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The internal renewable surface water resources are estimated at 3.948 km3/year and the internal renewable groundwater resources at 4.311 km3/year. The overlap between surface water and groundwater is estimated at 1.400 km3/year. This gives a total of 6.859 km3 of annual internal renewable water resources (IRWR) (Table 2).
The rivers in Armenia are tributaries of the main rivers of the southern Caucasus, namely the Araks and the Kura. About 76 percent of the total territory is part of the Araks basin and 24 percent of the Kura basin (UNDP/GEF, 2006). Total outflow is equal to the IRWR. The outflow to Georgia through the Debet River is estimated at about 0.89 km3/year and the outflow to Azerbaijan through the Agstay River at about 0.35 km3/year; both these rivers are located in the Kura basin. The total outflow to Azerbaijan through the Araks and its tributaries (Arpa, Vorotan, Vokhchi) is estimated at about 5.62 km3/year. The Araks River forms the border between Turkey and Armenia and further downstream, between the Islamic Republic of Iran and Armenia, it flows into Azerbaijan, joining the Kura River about 150 km before its mouth at the Caspian Sea. The border flow of the Akhuryan (with Turkey) is estimated at 1.03 km3/year and the Araks at 0.79 km3/year. Half of the border flow is accounted for in Armenia’s water balance, bringing the total actual renewable water resources to 7.769 km3/year.
The 14 sub-basins of the two main river basins (Kura and Araks) have been grouped into five basin management areas: Akhuryan, Northern, Sevan-Hrazdan, Ararat and Southern basins (USAID, 2006). About 9 500 rivers and streams with the total length of 23 000 km flow in Armenia. Out of that number 379 rivers are around 10–100 km long, and seven, namely the Akhuryan, Debet, Vorotan, Hrazdan, Aghstev, Arpa and Metsamor-Kasakh, are longer than 100 km. The annual distribution of river flow generated in the country by the 14 river basins and their characteristic features are presented in Table 3 (UNDP, 2006). Armenian rivers are typically of a mountainous nature with sharp seasonal variations, spring freshets and low water flow in summer.
Armenia has more than 100 small lakes, some of which regularly dry out in the dry season. The Sevan and Arpi lakes are the most important in terms of size and economic importance. The Hrazdan and Akhuryan rivers originate from these two lakes, the largest of which is Lake Sevan, located in the centre of the country. It lies at 1 900 m above sea level, which makes it a strategic source of energy and irrigation water. The level of the lake, originally with a surface area of about 1 414 km2 and 58 km3 of stored water, has fallen since the 1930s due to the lake’s increasing use for irrigation and domestic water supply. By 1972, its level had fallen by almost 19 m and its surface area had been reduced to 1 250 km2. At present, it covers an area of about 1 200 km2, has a volume of approximately 34 km3, and plays a central and important hydrological role in the country. It serves the densely populated Hrazdan river basin and the Ararat Valley where Yerevan, the capital, is situated. Through its regulated surface outflow into the Hrazdan River, the lake’s water provides a substantial amount of hydropower and irrigation to croplands in the Ararat Valley. The lake is also an important recreational, natural habitat and cultural resource for the Armenian population (MNP, 2005). Since 1960, two inter-basin transfer schemes were implemented to restore the ecology of the lake and its storage capacity as a strategic water reserve for multipurpose use. A 48 km tunnel was built from 1963 to 1982 to divert some 250 million m3 of water annually from the Arpa River to Lake Sevan. A similar project, to divert 165 million m3 of water annually from the upper Vorotan River to the Arpa River through a 22 km tunnel, was completed in 2004. In the last few years, the lake’s level has risen by about 2.7 m as a result of favourable meteorological conditions and improved management. Electricity generation at the Sevan-Hrazdan Cascade is currently tied to irrigation releases. During the last few years, irrigation releases have ranged from 120 to 150 million m3. The second most important lake is Lake Arpi. It is located in the western part of the Ashotsk depression at an altitude of 2 020 m above sea level. With the construction of a dam to solve irrigation problems, the lake became a reservoir.
Most of the reservoirs were constructed during the Soviet period. In 2004, some 83 reservoirs were operating in Armenia and total capacity was estimated at 1 399 million m3, of which approximately 1 350 million m3 was stored in reservoirs with a capacity of over 5 million m3 each. Most of the water is used for irrigation. Some reservoirs are used for hydropower, recreation, fisheries and environmental protection. In 1995, about 145 million m3 was used for municipal and industrial purposes. The largest reservoir is on the Akhuryan River, which forms the border with Turkey. It has a storage capacity of 525 million m3, is shared with Turkey, and provides water for the irrigation of about 30 000 ha in Armenia. In contrast, many small off-channel reservoirs in the southwest of Aragats (Talish, Talin, Kakavadzor, Bazmaberd, Katnakhpyur), which accumulate spring tide waters, have a capacity of only 10 000–50 000 m3 (UNDP, 2006).