Changes in water requirements for production and virtual water trade of cotton in Central Asia

Cotton is an important agricultural export item in Central Asia. Given the increasing shortage of water resources, it is necessary to evaluate the water requirements for cotton production and the virtual water trade of cotton for sustainable water resource management. In this study, the water requirements and specific water demands of cotton in the five countries of Central Asia were estimated from the long-term meteorological data from 175 stations using the ArcGIS spatial interpolation and crop coefficient method along with the distribution of cotton cultivation. Parameters for the specific water demands and the production and trade data from the Food and Agriculture Organization (FAO) as well as the amounts, change trends, and water requirements for production and virtual water trade of cotton in 1992-2017 were analyzed. Finally, the impact of virtual water trade on the water resources in Central Asia was evaluated. The results showed that: 1) the cotton water requirement was 761.0-1033.9 mm, the specific water demand was 2834.4-5732.1 m³·t^-1, and the value of Uzbekistan cotton was 4263.8 m³·t^-1, which was dominant in the calculations of water requirement for cotton production and the amount of exported virtual water of cotton. The parameters for the specific water demand were reliable for the spatial-weighted calculation processes. 2) From 1992 to 2017, the water requirement for cotton production and the net exported virtual water amount of cotton trended downward due to decreases in the total production and export amounts of cotton. The decrease in total cotton production resulted from a decline in harvested area, and low yields in some Central Asian areas directly led to decreased water demand for production (from > 30 billion m³ to > 20 billion m³). An increase in cotton consumption was the main reason for declining virtual water exports in Central Asia; the net exported virtual water volume dropped from 20 billion m³ to 5 billion m³. From 1992 to 2017, the annual water requirement for production, net exported virtual water, and the ratio of net exported amount to production amount of cotton were 23.72 billion m³, 14.74 billion m³, and 62.1%, respectively. At present, approximately one-third of the water requirement for cotton production is exported via virtual water trade despite falling cotton exports. 3) In Central Asia, 6.5% of the renewable water resources (not including water loss in farmlands and conveyance loss in irrigation events) was exported in the form of cotton exports in 1992-2017. In Uzbekistan and Turkmenistan, 18.4% and 12.7% of the renewable water resources were exported, respectively, which negatively impacted the regional water resources and induced water shortages.