Modulation of compound extremes of low soil moisture and high vapour pressure deficit by irrigation in India

Abstract

Compound extremes of low soil moisture and high vapour pressure deficit (VPD) have implications on terrestrial carbon sequestration, vegetation growth, and net primary productivity. Yet, the role of irrigation on the occurrence of compound extremes remains unexplored in India. Here, we use satellite observations, reanalysis datasets, and high-resolution simulations from the Weather Research and Forecasting (WRF) model to show that irrigation significantly reduces the frequency of compound extremes of low soil moisture and high vapour pressure deficit in India. The Indo-Gangetic plain witnessed a significant (p <0.05) increase in mean annual soil moisture while a substantial reduction in VPD during 1979-2018. Moreover, irrigation caused considerable cooling over the Indo-Gangetic plain during 1982-2018. The increase in soil moisture and the decline in VPD partly contributed to a substantial rise in Normalized Difference Vegetation Index (NDVI) and Net Primary Productivity (NPP). We conducted high-resolution simulations using the WRF model to examine the role of irrigation in modulating the frequency of compound extremes during 1979-2018. The WRF simulations under the irrigation-on show a considerably reduced frequency of the compound extremes compared to the irrigation-off (control) simulations. Irrigation modulates compound extremes by changing the land surface energy budget and planetary boundary layer height. Therefore, there is a need to consider irrigation for reliable projections of compound extremes under a warming climate in intensively irrigated regions.