Estimating runoff from ungauged catchments for reservoir water balance in the Lower Middle Zambezi Basin

The Lower Middle Zambezi Basin is sandwiched between three hydropower dams; Kariba, Kafue (Itezhi-tezhi) and Cahora Bassa. The operation of the upstream dams impacts on the inflows into the downstream Cahora Bassa Dam which, in turn, affects the area inundated upstream of the Cahora Bassa Dam. This study applied a rainfall-runoff model (HEC-HMS) and GIS techniques to estimate both the gauged and ungauged runoff contribution to the water balance of Cahora Bassa. The rivers considered in the study are the Zambezi, Kafue, Luangwa, Chongwe, Musengezi and Manyame. Missing data were generated using the mean value infilling method. The DEM hydro-processing technique was used to determine the spatial extent of the ungauged area. A hydrological model, HEC- HMS, was used to simulate runoff from the ungauged catchments. Results from the study show that the ungauged catchment contributes about 12% of the total estimated inflows into the Cahora Bassa Dam. Averaged results over 30 years show total inflows of 71.73 x 109 m3/yr, total outflows of 52.25 x 109 m3/ yr and a residual storage of 20 x 109 m3/yr. The study successfully estimated the water balance of the Middle Zambezi Basin which, in turn, may help to inform the operation of the Cahora Bassa Dam and management of artificial floods in the basin.Keywords: Cahora Bassa, DEM hydroprocessing, HEC-HMS, remote sensing, reservoir operation, runoff simulatio

Assessing the variation of river channel reach inflows on transmission losses

Arid and semi-arid regions are characterized by limited water availability throughout the year and highly variable streamflows. River channel transmission losses add another uncertainty to the complex flow regimes. However, the contribution of different factors influencing to transmission losses is poorly understood. In this work we determined whether variations in river channel transmission losses along five reaches of varying lengths could be related to reach inflows in Runde River catchment in Zimbabwe. We directly estimated transmission losses as the difference between reach inflow and outflow discharges. Using simple bivariate regression equations, channel transmission losses were modelled as response variables while reach inflows were the predictor variables. Our results indicate statistically significant positive relationships (p = .000, R2 > 0.05) between inflows and transmission losses for all minor, moderate, and major flow events. This simple approach can be applied in similar settings to understand the variations in transmission losse

Towards affordable 3D physics-based river flow rating: application over the Luangwa River basin

Uncrewed aerial vehicles (UAVs), affordable precise global navigation satellite system hardware, multi-beam echo sounders, open-source 3D hydrodynamic modelling software, and freely available satellite data have opened up opportunities for a robust, affordable, physics-based approach to monitoring river flows. Traditional methods of river discharge estimation are based on point measurements, and heterogeneity of the river geometry is not contemplated. In contrast, a UAV-based system which makes use of geotagged images captured and merged through photogrammetry in order to generate a high-resolution digital elevation model (DEM) provides an alternative. This UAV system can capture the spatial variability in the channel shape for the purposes of input to a hydraulic model and hence probably a more accurate flow discharge. In short, the system can be used to produce the river geometry at greater resolution so as to improve the accuracy in discharge estimations. Three-dimensional hydrodynamic modelling offers a framework to establish relationships between river flow and state variables such as width and depth, while satellite images with surface water detection methods or altimetry records can be used to operationally monitor flows through the established rating curve. Uncertainties in the data acquisition may propagate into uncertainties in the relationships found between discharge and state variables. Variations in acquired geometry emanate from the different ground control point (GCP) densities and distributions used during photogrammetry-based terrain reconstruction. In this study, we develop a rating curve using affordable data collection methods and basic principles of physics. The basic principal involves merging a photogrammetry-based dry bathymetry and wet bathymetry measured using an acoustic Doppler current profiler (ADCP). The output is a seamless bathymetry which is fed into the hydraulic model so as to estimate discharge. The impact of uncertainties in the geometry on discharge estimation is investigated. The impact of uncertainties in satellite observation of depth and width is also analysed. The study shows comparable results between the 3D and traditional river rating discharge estimations. The rating curve derived on the basis of 3D hydraulic modelling was within a 95 % confidence interval of the traditional gauging-based rating curve. The 3D-hydraulic-model-based estimation requires determination of the roughness coefficient within the stable bed and the floodplain using field observation at the end of both the dry and wet season. Furthermore, the study demonstrates that variations in the density of GCPs beyond an optimal number have no significant influence on the resultant rating relationships. Finally, the study observes that which state variable approximation (water level and river width) is more accurate depends on the magnitude of the flow. Combining stage-appropriate proxies (water level when the floodplain is entirely filled and width when the floodplain is filling) in data-limited environments yields more accurate discharge estimations. The study was able to successfully apply advanced UAV and real-time kinematic positioning (RTK) technologies for accurate river monitoring through hydraulic modelling. This system may not be cheaper than in situ monitoring; however, it is notably more affordable than other systems such as crewed aircraft with lidar. In this study the calibration of the hydraulic model is based on surface velocity and the water depth. The validation is based on visual inspection of an RTK-based waterline. In future studies, a larger number of in situ gauge readings may be considered so as to optimize the validation process.</p

