Shifts in Irrigation Water Demand and Supply Pattern During Sensitive Crop Growth Phases in South Asia

This study investigated climate change-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. The study explores the usage of irrigation water during climate-sensitive crop growth phases, supposed to be crucial for long-term integrated crop water management. A hydrology vegetation model LPJmL is forced with reference data for the period (1981-2010) and for scenario period (2011-2100) with an ensemble of eight downscaled (5 arc-min) GCM’s using two climate projections (RCP4.5 and RCP8.5) combined with two socio-economic pathways (SSP1 and SSP3). 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 of wheat crop can be explained by a shortening of the growing season length as a result of rising temperatures and increased precipitation. Whereas, an increase in irrigation demand for rice is the 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 as a result of a warmer and drier growing period, causing a significant increase in demand for groundwater, mainly for the irrigation of rice. Our results show the importance of crop water assessments during the sensitive crop growth phases of wheat and rice which varies in space and time. Including crop growth phases specifically in climate impact assessments of regional and global projection will help to improve the existing water management strategies and adaptation practices in the region