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Annual rain volume all over the country varies between 67 and 93 km3. Precipitation falls more on the western highlands, southwest highlands and the upper plateaus. It then gradually becomes lower towards the east. The ratio between the rainfall and potential evaporation reaches around 0.03–0.25 in the Rub Al Khali Desert.
The country can be subdivided into four major drainage basins, grouping numerous smaller wadis:
- the Red Sea Basin
- the Gulf of Aden Basin
- the Arabian Sea Basin
- the Rub Al Khali Interior Basin
The floods of the wadis are generally characterized by abruptly rising peaks that rapidly recede. Between the irregular floods the wadis are either dry or carry only minor base flows. Surface water resources have been estimated at 2 km3/year, but this quantity corresponds to the runoff from major rivers and does not include the runoff produced within the smaller catchments. Renewable groundwater resources have been estimated at l.5 km3/year of which a large part, estimated at 1.4 km3/year, probably comes from infiltration in the river beds. Total internal renewable water resources are thus estimated at around 2.1 km3/year (Table 2).
Surface runoff to the sea measured in some major wadis is estimated at 270 million m3/year and groundwater outflow to the sea at 280 million m3/year. There might be some groundwater flowing into Saudi Arabia but no data are available. The existence of surface drainage crossing into Saudi Arabia suggests that some sharing of surface flows could be possible, but details are not known.
The volume of groundwater reaches around 10 km3, of which 1 km3 in the Al-Masila Basin, 2.5 km3 in the Tihama Basin and the remaining distributed over the other regions.
Yemen has a long history of dam construction and the ancient civilization was founded upon the great dam of Ma’areb, the destruction of which marked the end of its existence. After the revolution, the government carried out the reconstruction of the Ma’areb Dam financed by the United Arab Emirates. The new dam has a capacity of 400 million m3. The remaining dams have a total capacity of 62.5 million m3, giving a total dam capacity of 462.5 million m3.
There are over a thousand hydraulic structures falling into three different categories:
- Dams: 347 storage dams have been constructed in the upper lands to store rainfall water for irrigation and for domestic use, and to recharge sub-aquifers. There are three types: large dams with a capacity above 500 000 m3, medium dams with a variable capacity from 200 000 to 500 000 m3 (71 dams of this type have been constructed) and small dams with a capacity of less than 200 000 m3.
- Spate water diversion structures: 33 of these structures have been constructed in the main wadis for spate water regulation and diversion.
- Small water harvesting structures: this category includes cisterns, pits and reservoirs with a storage capacity ranging from 500 m3 to 50 000 m3.
Thirteen wastewater treatment plants (WWTP) are in operation. They are concentrated in the capitals of the governorates and in some secondary cities. However, while the cities are growing fast, the capacity of the plants has not increased. For example, in Sana’a the WWTP was designed to treat 25 000 m3/day of wastewater, but now it receives more than 50 000 m3/day. Similarly, in Ibb city the WWTP was designed to receive 5 000 m3/day, but now it receives more than 10 000 m3/day. These examples reflect the insufficient treatment leading to the production of bad quality water that is not suitable for irrigation. The Ministry of Agriculture and Irrigation considers this water to be harmful and it should be appropriately treated in a way that prevents environmental pollution. In 2000, the total volume of produced wastewater was 74 million m3 and the treated wastewater was 46 million m3 in 1999, while the amount of treated wastewater used in agriculture was only 6 million m3/year in 2000.
In 2002, the total installed gross desalination capacity (design capacity) was 76 596 m3/day or 28 million m3/year (Wangnick Consulting, 2002). The production of desalinated water reached 25.1 million m3 in 2006, an increase of 151 percent compared with 1989, contributing to the water supply of Aden city.