DIRECT MEASUREMENT OF EVAPORATION OF INTERCEPTED WATER FROM FOREST CANOPIES

(1) A new method has been used to measure directly evaporation of intercepted water from forest canopies. Cut trees were suspended from a load cell in stands of widely spaced trees, sprayed with simulated rain and weighed during evaporation of the intercepted water. (2) Values of boundary layer conductance were calculated directly. It was found that the boundary layer conductance per tree increased with spacing from 0.82 m3 s-1 in the 2-m spacing treatment to 5.92 m3 s-1 in the 8-m spacing treatment, whereas boundary layer conductance per unit ground area declined from 0.24m s-1 to 0.09m s-1 as the density of the trees decreased. (3) Values of canopy storage capacity were also calculated directly and found to be a property of individual trees and unaffected by tree density. (4) The assumption incorporated in the Rutter model of interception loss that the evaporation from the wet canopy is given by the Penman equation was found to be correct. The second assumption in the Rutter model that the actual evaporation from the partially wet canopy is adequately given by the Penman equation multiplied by C/S (where C is the amount of water retained on the canopy and S is the canopy storage capacity) was also verified.