On Penman's equation for estimating regional evaporation

Abstract

Abstract The form of Penman's evaporation equation most extensively applied in hydrometeorology is discussed. It is shown that this equation underemphasizes the importance of ventilation relative to radiation in maintaining regional evaporation. A generalized ventilation term is proposed and a modified equation for evaporation derived. This new equation is calibrated to give the same annual total for short vegetation as Penman's original version. It predicts both an enhancement of winter evaporation at the expense of summer evaporation and a significant increase in annual total evaporation with vegetation size, especially in regions of frequent substantial rainfall.