Variations in and predictions of irrigation water requirements of cotton and winter wheat in Central Asia

Agricultural irrigation consumes most of the fresh water in Central Asia (CA). Therefore, changes in irrigation water use have direct effects on water resources, water management, and water resource allocation. The crop water requirement (CWR) is a direct indicator of agricultural irrigation; the CWR of cotton and winter wheat (the two main crops of CA) was investigated in this study. Based on the CWR method proposed by the Food and Agriculture Organization (FAO), the CWR and total irrigation water of cotton and winter wheat in irrigated croplands in CA were calculated from 2006 to 2015. Changes in the irrigation cropland area and cotton and winter wheat CWR from 2006 to 2015 were also analyzed. To assess the near-future status of agricultural water resources in CA, the irrigated cropland area in 2030 was predicted via the CA_Markov method, and the CWR of the two crops in 2030 was explored. Land cover data from the European Space Agency Climate Change Initiative (ESA CCI) was used to identify the irrigation cropland. Digital Elevation Model (DEM) data from the Shuttle Radar Topography Mission (SRTM), population density data from the Socioeconomic Data and Applications Center, and river vector data from Natural Earth were used to predict the irrigated cropland area in 2030. The results showed that irrigated cropland in CA increased by 492 km² from 2006 to 2015, with increases in Kazakhstan, Kyrgyzstan, and Turkmenistan, and decreases in Tajikistan and Uzbekistan. The CWR of cotton and winter wheat tended to increase in most areas. The CWR of cotton increased rapidly from 2006 to 2015 (2.5-4.3 mm·a^-1) in Turkmenistan and eastern Kazakhstan but decreased in Kyrgyzstan. For winter wheat, the largest CWR increase (10 mm·a^-1) was in Turkmenistan, and no changes were observed in the other four CA countries. In 2030, the irrigated cropland in Turkmenistan was predicted to increase by 30.5% compared with 2015, and Turkmenistan is the only country to increase irrigated croplands in the near future. This will lead to a notable increase in the CWR of cotton and winter wheat. The results showed that there will be an increase of 28 km³ for the total irrigation water of cotton and 17 km³ for winter wheat in Turkmenistan in 2030. The other four countries were predicted to have decreased irrigation cropland compared with 2015, with decreases of 7.4%, 5.8%, 3.4%, and 0.9% for Uzbekistan, Tajikistan, Kazakhstan, and Kyrgyzstan, respectively. Although Uzbekistan is predicted to decrease irrigation cropland in 2030, the increase in winter wheat CWR will increase the total irrigation water by 10-14 km³. Therefore, Uzbekistan and Turkmenistan will face severe shortages in irrigation water resources in the near future.