|Howell T A,|
|Dusek D A,|
Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 16, 1994
Publication Date: N/A
Interpretive Summary: Vapor pressure deficit (VPD) is a parameter computed using air temperature (AT) and relative humidity (RH). When RH is high, VPD is small; and when RH is low, VPD is large. The methods used to calculate VPD and the types of data used as input are varied. VPD must be computed for most crop water use models and many crop simulation modes. The VPD method that used minimum and maximum air temperature and mean daily dew point temperature (DP) was the most accurate in this semi-arid climate. However, DP requires hourly values for RH and AT determined from frequent sampling (about once a minute).
Technical Abstract: Vapor pressure deficit (VPD) is a required computed input for combination evapotranspiration (ET) equations like the Penman or Penman-Monteith models, and VPD affects water-use and radiation-use efficiency of crops. This study was conducted to determine which VPD calculation methods performed the best for the semi-arid environment in the Southern Great Plains. Air temperature and relative humidity were measured with an electronic sensor in a standard weather station shelter in an irrigated weather station site near Bushland, Texas, during 1992 and 1993. Temperature and relative humidity were measured at 6 s intervals, and averages were recorded for each 15-min period, for each day, and daily maximums and minimums were also recorded. VPD, actual vapor pressure, and dew point temperatures were computed for each 15-min period and averaged for each day. VPD methods that used mean daily dew point temperature for daily actual vapor pressure performed well. VPD methods that used hybrid calculations based on maximum and minimum air temperature and relative humidity performed the worst. VPD methods using one-time-of-day dew point temperatures as recommended by ASCE Manual No. 70 should be used with caution in an environment like the Southern High Plains. Weather data sets containing maximum and minimum air temperature and daily mean dew point temperature should represent the most accurate calculations of VPD in this type of environment.