TECHNIQUE FOR THE DIRECT MEASUREMENT OF WATER STORAGE ON A FOREST CANOPY

The amount of water stored on a forest canopy is an important and useful hydrometeorological variable. As a result of the developments described in this paper canopy storage may now be regarded as a continuously measurable quantity. A novel method is presented for the continuous measurement of canopy storage, using the cantilever deflection of a water-laden branch. A description of the necessary instrumentation is presented and the validity demonstrated. Calibration methods and results are discussed, and the extrapolation from individual branches to a complete canopy is considered. A description is given of a continuous two-year experiment undertaken in the Hafren Forest, Wales, and a sample of the recorded data is shown and analysed for canopy water storage. In particular, a canopy storage capacity of 2.5 ± 0.4 mm is found for a closed Sitka spruce canopy. A preliminary analysis of dynamic events has allowed a test of some of the hypotheses made in the Rutter model. In particular the exponential form of evaporation from a completely wet but unsaturated canopy has been verified, and aerodynamic resistance values are calculated directly. © 1979.