Transformation of the kinetic energy of rainfall with variable tree canopies

Abstract

This thesis defines a physically based model describing the kinetic energy of throughfall from any vegetation canopy. Empirical measurements of the drop-size distribution of rainfall and sub-canopy throughfall were used to develop the model which was tested in the context of splash erosion. Comparisons are made for individual storms between rain falling in the open and through a canopy. Three canopies were used, one oak and two tropical rain forest differing in height. Through each storm raindrop sizes were frequently measured using the paper-staining technique. Kinetic energy/mm/m2 was calculated from the drop sizes, their velocities and amount of rain or throughfall. The velocities were assumed to depend on the height of fall. In the rain forest sites splash cups surrounded by uniform areas of sand were used to measure the material splashed. The oak canopy data was used to examine the validity of a working hypothesis relating qualitatively the size of throughfall drops to the saturation of the canopy. It was confirmed that the canopy changed the drop-size distribution of the rain and consequently changed the kinetic energy/mm/m2. The sequences of drop-size distribution change proposed by the hypothesis were related to the cumulated canopy storage. The tropical rain forest results confirmed these findings and extended them. Although rainfall kinetic energy/mm/m2 may be predicted from rainfall intensity, throughfall kinetic energy/mm/m2 was independent of intensity and the frequency distribution of the energy of throughfall samples was bimodal, with a high energy group which was commonly higher than that of the rainfall. The probability of a thoroughfall sample being in either energy group depended on the cumulated canopy storage or the percentage storm duration elapsed. The relative magnitude of rainfall and throughfall total kinetic energy depended on the saturation of the canopy and on the canopy height and for some storms the throughfall energy was higher than the rainfall. Soil splash increased with increasing kinetic energy. The model predicting throughfall energy requires inputs of canopy height, rainfall intensity and the frequency distribution of energy of discrete samples of throughfall. The model is most sensitive to canopy height.