Studies of the natural evaporation and energy balance

Abstract

The natural evaporation from agricultural land has been evaluated on an hourly basis continuously for 29 months. The site is situated near Kristianstad in southern Sweden (56° N). The method employed is a “bulk aerodynamical method” with stability dependent coefficient. The key parameters are: wind speed at 12 m above ground, temperature and humidity at 1.5 and 12 m. The measurements are shown to be representative of areas extending about 1 kilometre upwind. The accuracy of individual hourly values is not very high according to inadequacy of the humidity profile equipment, but the values are progressively more significant as averaging time increases.

Examples of series of hourly, daily and monthly values are presented and discussed in terms of the influence of different weather elements.

Monthly values of the “potential evapotranspiration” has been derived with the Thorntwaite and Penman formulae. It is found that the actual evaporation during spring and early summer in this area is in the mean only half that obtained with the Penman formula but almost equal to the Thorntwaite values. During late summer and autumn the Penman values are about 1.3 times the actual, but the Thorntwaite values are about double the actual. During the late autumn and winter period the actual evaporation is considerably greater than the values obtained with the formulae.

A rough seasonal energy balance of the surface is obtained. It is found that in this area nearly equal amounts of the net radiation goes to evaporation and to heating of the atmosphere during spring and early summer. During late summer progressively more and more of the net radiation energy goes to evaporation. During the winter large amounts of heat is supplied by the atmosphere to account for local radiational loss and also for evaporation.