© 1996 Heron Publishing—Victoria, Canada
Rainfall interception and drop size—development and calibration of the two-layer stochastic interception model
Ian R. Calder
Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, U.K. / Received March 2, 1995
Summary
This paper reviews the development of the stochastic interception model from the original, single-layer, drop-size-dependent
model to the two-layer model that recognizes that vegetation canopies are wetted through both the primary impact of raindrops
to the top layer of the canopy and secondary impacts from drops falling from the vegetation to lower layers of the canopy.
It is shown that drop volumes of primary raindrops can be calculated from the Marshall-Palmer distribution and drop volumes
of secondary drops can be estimated from disdrometer measurements of the characteristic volume appropriate to the particular
vegetation species. It is recognized that, in addition to the volume-dependent stochastic wetting effect, there is also another
drop-size-dependent wetting effect that is related to the kinetic energy of the raindrops, which reduces the maximum storage
that can be achieved on the canopy.
The predicted wetting functions for canopies of different density are described and compared with observations made with the
use of a rainfall simulator. It is also shown that the species-dependent model parameters can be determined from measurements
made with the rainfall simulator. The improved performance of the model compared with conventional interception models is
demonstrated for a tropical forest in Sri Lanka. Application of the two-layer model may explain why interception losses from
coniferous, fine-leaved forests in the temperate, low-intensity rainfall climate of the uplands of the U.K. are among the
highest in the world, whereas interception losses from tropical broad leaved forest in high-intensity rainfall climates of
the tropics are among the lowest.
Keywords:
canopy wetting, drop volume, kinetic energy, Marshall-Palmer distribution, rainfall simulator.
