A small portable rainfall simulator for reproducible experiments on soil erosion

► Requirements for small portable rainfall simulations in soil erosion and soil hydrology studies. ► Methodical upgrade of a small portable rainfall simulator based on these requirements. ► Construction and implementation of new water flow control and pump system. ► Testing of full cone nozzles for the use in small portable rainfall simulators. ► Detailed investigation of simulated rainfall characteristics. The importance of distinguishing and discretely studying the subprocesses of runoff generation and erosion has led to the development of rainfall simulations on small plots. We methodically upgraded a small portable rainfall simulator with particular respect to (1) rainfall characteristics that include homogeneous spatial rainfall distribution and drop spectrum, (2) handling, and (3) control of test conditions. We measured simulator characteristics with rain gauges, calibration plate and Laser Precipitation Monitor by Thies (LPM). The upgraded small rainfall simulator, and measurements of the improved rainfall characteristics are presented in this paper. The upgraded configuration shows the desired improvements: regarding drop size distribution, a close relationship to natural rainfall (Marshall & Palmer Distribution) can be observed. Due to low fall heights, measured drop fall velocities are slow; maximum velocities range between 3.4 and 5ms−1. Mean kinetic energy expenditure, mean kinetic energy per unit area and unit depth of rainfall and mean momentum are 214Jm−2h−1, 5.8Jm−2mm−1 and 0.016kgms−1, respectively. The spatial rainfall distribution of the upgraded simulator is homogenous with a Christiansen-Uniformity Coefficient of 91%. The measured variables show extremely low variation throughout all tests and should therefore be reproducible in field investigations at any time.