Aerodynamic and surface factors in evaporation

Abstract

Measurements of Z o and other aerodynamic qualities of three crops in Germany (pine forest, lucerne, potatoes) are combined with British records and with those of Tanner and Pelton, permitting the following generalizations. For pliable agricultural crops the roughness length decreases with increasing wind speed (to ½ for 1 to 3 m sec −1 ), and in this range, Z o ≅ crop height/10. Calculations of eddy conductivity (1/ r a ) give values 3 times that of open water for the farm crops, and 30 times that of open water for the trees. From measured components of the energy balance the surface resistances of the crops are calculated using Monteith's empirical relation. Values of r s for the trees‐between 1.0 and 1.5 sec cm −1 ‐ are always 2 to 4 times as great as for the farm crops, indicating very effective stomatal control of transpiration by pine leaves. Including the two specific crop parameters in Penman's combination formula gives estimates of evaporation for open water, pine forest, potatoes, and lucerne that agree within 5–15% with measurements in southern England and in California.