Estimating Raindrop Kinetic Energy: Evaluation of a Low-Cost Method

Abstract. The Loess Plateau of China is regarded as the most intensively eroded region in the world and soil erosion caused by raindrop impact is a common occurrence on agricultural land within this region. Therefore, understanding the influence of rainfall energy on the soil surface is needed to improve prescriptions for best management practices aimed at mitigating erosion. Disdrometers for measuring rainfall energy are presently available; however, these are relatively expensive and their use may not be justified for determining raindrop energy for predictive soil erosion models in regions where there are limited economic resources. To overcome this constraint, a device was tested for evaluating size and velocity of water drops during rainfall events. This device utilized two rotating disks combined with filter paper to obtain raindrop diameter and velocity which can then be used for determining the kinetic energy of falling raindrops. With this device, raindrop diameter was determined from the resultant raindrop stain left on the filter paper during rainfall events and velocity was calculated from the time it took a falling raindrop to travel between the pair of rotating disks. Measurements were taken for approximately 10 minutes during each of six rainfall events of different intensities over a three month period (from June to August of 2013). The smallest raindrop measured was 0.39 mm diameter and the largest was 5.92 mm diameter. The event average raindrop diameter increased with increasing event rainfall intensity. The minimum raindrop impact velocity was 1.47 m s-1, the maximum was 9.45 m s-1, and the event average terminal velocity increased as event rainfall intensity increased. Estimated raindrop kinetic energy ranged from 0.04 × 10-6 J to 4728.21 × 10-6 J, with event mean raindrop kinetic energy ranging from 40.33 x 10-6 J to 276.94 × 10-6 J. The relationship between estimated event rainfall kinetic energy and event rainfall intensity was represented by an exponential function. The disk device was also compared to an optical disdrometer. The data collected for rainfall intensity, raindrop diameter, and velocity were statistically similar between the two devices. Results from this study show that this low-cost method can be used to estimate rainfall kinetic energy in the Loess Plateau region of Northwest China. Keywords: Loess Plateau, Raindrop diameter, Raindrop velocity, Rainfall intensity.