Opposing evapotranspiration responses to rising vapor pressure deficit in mainland Southeast Asia

Vapor pressure deficit (VPD) is projected to strongly increase over land areas under future global warming due to rising air temperature and declining relative humidity. Understanding the impacts of VPD on the water cycle, particularly evapotranspiration (ET), is therefore critical for delineating better water management strategies. It is commonly assumed that higher VPD enhances the atmospheric demand for water and increases ET – especially in non-water-limited regions. However, this assumption disregards plant physiological and biophysical controls that can override atmospheric demand for water. Elevated VPD can lead to stomatal closure and a decrease in transpiration and thus ET. Moreover, higher VPD always co-occurs with warmer temperatures, which may exacerbate plant water stress or inhibit enzyme activity, further suppressing plant growth and reducing ET. When plant controls dominate over atmospheric demands, ET may decrease with increasing VPD at the annual scale. Here, we tested this hypothesis across mainland Southeast Asia using a mechanistic model (T&C) and remote sensing products. After a model testing with available flux tower data, we run T&C at very high resolution (1km²) over a domain of 2.93 million of km² for a period of 13 years (1998-2010). We found that around 30% of mainland Southeast Asia exhibits decreasing ET with higher VPD. Specifically, when the background VPD (mean annual) exceeds ~1150 Pa, ET starts decreasing and decreases faster with higher VPD. Under future global warming and rising VPD, such ET reductions may lead to diminished land-atmosphere moisture exchange, potentially amplifying local atmospheric dryness. These findings provide a new perspective on the nonlinear responses of ET to VPD and improve our understanding of hydrological response under future climate change over a large and understudied area such as Southeast Asia.