By Carlo Buontempo, Met Office
At around 6000 km in length, the Nile is the longest river in the world. It represents a crucial source of freshwater for the 11 Nile riparian countries and it provides most of the freshwater available to Egypt.
Given this dependence it isn't surprising that Egypt is particularly interested in understanding the future of the river. Dealing with the variability of Nile flow has been a constant theme throughout the long history of the country and some sort of resilience is part of the cultural inheritance of modern Egyptians.
At the same time, the recent demographic changes and the likely future growth in population will provide extra challenges for this North African country. Understanding how much water will flow in the river in future decades is one of main concerns of the Ministry of Water Resources and Irrigation (MWRI) in Cairo.
The construction of the high Aswan dam in the 1960s (which has been filling up until 1976) significantly changed the way water resources are dealt with in Egypt. The river can now be considered fully managed downstream of the dam. This means that most of the questions about future runoff can be basically mapped onto the expected inflow into Lake Nasser, the artificial basin crated by the Aswan dam.
The Danish Hydrological Institute (DHI), Met Office Hadley Centre and MWRI have worked together to better understand how climate change could affect the region and in particular its water resources. The project, funded by the Spanish Millennium Development Goal and managed by the UN through UNDP, provided a unique opportunity to put together crucial expertise on local environment, hydrological processes and climate change.
CLIMATE CHANGE CHALLENGE
Although a number of studies have looked at how climate change may affect Nile river runoff, the task has proven to be challenging. The flow of the river appears to be very small when compared to the huge catchment the river drains.
The observed runoff effectively represents a fine balance between huge forces of nature. The tropical precipitation that falls on the highlands of western Ethiopia provides most of the water of the main Nile north of Khartoum and is effectively counterbalanced by the large evaporation that occurs in the desert.
This translates into a very non-linear response to change in rainfall occurring on the Ethiopian highlands. Sensitivity studies conducted by scientists at MWRI in Cairo have shown how sensitive the Nile runoff is to changes in precipitation along the Blue Nile catchment.
In the dry season, most of the water in the main Nile is supplied by the White Nile which comes from Lake Victoria, while the Blue Nile and Atbara river, which together drain most of the west Ethiopian plateau, account for as much as 90% of the flow of the main Nile in flood conditions.
Given the limited multi-annual variability of the White Nile flow compared to that of the Ethiopian river, the latter is usually considered the most important subcomponent of the Nile basin as far as the water resources of Egypt are concerned.
While most of the previous studies in the region used Global Climate Model (GCM) models to assess how climate change may affect Nile runoff, simulations of regional climate models were used in this project. These are effectively high-resolution versions of the global model that are able to capture some of the local climate forcing not 'visible' at the GCM scale.
LOCAL LEVEL IMPACTS
For the project five different simulations of Met Office global model were downscaled using the Met Office regional climate model, PRECIS. These were chosen from a larger ensemble of 17 simulations on the basis of their ability to reproduce some of the key features of the regional climate such as the African Monsoon or the seasonal cycle of temperature in the Nile basin.
MWRI used the outputs of these simulations to generate synthetic time series of precipitation and temperature for future decades. These time series were then used as input in the Nile Forecast System, the operational model used at MWRI for planning and assessing future changes in the runoff. The results suggest that a small increase in the river runoff for the 2050s appears to be more likely than a decline.
The management of water in a cross-boundary basin is a challenging exercise. It is important that decisions are informed by the best available scientific evidence. This is one of the reasons why DHI and Met Office are currently involved in a follow-up project involving not only Egypt but all the riparian countries via the Nile Basin Initiative.
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Carlo Buontempo is a senior climate change consultant at the Met Office. The opinions expressed are his own.