Modelling the physical characteristics of simulated rainfall: A comparison with natural rainfall

The drop size distributions and the drop velocity function that characterize simulated rainfall at three intensities are modelled in terms of expressions developed for natural rainfall. As a result, the continuous relationship between kinetic energy per unit of mass (or time) and intensity is defined for the simulated case, and is compared with that for natural rainfall at similar intensities. The similarity characterizing drop size distributions of natural rainfall is found also in simulated rainfall. However, the drop fragmentation process in the simulator differs from the process that occurs in natural rainfall, and the dimensionless expression of the resulting drop size distributions differs from the theoretical function proposed by Assouline and Mualem (1989, Trans. ASAE, 32(4): 1216-1222). For a falling distance of 5.5 m, the velocity at the soil surface of the simulated drops is similar to the terminal velocity of natural raindrops of equivalent diameters. The simulated rainfall kinetic energy per unit of mass is close to the natural rainfall value at similar intensity, and the maximal difference is about 10%. However, the differences in the respective drop size distributions induce different trends in the kinetic energy-intensity relationship. The simulated rainfall kinetic energy per unit of time is not a linear function of the intensity. Therefore, cumulative rainfall is not equivalent to cumulative kinetic energy, and cannot be used as the independent variable in models dealing with processes affected by the rainfall energy without error. (C) 1997 Elsevier Science B.V