Evaporation of well-watered surfaces: the Dalton-Schmidt paradox

Henk de Bruin, Wageningen University (emeritus)

For many practical applications such optimal water management of irrigated regions or water and energy balance studies, estimates are needed of evaporation of well-watered surfaces.; In 1805 Dalton published a formula for evaporation of open water, which suggests that wind speed and "atmospheric demand" are the physical driving forces of the evaporation process. However, 100 years ago, Schmidt showed, using thermodynamic arguments that the evaporation of the oceans is determined primarily by the available energy. This can be mentioned the Dalton - Schmidt paradox. Recent findings confirm Schmidt's thermodynamic approach, but Dalton's formula does yield good results also. The solution of the paradox is that surface temperature, appearing in Dalton's formula, is a dependent variable that will adapt to the available energy at given wind speed and air humidity of the air. Experimental evidence will be presented for three cases:

a) well-watered grass in the Netherlands without local advection;

b) irrigated grass in Spain under conditions with local advection;

c) a well-mixed 15 meter deep water reservoir in the Netherlands.