Detecting changes in irrigation water requirement in Central Asia under CO2 fertilization and land use changes

As one of the largest arid and semiarid areas in the world, Central Asia (CA) has been facing severe water crisis. Agricultural irrigation consumes most water resources there. However, it is not clear how the irrigation water requirement (IWR) varies spatially and temporally in CA, especially under CO₂ fertilization and land use change. This study, for the first time, quantifies changes of IWR for two predominant crops (cotton and winter wheat) over CA under two climate change scenarios (RCP2.6 and RCP4.5, both of which consider CO₂ fertilization effects) and land use projections. Our results show that without considering atmospheric CO₂ concentration for estimating IWR would result in large errors and even different signs of the changes. In the future, IWR for cotton and winter wheat tends to increase in 2020s and 2040s but decrease in 2060s and 2080s under RCP2.6 and CO₂ fertilization. The change magnitude is less than 5%. Under RCP4.5 and CO₂ fertilization, most areas in CA exhibit an increase of less than 5%. The maximum increases of 5%-15% for cotton occur in  Tajikistan. The maximum increase of more than 50% for winter wheat occurs in Tajikistan under both climate scenarios. The IWR in Turkmenistan is most sensitive to land use change, with 33% increase compared with IWR in 2015. The other four countries have small differences (less than 10%) between 2015 and 2030. Severe water security pressure is predicted in Turkmenistan and Uzbekistan and the smallest in Tajikistan. This study provides a comprehensive evaluation of IWR for the Central Asian countries in the future and helps the decision maker for sensible water management.