Intensive irrigation in India has been demonstrated to decrease surface temperature, but the influence of irrigation on humidity and extreme moist heat stress is not well understood. Here we analysed a combination of in situ and satellite-based datasets and conducted meteorological model simulations to show that irrigation modulates extreme moist heat. We found that intensive irrigation in the region cools the land surface by 1 °C and the air by 0.5 °C. However, the decreased sensible heat flux due to irrigation reduces the planetary boundary layer height, which increases low-level moist enthalpy. Thus, irrigation increases the specific and relative humidity, which raises the moist heat stress metrics. Intense irrigation over the region results in increased moist heat stress in India, Pakistan, and parts of Afghanistan—affecting about 37–46 million people in South Asia—despite a cooler land surface. We suggest that heat stress projections in India and other regions dominated by semi-arid and monsoon climates that do not include the role of irrigation overestimate the benefits of irrigation on dry heat stress and underestimate the risks.

印度的集约灌溉已被证明可以降低地表温度，但对湿度和极端湿热胁迫的影响尚不十分清楚。在这里，我们分析了原位和基于卫星的数据集的组合，并进行了气象模型模拟，以表明灌溉调节了极端潮湿的热量。我们发现，该区域的密集灌溉使陆地表面的温度降低了1°C，空气冷却了0.5°C。但是，由于灌溉而导致的显热通量的减少会减小行星边界层的高度，从而增加低层湿焓。因此，灌溉增加了比湿度和相对湿度，从而提高了湿热应力指标。该地区的密集灌溉导致印度，巴基斯坦，尽管地表温度较低，但阿富汗和部分阿富汗地区（南亚约有3,746万人受到影响）。我们建议印度和其他以半干旱和季风气候为主的地区的热应力预测不包括灌溉的作用，这高估了灌溉对干热胁迫的好处，而低估了风险。