Thermodynamic controls on vapor pressure deficit during droughts

 

Vapor pressure deficit (VPD) is widely used as a measure of atmospheric dryness and evaporative demand in drought studies, yet its interpretation as an independent drought driver remains unclear because of its close coupling to soil-moisture and radiation. Disentangling the atmospheric forcing from land-surface controls on VPD is essential for correctly diagnosing the drought responses, attributing ecohydrological impacts, and interpreting land–atmosphere feedbacks under water-limited conditions. Here, we present an analytical thermodynamic framework that mechanistically describes VPD as a function of observed radiative and surface-evaporative conditions, requiring no additional parameters. This formulation links VPD to variations in lower-atmospheric heat storage reflected in diurnal air temperature range (DTR) and  saturation vapor pressure. The resulting analytical expression is decomposable and helps to disentangle the atmospheric and land-surface drivers of VPD. When applied over global land, the approach reproduces observed spatial and temporal variability in VPD with R² of 0.9 and 0.8 respectively. It captures observed responses of VPD to solar radiation, clouds, and evapotranspiration across diverse climate and moisture regimes. Our results demonstrate that much of the variability in VPD during dry periods emerges as a thermodynamic response to surface water limitation rather than purely atmospheric forcing. This coupling provides a mechanistic basis for interpreting VPD as both a driver and an indicator of ecohydrological drought responses, with important implications for diagnosing drought stress, understanding land–atmosphere feedbacks, and improving projections of ecosystem vulnerability under climate change.