A geospatial web-based integrative analytical tool for the water-energy-food nexus: the iWEF 1.0

Introduction The water-energy-food (WEF) nexus has evolved into an important transformative approach for facilitating the timely identification of trade-offs and synergies between interlinked sectors for informed intervention and decision-making. However, there is a growing need for a WEF nexus tool to support decision-making on integrated resources management toward sustainable development. Methods This study developed a geospatial web-based integrative analytical tool for the WEF nexus (the iWEF) to support integrated assessment of WEF resources to support resilience building and adaptation initiatives and strategies. The tool uses the Analytic Hierarchy Process (AHP) to establish numerical correlations among WEF nexus indicators and pillars, mainly availability, productivity, accessibility, and sufficiency. The tool was calibrated and validated with existing tools and data at varying spatio-temporal scales. Results The results indicate the applicability of the tool at any spatial scale, highlighting the moderate sustainability in the management of WEF resources at various scales. The developed iWEF tool has improved the existing integrative WEF nexus analytical tool in terms of processing time and providing geospatial capabilities. Discussion The iWEF tool is a digital platform that automatically guides policy and decision-making in managing risk from trade-offs and enhancing synergies holistically. It is developed to support policy and decision-making on timely interventions in priority areas that could be showing signs of stress

Water productivity in rainfed agriculture : redrawing the rainbow of water to achieve food security in rainfed smallholder systems; Dissertation, UNESCO-IHE Institute for Water Education, Delft.

The research has been conducted in the semi-arid Makanya catchment of northern Tanzania. It has successfully applied different analytical techniques to better understand soil and water interactions at field scale. It has been successfully demonstrated that there is indeed scope to increase crop water productivity provided the local farmers adopt more efficient cultivation techniques. Substantial yield increases occur as a result of diverting runoff and these further improve when ohter techniques such as ripping, application of manucure and cover cropping are introduced. This confirms no single solution exists to solve the problem of low yields in rainfed farming systems. However, even with the promising results, the research has shown that there is room to further improve the efficiency of crop water use through improvement in research approaches and exploration of better techniques

Water insecurity in Zimbabwe’s towns and cities: Challenge for institutions

There is widespread concern over water insecurity in most towns and cities in Zimbabwe. Some households have gone for years without receiving water yet most reservoirs supplying such towns or cities have been recording decent storage levels throughout the years. This suggests that the collapse of water service provision in the country is not related to shortage of quality raw water, but indicates dilapidation of water infrastructure due to a combination of lack of maintenance, lack of timely investments in infrastructure and general collapse of the water governance structures. In the absence of reliable and safe water sources, communities resort to unsafe sources of water resulting in increased exposure to diseases. The related collapse of wastewater treatment systems has also resulted in many towns and cities discharging almost untreated sewage into public watercourses. Within these municipalities are institutions that also rely on reliable municipal water supplies. Such institutions include schools, colleges, hospitals, clinics and hotels which house large concentrations of populations at given times. Failure to secure reliable and safe water for such institutions threatens operations and may even expose such populations to diseases related to poor sanitation and hygiene. Institutions are therefore increasingly seeking own secure sources of water with groundwater being the immediate option. This paper supports the development of groundwater sources to improve institutional water security but also recommends that such options should be operated as emergency alternative sources to guarantee water security in the event of failure by traditional sources. However, this should be accompanied by strict monitoring of abstractions and water quality so as to safeguard human and environmental health.,Zimbabwe Institution of Engineer

Water productivity in rainfed agriculture; redrawing the rainbow of water to achieve food security in rainfed smallholder systems

The challenge of water scarcity as a result of insufficient seasonal rainfall and dry spell occurrences during cropping seasons is compounded by inefficient agricultural practices by smallholder farmers where insignificant soil and water conservation efforts are applied. The hypothesis of this research is that many of the past research efforts have taken a fragmented approach to deal with the challenges facing subsistence farmers in rainfed systems The research has been conducted in the semi-arid Makanya catchment of northern Tanzania. The research has successfully applied different analytical techniques to better understand soil and water interactions at field scale. It has been successfully demonstrated that there is indeed scope to increase crop water productivity provided the local farmers adopt more efficient cultivation techniques. Substantial yield increases occur as a result of diverting runoff and these further improve when other techniques such as ripping, application of manure and cover cropping are introduced. This confirms that no single solution exists to solve the problem of low yields in rainfed farming systems. However, even with these promising results, the research has shown that there is room to further improve the efficiency of crop water use through improvement in research approaches and exploration of better techniques.Water ManagementCivil Engineering and Geoscience