Climate-Induced Shifts in Irrigation Water Demand and Supply during Sensitive Crop Growth Phases in South Asia

Abstract

Abstract This study investigated climate-induced shifts in irrigation water demand and supply of the major staple and water-intensive crops (wheat and rice) in the Indus, Ganges and Brahmaputra (IGB) river basins of South Asia. It explores irrigation water usage during climate-sensitive crop growth phases (i.e., vegetative and reproductive which required ~ 60% of the total crop water demand), which is supposed to be crucial for long-term integrated crop water management. A hydrology vegetation model LPJmL is forced with an ensemble of eight downscaled (5 arc-min) GCM’s using a mix of two emission scenarios i.e., RCP4.5-SSP1 and RCP8.5-SSP3. To investigate phase-specific shifts in crop water use during the period 1981-2100, trend analysis is performed. It shows a significant (p<0.001) increase in irrigation water demand during the vegetative phase of wheat (6 mm) and reproductive phase of rice (26 mm) and a decrease during the reproductive phase of wheat (13 mm) and vegetative phase of rice (11 mm) in selected study sites. The large decrease in projected irrigation demand for wheat can be explained by a shortening of growing season length as a result of rising temperatures and increased precipitation. Whereas, an increase in irrigation demand for rice is a combined effect of higher temperatures and less precipitation during the reproductive phase in the region. At the same time, irrigation supply by surface water and groundwater is likely to change in future due to warmer and drier growing periods, causing a significant increase in groundwater irrigation, mainly for rice. Our major research findings show the importance of crop water assessments during the sensitive crop growth phases of wheat and rice which vary in space and time. Including crop phase-specific, climate impact assessments of regional and global projection will help improve the region's existing crop-water management strategies and adaptation practices.