Investigating soil moisture-heat stress coupling using an idealized model

The lethality of heat stress depends on both temperature and humidity. Wetter soil reduces temperature but increases humidity making the collective impact on heat stress unclear. Several studies using complex models have shown that irrigation can amplify moist heat stress. Our aim here is to improve process-level understanding of soil moisture-heat stress linkages and how these linkages are modulated by external environmental conditions (e.g. temperature and humidity gradients). Here we employ an idealized model by coupling a bulk convective boundary layer model to Penman-Monteith equations with moist heat stress represented by equivalent potential temperature. We find that a wet soil or low canopy resistance amplifies heat stress by suppressing planetary boundary layer (PBL) growth and cutting off dry air entrainment. The overall effect can be decomposed into two components (geometric and flux) and their impacts separately identified using this idealized framework. The geometric term reflects the impact of spreading the same amount of heat flux into a shallower PBL. The flux term reflects changes in dry air entrainment in the absence of PBL height variations. Soil moisture's control on heat stress highly depends on the lapse rate of potential temperature and specific humidity within the air above PBL top, and is strongest under weak stability and large vertical humidity gradient.