Can rainfall adequately replenish soil moisture to realize the cooling potential of urban green spaces in the rain-abundant city?

In rain-abundant cities, can natural rainfall sustain soil moisture in urban green spaces (UGSs) to ensure continuous microclimate regulation? This study, based on the field observation of a small UGS in Guangzhou and numerical microclimate simulations, explores the effects and spatiotemporal limitations of rainfall on soil moisture availability and quantitatively assesses the impact of irrigation on enhancing microclimate regulation. The results show that while rainfall events significantly increase the soil water content (SWC) in UGSs, the soil moisture replenished by rainfall is gradually consumed through vegetation transpiration. Excessively long rainfall intervals may even lead to a decrease in soil moisture to the extent that vegetation transpiration is limited by soil moisture stress. Within 0.9 to 2.5 days following rainfall events before heatwaves, SWC drops to levels that impose soil moisture stress on vegetation transpiration. Numerical simulations reveal a significant nonlinear relationship between irrigation effectiveness and background climatic conditions. For instance, when the average incoming shortwave radiation during rainfall intervals falls below 400 W·m⁻², irrigation is not required in UGSs to enhance their cooling capacity. Furthermore, irrigation should be initiated when soil moisture drops to levels where limitations on vegetative transpiration would compromise the cooling potential of the UGS. Therefore, the time to start irrigation is influenced by cumulative background meteorological conditions. For UGSs to achieve an additional 0.2 °C cooling effect through irrigation, the cumulative incoming shortwave radiation needs to reach 14.37 MJ·m⁻². This study advances the understanding of the spatiotemporal boundaries of rainfall in the microclimate regulation of UGSs, providing scientific guidance for developing integrated water management strategies in UGSs.