Seasonal and climatic variation of weighted VPD for transpiration estimation

Vapor pressure deficit (VPD) is one of the critical variables that drives evapotranspiration, and is of fundamental importance in crop physiology and modeling in the face of climate change. Unfortunately, direct records of atmospheric moisture are rarely available at short temporal scales, e.g. hourly, and country or regional scales. Most models use approximations to estimate daily transpiration-weighted VPD. Tanner and Sinclair (1983) suggested an approach to calculate weighted daily VPD as a fraction (0.75) of the difference between daily maximum and minimum vapor pressure based on estimates calculated from daily maximum and minimum temperatures, respectively. A test of the Tanner-Sinclair suggestion is reported by obtaining daily weighted VPD from hourly measurements of humidity and temperature. The objective of this study was to assess the fractional value to obtained daily weighted VPD estimations. This study was based on ten years of hourly weather data collected at thirty five stations across the wide diversity of environments that exist in France.