Climate change and water resources management in arid and semi-arid regions: prospective and challenges for the 21st century

The overgrowing population and the recent droughts are putting water resources under pressure and calling for new approaches for water planning and management if escalating conflicts are to be avoided and environmental degradation is to be reversed. As countries are using their water resources with growing intensity, poor rainfall increasingly leads to national water crises as water tables fall and reservoirs, wetlands and rivers empty. Global warming could cause further changes, further variability and further uncertainty. The UK Hadley Centre's global climate model was run at a spatial scale of 2.5 by 3.75 degrees (latitude and longitude) grid squares to simulate the global climate according to scenarios of greenhouse gas concentration emission. Runs of the model assuming the emission scenario proposed by the Intergovernmental Panel on Climate Change in 1995 are analysed here for the 2050s time horizon. Outputs provide estimations of climate variables, such as precipitation and temperature, at a monthly time step. Those results, assumed representative of future climatic conditions, are compared to mean monthly values representative of the current climate and expressed in terms of percentage change. The results show that, for the dry season (April-September), by the 2050s, North Africa and some parts of Egypt, Saudi Arabia, Iran, Syria, Jordan and Israel, are expected to have reduced rainfall amounts of 20-25% less than the present mean values. This decrease in rainfall is accompanied by a temperature rise in those areas of between 2 and 2.75 degrees C. For the same period, the temperature in the coastal areas of the Mediterranean countries will rise by about 1.5 degrees C. In wintertime, the rainfall will decrease by about 10-15% but would increase over the Sahara by about 25%. Given the low rainfall rate over the Sahara, the increase by 25% will not bring any significant amount of rain to the region. In wintertime, the temperature in the coastal areas will also increase but by only 1.5 degrees C on average, while inside the region it will increase by 1.75-2.5 degrees C. In southern Africa (Angola, Namibia, Mozambique, Zimbabwe, Zambia, Botswana and South Africa), results suggest an increase of the annual average temperature ranging between 1.5 and 2.5 degrees C in the south to between 2.5 and 3 degrees C in the north. The summer range is between 1.75 and 2.25 degrees C in the south, and increases towards the north to between 2.75 and 3.0 degrees C while the winter range is between 1.25 and 2 degrees C in the south, and increases towards the north to between 2.5 and 2.75 degrees C. On the other hand, the annual average will decrease by 10-15% in the south and by 5-10% in the north. The annual average decrease is 10%. However, some places will have an increase i.e. by 5-20% in South Africa in wintertime. In the Taklimakan region (Tarim Basin) west of China, the annual average temperature is shown to increase by 1.75-2.5 degrees C. Annual average rainfall should increase by 5- > 25% in most of the region but decrease by 5-10% in some small parts. In summer, an increase by 5-15% is indicated in most of the region, and an increase by up to 25% or more during the wintertime. In the Thar Desert (India-Pakistan-Afghanistan), estimations suggest that the annual average increase in temperature ranges from 1.75 to 2.5 degrees C, ranging from 1.5 to 2.25 degrees C in winter and from 2 to 2.5 degrees C in summer. Annual average precipitation is shown to decrease by 5-25% in the region. The winter will have values closer to the annual average but the summer will have more decrease and most of the region will see a decrease closer to 25%. In the Aral Sea basin (Kazakhstan, Turkmenistan and Uzbekistan) estimates suggest an annual average increase in temperature ranging from 1.75 to 2.25 degrees C, higher in summer (between 2 and 2.75 degrees C) than in winter (between 1.5 and 2 degrees C). Rainfall should increase by 5-20% annually, in summer increasing by 5-10% in the north but decreasing by up to 5% in the south, while in wintertime, both south and north should undergo increases of 5-10% and 20-25%, respectively. In Australia, results indicate an increase in the annual average temperature ranges of 1-1.5 degrees C in the south to 2.5-2.75 degrees C in the north, slightly higher during the summer than in the winter. The summer range is between 1 and 2 degrees C in the south and increases towards the north to 2.5-3.0 degrees C while the winter range is between 1 and 1.5 degrees C in the south, and increases towards the north to between 2 and 2.25 degrees C. Rainfall annual average is shown to decrease by 20-25% in the south and by 5-10% in the north. Given the above-mentioned facts, in order to meet the water demands in the next century, some dams and water infrastructure will be built in some countries and a new paradigm by rethinking the water use with the aim of increasing the productive use of water will have to be adopted. Two approaches are needed: increasing the efficiency with which current needs are met and increasing the efficiency with which water is allocated

among different uses. In addition, non-conventional sources of water supply such as reclaimed, recycled water and desalinated brackish water or seawater is expected to play an important role.