Monthly observed rainfall data was collected mainly from the NBI database which was originally gathered from the member countries, other sources being GHCN, NBE, MWE, and NBRP. The selection of the stations for use in this atlas was based on the length of the records, quality of the data, and the spatial location of the station with an idea to get a fair spatial distribution of the stations across the basin. Most of the data used had been quality controlled by the Nile Basin Initiative Secretariat. Spatially, there is a gradual decrease of rainfall amounts from upstream to downstream with some upstream areas registering up to monthly maximums of 700mm in the rainy seasons (March- May) and the lower arid parts of the basin registering maximums of up to 60mm in the wet season (July – September). There are two distinct wet seasons separated by dry seasons in the equatorial lakes region, which gradually transform into a single wet season, followed by a dry one in the other parts of the basin.
This section of the atlas presents the monthly rainfall distribution over the Nile Basin presented at sub-basin level and clearly indicating the variations in seasons and rainfall amounts. The background map for the sub-basins shows the average rainfall distribution as derived from satellite data. The mean monthly rainfall distribution based on satellite data (TRMM: 3B43 v7) for the period 2000-2012 is presented for comparison purposes.
Average Country Rainfall
On average, annual rainfall in the Nile basin is approximately 650 mm. However, rainfall differs substantially by country, with low rainfall in Egypt and high rainfall in Ethiopia and the countries of the Equatorial Lakes Plateau.
Egypt is the country with the least rainfall averaging 200 mm per year. The capital city, Cairo, receives about 25 mm per year. Ninety per cent of the country receives rain only once every couple of years. An estimated 30% of Sudan is desert, where drought is common. Rainfall here averages about 254 mm per year. This area borders a semi-arid Sahelian region of low mountains in the central area of the Sudan, giving way to a swamp-covered south which receives approximately 1015 mm of rain a year.
Rainfall in Ethiopia ranges from 510 mm up to 1 525 mm in the rainy season from mid-June to September. In absolute terms, there is an overall large amount of rainfall in Ethiopia, but the effects vary widely, and are often not beneficial. For example, heavy downpours in the rainy season cause severe erosion leading to losses in soil fertility and productivity; while the rest of the year is extremely dry, making farming almost impossible without irrigation. Precipitation is generally higher in the upstream countries.
The climate in Burundi is tropical and moderated by its altitude. The average annual rainfall ranges from 1000 mm to 1 500 mm. Rainfall in the DRC falls throughout the year and ranges from 1524 mm in the north, to 1270 mm in the south. 20% of Uganda is covered by open water and precipitation is between 1000 mm and 1500 mm a year. In some of the countries, the amount of precipitation received in their portion of the Nile basin exceeds the national average. This fact is important for policy and decision making especially in countries where there is a lack of additional water resources. Examples include Egypt, Sudan and Ethiopia which do not have significant water resources within their borders outside of the Nile and its tributaries.
| Country | Avg. Country Rainfall (mm/yr.)3 | Avg. Nile rainfall (mm/yr.) |
| Burundi | 1275 | 1202 |
| DR Congo | 1543 | 1146 |
| Egyp | 51 | 19 |
| Ethiopia | 848 | 1184 |
| Kenya | 630 | 1149 |
| Rwanda | 1212 | 1137 |
| Sudan | 250 | 487 |
| South Sudan | 900 | 900 |
| Tanzania | 1071 | 1043 |
| Uganda | 1180 | 1193 |
3 Source: World Development Indicators, 2015