An improved method to estimate actual vapor pressure without relative humidity data

Abstract

Estimating actual vapor pressure (ea) without relative humidity (RH) data continues to draw research attention. One of the accurate ways to estimate ea is through estimation of dew point temperature (Tdew) from minimum (Tmin) or mean temperature (Tmean). Two existing methods have been largely used to estimate ea. The first method (method I) assumes that Tdew is close to Tmin. The other one (method II) adjusts Tdew from Tmin with piecewise correction factors (aT) from sub-humid to hyper arid regions, and from Tmean with a fixed correction factor, aD, in humid regions. Here, two methods are proposed to estimate ea. The first method (method III) adjusts Tdew from Tmin with dynamic aT based on the correlation function between aT and aridity index (AI) regardless of climate zones. The second method (method IV) adjusts Tdew from Tmin with dynamic aT when AI < 1.00 and from Tmean with aD when AI ≥ 1.00. The performance of four methods was evaluated based on data from 886 meteorological stations distributed from hyper-arid to humid regions. Results showed that there was a significant logarithmic correlation function between aT and AI, but no significant correlation between aD and AI. Daily values of ea estimated by method I were greatly overestimated in semi-arid to hyper-arid regions, but were reasonably estimated in humid regions. The accuracy of method II was improved in hyper-arid to dry sub-humid regions but decreased in humid regions, compared to method I. The proposed methods (III and IV) further improved the accuracy and produced reasonable estimation of daily ea in hyper-arid to humid regions, and method III produced a slightly better performance than method IV. Similar results were also observed for estimation of monthly ea. Therefore, the proposed method III is highly recommended to estimate daily and monthly ea when RH data are unavailable.