Equilibrium Evapotranspiration as Influenced by the Correlation of Temperature with its Vertical Gradient

Abstract

The correlation among meteorological parameters makes the equilibrium evapotranspiration estimated from long-term averages of the parameters involved ambiguous to a certain extent. An equation is derived for incorporating the correlation effect in equilibrium evapotranspiration. In the equation the rate of change of saturation vapour pressure with temperature s appears as that multiplied by a factor, which is unity only for a null correlation between vertical temperature gradient and s.The correlation coefficient calculated by use of the data of temperature, net radiation and wind speed above a paddy field shows a null correlation for the period of ten days and a significant negative correlation for the periods of 100 and more days. The ratio of the mean equilibrium evapotranspiration from its daily values to that calculated from the mean values of relevant meteorological parameters is, for periods of 10 to 200 days, in good agreement with the prediction through correlation of pertinent parameters. The correlation effect on equilibrium evapotranspiration is found to be about 5% for the period of 100 days.