Irrigated cropland expansion exacerbates the urban moist heat stress in northern India

Abstract

Agricultural irrigation has significantly reshaped the land surface energy and water balance. Previous studies have well investigated its cooling effect on air temperature (T air). However, its effect on increasing air humidity which can intensify the humid heat was often overlooked, particularly over urban areas with high population density, high T air, and consequently greater exposure to moist heat stress. In this study, using state-of-the-art reanalysis data at a high spatial resolution (∼9 km), we evaluated how changes in area equipped for irrigation (AEI) around a city affect urban moist heat stress (UMHS) in more than 1000 cities in China and India. In addition to T air, wet-bulb temperature (T WB) and wet-bulb globe temperature (T WBG), which consider humidity and are closer to the perceived temperature, were assessed. We found that although AEI expansion lowers urban T air, irrigation increases T WB and T WBG due to increased air humidity, thereby exacerbating the UMHS. This ‘warming’ effect of irrigation is more evident in northern India where AEI has expanded significantly in recent decades, and is prominent in the pre-monsoon and post-monsoon seasons, when precipitation and air humidity are low. However, this effect is not evident in China due to the lower intensity of AEI expansion and differing climatic conditions. Overall, this study highlights the side-effect of irrigation on regional climate, providing crucial information for better understanding urban heat stress and for future city planning.