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Two river basins are found in Uzbekistan, which form the Aral Sea basin:
- Amu Darya basin – covers 81.5 percent of the country. The entire main Amu Darya river can be divided into three reaches: the upper reach borders Afghanistan and Tajikistan, where most of the water flow is generated; the middle reach first borders Uzbekistan and Afghanistan and then enters Turkmenistan; and the lower reach, in Uzbekistan, before the river discharges into the Aral Sea. The main tributaries within Uzbekistan are the Surkhandarya, Sherabad, Kashkadarya and Zeravshan rivers. The Surkhandarya and Zeravshan rivers originate in Tajikistan. The Zeravshan was the largest tributary of the Amu Darya before it began to be tapped for irrigation. Even the remaining flow evaporates in the Kyzylkum desert near the city of Bukhara. The total flow produced in the Amu Darya basin is an estimated 78.46 km3/year on average, calculated by adding the internal renewable surface water resources (IRSWR) of the different countries in the basin: Tajikistan 59.45 km3/year, Kyrgyzstan 1.93 km3/year, Afghanistan 11.70 km3/year, Uzbekistan 4.70 km3/year and Turkmenistan 0.68 km3/year, while the 5 and 95 percent probabilities are an estimated 108.4 and 46.9 km3/year respectively. The period April-September accounts for 77–80 percent and the period December–February for 10–13 percent of annual flow. This intra-annual flow distribution is favorable for irrigated agriculture. Because of significant losses when the river flows through the desert, and because of major water withdrawal by agriculture, the flow reaching the Aral Sea is limited to less than 10 percent of this figure in the driest years. About 4.7 km3/year, or 6 percent of the average total surface water resources of the Amu Darya river basin, are generated within Uzbekistan.
- Syr Darya basin – covers 13.5 percent of the country. The entire main Syr Darya river can be divided into three reaches: the upper is in Kyrgyzstan, where most of the water flow is generated; the middle in Uzbekistan and Tajikistan; and the lower reach in Kazakhstan, before it discharges into the Aral Sea. The main tributaries within Uzbekistan are the Chirchik and Akhangaran rivers, which rise in Kyrgyzstan. The total flow produced in the Syr Darya basin is an estimated 36.57 km3/year, calculated by adding the IRSWR of the different countries in the basin: Kyrgyzstan 27.42 km3/year, Tajikistan 1.01 km3/year, Uzbekistan 4.84 km3/year and Kazakhstan 3.3 km3/year, while the 5 and 95 percent probabilities are an estimated 54.1 and 21.4 km3/year respectively. Because of significant losses in the desert areas of its course, and because of major water withdrawal by agriculture, the flow reaching the Aral Sea is limited to less than 5 percent of this figure in the driest years. About 4.84 km3/year, or 13 percent of the average surface water resources of the Syr Darya river basin are generated within Uzbekistan.
Uzbekistan has thousands of small streams that disappear in the desert, many having been emptied by irrigation (OrexCA, 2011).
The total river flow generated inside Uzbekistan is thus estimated at 9.54 km3/year of which 49 percent from the Amu Darya river basin and 51 percent from the Syr Darya river basin.
Surface water resources allocated to Uzbekistan are calculated every year, depending on climatic conditions and existing flows. However, the estimated average surface runoff from upstream countries is as follows (Table 2):
- Amu Darya basin: Based on an agreement between Uzbekistan and Turkmenistan signed in January 1996, which supplemented the ‘1992 Five Central Asia Countries Agreement’, half of the water is allocated to Uzbekistan and half to Turkmenistan. Thus, of the average flow of 44 km3/year, 22 km3/year are reserved for Uzbekistan and 22 km3/year for Turkmenistan (of which 0.68 km³/year are Turkmenistan’s IRSWR). This means that of the 43.32 km3/year allocated flow from the Amu Darya river basin from Tajikistan into Uzbekistan, 21.32 km3/year (=22–0.68) is transit flow to Turkmenistan.
