Estimating Air Vapor Pressure in a Semiarid Region Using FAO-56 Methodology

Abstract

This paper presents three models for the estimation of vapor pressure from temperature and humidity data as suggested by Food and Agriculture Organization (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) that have been compared for their performance using daily data from 2002 through 2007 for a semiarid climatic region of Udaipur in Rajasthan, India. Vapor pressure data obtained from dew-point temperature has been used as actual vapor pressure. Model 1 performed better than the remaining two models. Lower values of vapor pressure are better modeled by the three models than higher values. The average absolute error of Model 1 varied from 5.09 to 15.59%, of Model 2 from 5.59 to 19.73%, and that of Model 3 from 5.62 to 37.21%. Estimates of vapor pressure obtained from the three models were also used to estimate open-water evaporation determined by the modified Penman method; the results were compared. Errors in vapor pressure data did not significantly affect estimates of evaporation, indicating that other factors such as energy budget played a much more significant role than mass transfer influence for this arid basin. Errors in evaporation estimation were