scale irrigation schemes. During low-flow conditions, supply will run short of demands and the lake water level can drop by 0.3 m from the natural outlet level (1785 m a.s.l). Lake water levels and long-term lake level fluctuations have been affected significantly due to regulation with subsequent impacts on the lake ecology. An upstream–downstream cooperation, transparency and participation in the decision making and establishment of an adequate data acquisition system are critically important elements in the management of water resources in the basin.

The population of Ethiopia is growing rapidly and consequently, the country may face aggravated food insecurity. Therefore, improving agricultural productivity is critical to feed its inhabitants. The occurrence of drought and poor irrigation systems lead to a real burden on water resources. Thus, optimal irrigation planning and management should be considered. In this study, the linear programming (LP) model was applied to optimally allocate water and land for different field crops at Koga irrigation scheme. The model was applied for an optimally defined cropping pattern and three levels of deficit (10%, 20%, and 30% of ET) irrigation. The objective function of the study was to maximize net benefit and water productivity. The model was developed using constraints of water, land and different field crop diversifications. CROPWAT 8.0 was used for the estimation of crop water requirements using local climate and soil data. The result showed that using 20% deficit irrigation for the identified optimal cropping pattern, 7361 ha of land (2018 ha additional land from the irrigated land of the scheme in 2016/17) could be irrigated. Under this scenario of deficit irrigation, a net benefit of 284.7 million Birr and water productivity of 5.3 Birr m⁻³ have been achieved. This indicates that using 20% deficit irrigation, the net benefit increases by 23% and the water productivity by 55% as compared to the existing irrigation practice. Wheat, barley and pepper were not found to be attractive crops in the cropping pattern system. The actual irrigation system has been found poorly managed in addition; the cropping system and the irrigation activity were operated below the optimal. Therefore, the practice of deficit irrigation (recommended 20% deficit irrigation) can substantially increase the net benefit and water productivity of the scheme.