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Understanding future river flows in Lake Victoria Basin

The Katonga River opens through a wide mouth into the north-western shores of Lake Victoria, about two hours’ drive from the Ugandan capital of Entebbe. Because of the large surface area of the river, and its surrounding wetlands, it is prone to lose a considerable volume of water through evaporation. In a changing climate, rising regional temperatures are likely to increase the rate of evaporation from here, impacting on water flow.

However, a likely increase in rainfall associated with more intense storm events is also likely to flush more water into the river catchment during the region’s two rainy seasons.

Which force will win out in the end – evaporation or rainfall – and how will that impact on flows in this river in future?

Water resource managers in the region need to know what sort of future to plan for, and a recent collaboration between climate modellers and hydrogeologists might help improve the understanding of likely future changes in river flows across the Lake Victoria Basin (LVB) in East Africa.

Future Climate for Africa researchers have been working on refining a new Africa-focused climate model which can better reflect likely changes in future rainfall trends across the region, something which previous global-scale models had difficultly doing. The Pan-African Convection-Permitting Regional Climate (CP4-Africa) model specifically captures how convection – the upward flow of warm air in the atmosphere – drives cloud formation, rainfall, and thunderstorm activity across the region. The model’s outputs suggest that there will be a greater increase in the intensity and severity of storms across the Lake Victoria Basin region than predicted by global climate models that do not include convection. This prediction will see a potential increase in the likelihood of high flows during the rainy seasons.

This information is now feeding in to a second modelling process, done in conjunction with the British Geological Survey (BGS), to understand what the increase in extreme rainfall events will mean for river flows and water resources in the East African basin.

‘The CP4-Africa model shows that there won’t be a large change in the timing of the two rainy seasons in Uganda, for instance, but there may be an increase in the intensity of rain storms,’ explains BGS hydrogeologist Dr Dan Lapworth.

This information now feeds into a river flow model used by the BGS team, which is able to estimate how flows in rivers may change following extreme storms.

‘We have a river flow model that is calibrated using historic observations of river flows across the basin for the past 30 to 40 years,’ says Lapworth. ‘With the rainfall output from the CP4-Africa model, we can simulate how these rain events will impact on future river flows in the Katonga catchment and others across the basin.’

According to Dr Matthew Ascott, another hydrogeologist with the BGS, the model for the Katonga River indicates that increased rainfall within the catchment will be enough to overcome the increase in evaporation, therefore leading to greater river flows overall.

‘Even if evaporation increases from the river system because of rising temperatures, the increase in rainfall during extreme storms across the river’s catchment will nevertheless result in more water entering the river and keep flows high.’

The information from the modelling processes is being passed on to national water resource managers across the basin to assist with the region’s future water planning.

The Ugandan Ministry of Water and Environment (UMWE) is one of the national institutions hoping to benefit from this improved modelling. The ministry oversees the large-scale management of water resources in the country, which is divided into different water management zones.

‘The ministry is integrating climate change information into their plans for each of these zones, so that they can plan for better water access and resilience in future,’ says Lapworth.

This will allow for national-level intervention which will feed through to more effective planning within the agriculture sector, those managing water supply, soil erosion, wetland ecosystem and catchment management, and for planning relating to large-scale infrastructure investment.

Changes in water flows in the Katonga River have been relatively under-researched, according to the BGS team.

‘Through consultation with the UMWE, we focused initially on this catchment because the hydrology here is interesting, typical of several other catchments, and it wasn’t well studied,’ says Lapworth. ‘There are a lot of wetlands along the river, and high levels of evaporation, which are difficult physical processes to model numerically.’

Initial model results suggests that extreme river flows in the Katonga may increase in magnitude compared with current trends, and the use of large scale global climate models may underestimate flow extremes in some cases.

The BGS team is working with modellers in the UMWE so that they are better able to interpret the modelling information themselves, and also translate these outputs into accessible climate information which policymakers can better understand and implement in their planning processes. The work is ongoing.

The work reported on in this story is part of the FCFA HyCRISTAL (Integrating Hydro-Climate Science Into Policy Decisions for Climate Resilient Infrastructure and Livelihoods in East Africa) research consortium. HyCRISTAL focuses on this rapidly developing region, which is close to the equator. Most of the region has two rainy seasons per year, but includes areas, which have a single dominant rainy season. HyCRISTAL’s focus is the East African Community states (Burundi, Kenya, Rwanda, Tanzania and Uganda), as well as Somalia and Ethiopia.

This article was written by Leonie Joubert.