- Syr Darya basin: 22.33 km3/year from Kyrgyzstan, of which 11.8 km3/year is transit flow to Tajikistan, of which 11.54 km3/year again is transit flow to Uzbekistan, of which finally 10 km3/year is reserved for Kazakhstan;
There are 94 major aquifers in Uzbekistan. The renewable groundwater resources are an estimated 8.8 km3/year, of which 2 km3/year are considered an overlap with surface resources. The IRWR are therefore an estimated 16.34 km3/year and total actual renewable water resources (TARWR) are 48.87 km3/year, equal to total actual renewable surface water resources (TARSWR) of 42.07 km3/year, taking into consideration the allocation mechanism between the different countries, plus renewable groundwater resources of 8.8 km3/year minus the overlap of 2 km3/year, (Table 2 and Table 3).
Between 1990 and 1994, return flow on the Uzbekistan territory was an estimated 32.4 km3/year, of which 21.5 km3/year in the Amu Darya river basin and 10.9 km3/year in the Syr Darya river basin. This total comprises 30.9 km3/year of drainage flow from irrigated areas (of which 2.55 km3/year is the result of vertical drainage from pumping) and about 1.5 km3/year of untreated municipal and industrial wastewater. The main portion of the return flow, 49 percent or 15.9 km3/year, returned to rivers: 9.5 km3/year in the Amu Darya basin and 6.4 km3/year in the Syr Darya basin. About 37 percent or 12 km3/year ended up in natural depressions (Arnasay, Parsankul, Sarykamish and lake Sudochie) from which most water evaporates.
More than 4.5 km3/year or 14 percent were used for irrigation: 2.9 km3/year without treatment, mainly for cotton on light soils and 1.6 km3/year after in situ desalting treatment (phytomelioration). Around 2000, direct use of drainage water was an estimated 6.84 km3, of which 4.21 km3 from the Syr Darya and 2.63 km3 from the Amu Darya system. Around 2005, total return flow was an estimated 23 km3 (Abdullaev et al., 2009).
The collector-drainage water outflow has led to the creation of artificial lakes in natural depressions. The largest lakes are: Aydarkul, in the Arnasay depression in the middle reach of Syr Darya, which stored about 30 km3 in 1995; the Sarykamish and Sudochie lakes, both located in the lower reach of the Amu Darya, store 8 and 2 km3 respectively. Several lakes have formed in the centre of the country in the Amu Darya basin, the largest being Parsankul lake close to the Zeravshan river, which stores about 2 km3.
There are at least 50 reservoirs in Uzbekistan with a total capacity of over 22 km3. The largest reservoirs are multipurpose dams used for irrigation, flood control and hydropower production. In the Syr Darya basin, the largest reservoirs are the Charvak and Andijan reservoirs. The Charvak reservoir, which is one of the largest hydropower plants in Central Asia, is on the Chirchik river, near the capital Tashkent and has a capacity of 1.99 km3 and 600 MW. The Andijan reservoir on the Karadarya river in the Fergana valley, has a capacity of 1.9 km3. In the Amu Darya basin, the largest reservoir is the Tuaymuyun, in Khorezm vilayat, with a storage capacity of 7.8 km3, comprising four separate reservoirs. One reservoir in this system (Kaparas) is to provide drinking water for the Karakalpakstan area, which is experiencing severe environmental problems as a result of the shrinking of the Aral Sea. Most reservoirs were built more than 25 years ago. During this period, almost all were exposed to siltation, resulting in almost 20–25 percent loss of useful capacity.
Gross theoretical hydropower potential is an estimated 88 000 GWh/year and the economically feasible potential 15 000 GWh/year. In 1993 total installed capacity was1.7 GW, and in 1995 provided about 12 percent of the country’s electricity.
Extensive canal systems, such as the Amu-Bukhara canal and many others built during the Soviet period, have greatly altered water-flow patterns (OrexCA, 2011).