Potential evapotranspiration in central Iraq using the Penman method with modified wind function

Abstract

Radiation was correlated with hours of bright sunshine for Central Iraq. The resulting regression equation, which was in good agreement with other regressions derived for similar climatic regions, was used in Penman's combination equation. It was found that wind velocity and the aerodynamic component of pan evaporation were better correlated than wind velocity and pan evaporation divided by the vapour pressure deficit. The resulting wind functions for the standard U.S.W.B.-A pan and the sunken Colorado pan were incorporated in modified Penman equations for use in Central Iraq. Potential evapotranspiration calculated according to the modified equations was compared to potential evapotranspiration calculated from the standard Penman equation and from equations with modified wind functions reported in the literature, as well as with experimental data on water use by crops in Iraq. The equation modified for use in Iraq by means of the wind function for the A-pan resulted in a maximum value of the potential evapotranspiration of about 15 mm/day in July and August, which is a reasonable value for tall crops. The relative importance of the aerodynamic term in the combination equation is stressed for arid conditions where advection may be important and the vapour pressure deficit is great.