The Nile basin exhibits large variations in climate ranging from the tropical climate at the sources of the Blue and White Nile to the Mediterranean climate at the mouth of the Nile. This variation reflects the latitude range; 4oS to 32oN and the altitude range; from sea level to more than 3,000 m. The tropics; East African lakes region and southwestern Ethiopia, exhibit climates with well-distributed rainfall in excess of 1,000mm per year whereas northern Sudan all across Egypt, there is negligible rainfall (sometimes falling below 50mm a year) except for the Mediterranean coast which gets about 180mm a year. Depending on the location and altitude in the equatorial lakes region, there is generally little variation in the mean annual temperature ranging from 16 to 27oC whereas in the semiarid areas up to Egypt, the temperature ranges are quite high; 10 – 45oC.
There is evidence that the global climate is changing due to human-induced emissions of greenhouse gasses. The emissions lead to increasing atmospheric concentrations of these gasses. In turn increased concentration of these gases affects the global radiation balance. The general expectations that this will results in a warmer world and that the hydrological cycle will accelerate. On a global scale the climatic changes mean an increasing temperature, known as global warming, more precipitation and more evaporation. However, although the general trends are recognised, regionally the magnitude and even the direction of change are still far from clear.
Changes in climate in the Nile basin may lead to changes in the discharge of the river Nile. Such changes have occurred in the far past. Geological records of the Nile basin reveal an alternation between relative wet and dry periods during the last 20,000 years. Relatively wet periods appeared between 12,000 and 7500 BP and between 6000 and 2500 BP. The periods 20,000- 12,000 BP, 7500 – 6000 BP as well as the period 2500-1000 BP were relatively dry. More recently the observed discharges indicate that the last 3 decades of the 19th century were relatively wet. Well known is the very dry period between 1980 and 1990.
If the flow of the river Nile would change considerably, this will have effect on the water management in the Nile. Current dam operation and release strategies, that are able to meet the water demand in the various Nile countries under the actual conditions, may not be sufficient to meet the demands in future, this because both the supply as well as the demand changes in time.
This chapter presents the main climate variables for which data/information was gathered (rainfall, temperature, relative humidity, evapotranspiration, and wind speed) mainly in terms of their monthly distribution within the Sub-basins.