Fractional coefficient for estimating physiologically meaningful and diurnal transpiration cycle weighted VPD from daily ambient environmental data

Abstract

Appropriate representation of environmental conditions via meaningful metrics is critical for their effectiveness in modeling crop physiological and resource use processes. Vapor pressure deficit (VPD) is a primary determinant of crop water use efficiency, commonly reported as daily average. However, VPD can vary substantially during the day in relation to diurnal cycle of transpiration, in which case daily average VPD may not suffice. In such scenarios, integrating VPD over the course of diurnal transpiration cycle into a weighted VPD (VPDw) can be physiologically meaningful as a metric. The goal of this research is to characterize the fractional coefficient (Frac) that can be used to effectively calculate VPDw from daily temperature and relative humidity data without the need for sub-daily environmental data. Site-specific and seasonal estimates of Frac, which represent the relative positioning of VPDw to diurnal extremes of saturation vapor pressure are currently lacking for semi-arid regions. To address this, we quantified Frac at 101 sites in the U.S. Central Plains using long-term (1982–2019) hourly and daily radiation and VPD datasets. Frac varied substantially across the region, with growing season mean values ranging from 0.59 to 0.92. Summer months had the highest Frac values with relatively lower uncertainty as compared to lower Frac during winter. Local observations of Frac deviated significantly from previously assumed stationary estimates implying that site-specific and month-specific Frac may be necessary for robust water use estimations. VPDw measured at an experimental site in Central U.S. Plains explained greater variance in daytime crop transpiration than daily mean VPD in maize, soybean, and sorghum canopies. Site-specific Frac was also assessed to perform better than commonly used Frac estimate of 0.75 to explain day-to-day variation in transpiration, demonstrating effective use of long-term mean Frac values presented here to estimate VPDw using daily datasets.