The Impact of Agricultural Irrigation on the South Asian Monsoon Variability and Moisture Transport

Abstract:

Agricultural intensification in South Asia has resulted in the expansion and intensification of surface irrigation over the 20^th century. The most intensively irrigated areas are found in the northern Indian sub-continent, a region important to the onset and development of the South Asian Summer Monsoon (SASM). The resulting changes to the surface energy balance could affect the temperature contrasts between the land surface and the equatorial Indian Ocean, potentially altering the SASM circulation. Prior studies note apparent declines in the SASM intensity over the 20^th century and focus on how altered surface energy balances impact the rainfall distribution. Here, we use the coupled GISS ModelE-R general circulation model to investigate the impact of intensifying irrigation on the SASM circulation over the 20^th century, and the effect of irrigation compared to the impact of increasing greenhouse gas (GHG) forcing. We force our simulations with time-varying, historical estimates of irrigation, both alone and with 20^th century GHGs forcings. In the irrigation-only experiment, irrigation rates correlate strongly with lower and upper level temperature contrasts between the Indian sub-continent and the Indian Ocean (Pearson’s r=-0.66 and r=-0.46, respectively), important quantities indicating the SASM circulation strength. With GHG forcing, these correlations strengthen: r=-0.72 and r=-0.47 for lower and upper level temperature contrasts, respectively. Under irrigated-only conditions, the mean SASM intensity in the model decreases insignificantly. However, with irrigation and GHG forcing, inter-annual variability of the SASM circulation decreases by ~40%, consistent with trends in reanalysis products. These findings suggest that intensifying irrigation, in concert with increased GHG forcing, can reduce the variability of the simulated SASM circulation and alter the regional moisture transport by limiting the surface warming and reducing land-sea temperature gradients. Currently, irrigation is not generally included as a South Asian, or global, forcing in most climate model simulations or climate change analyses. However, including irrigation may be necessary to accurately simulate the historical trends and interannual variability of the SASM over the last 50 years.