Abstract:

The influence of canopy on the rainfall size distribution and on the raindrop kinetic energy was researched quantitatively and on the basis of effect mechanism for the tree species of Mao-bamboo (Phyllostachys pubescens),Chinese fir (Cunninghamia lanceolata),Black locust (Robinia pseudoacacia).Such research is the basis for further researches on the benefits of forest in terms of hydrology and soil protection.The study showed,that the raindrop size (D,raindrop diameter,mm) distribution is basically not influenced by the type and intensity of rainfall in open field,i.e.it is fairly stable.When the raindrop size distribution of canopy rainfall is expressed with the equation of F=1-exp(-(10D/α)n),the value of parameter α is 2.7718、2.5939、2.5112 and parameter n is 1.7593、1.9175、2.6126 for Mao-bamboo,Chinese fir,Black locust respectively.The raindrop kinetic energy per unit canopy rainfall is also basically not influenced by the type and intensity of rainfall outside of forest.When the falling height of raindrop (H,m) is big enough so that all raindrops can fall with their terminal velocity,the potential kinetic energy of raindrop per unit canopy rainfall is 24.163J·m^-2mm^-1,23.214J·m^-2mm^-1,23.713J·m^-2mm^-1for Mao-bamboo,Chinese fir,Black locust respectively.The difference among the 3 tree species is small.In this study the models to calculate the raindrop kinetic energy per unit canopy rainfall according to the falling height were established,they are e_c=24.129[1-exp(0.0023H²-0.3259H)],e_c=23.214[1-exp(0.0025H²-0.3409H)],e_c=23.713[1-exp(0.0015H²-0.3467H)] for Mao-bamboo,Chinese fir,Black locust respectively.The raindrop kinetic energy per unit canopy rainfall decreases with falling height markedly in the range of 0～5m;relative markedly in the range of 5～10m,and no more obviously in the range of above 10m.The relations between raindrop kinetic energy per unit rainfall in open field and the rainfall intensity for different rainfall types were researched.The method to evaluate the influence of forest canopy on the rainfall kinetic energy and the possible approach to do such evaluation in a regional scale were also discussed.

Key words: Forest hydrology, Rainfall kinetic energy, Canopy interception, Mao-bamboo, Chinese fir, Black locust