Comparison of Vapor-Pressure-Deficit Calculation Methods—Southern High Plains

Abstract

Vapor-pressure deficit ( VPD ) affects evapotranspiration, water-use efficiency, and radiation-use efficiency of crops. VPD calculation methods were evaluated for a semiarid environment in the Southern Great Plains. Air temperature and relative humidity were measured near Bushland, Texas, during 1992 and 1993. Temperature and relative humidity were measured at 0.17 Hz (6 s), averages were recorded for each 15-min period, and daily (24-hr) maximums, minimums, and averages were recorded. VPD , actual vapor pressure, and dew-point temperatures were computed and averaged for each 15-min period and day. Methods that used mean daily dew-point temperature to compute daily actual vapor pressure performed well, and methods that used hybrid calculations based on maximum and minimum air temperature and relative humidity performed the worst. Methods using one-time-of-day dew-point temperatures as recommended by the 1990 ASCE Manual No. 70 should be used with caution in this environment. Weather data sets containing maximum and minimum temperatures and daily mean dew-point temperature should provide the most accurate calculations of VPD in this environment.