Portable Rainfall Simulator and Plot Unit for Use in Field Studies of Infiltration, Runoff and Erosion

Abstract

The simulation of rainfall for studies of infiltration, surface runoff and erosion is not new. Early methods of applying rainfall used hand-operated watering cans (e.g. Duley & Hays 1932). In later work rainfall was delivered to the plots through specially designed upturned nozzles; here the spray also wets a variable area surrounding the plots and protective tents are commonly necessary to minimize wind effects. The most familiar are the Types F and FA Infiltrometers (Wilm 1943) and the Rocky Mountains Infiltrometer (Dortignac 1951). These units have proved of value in elucidating the contribution of rainfall, soil, vegetation and management effects to infiltration, runoff and soil loss (Meyer 1960), but there are several disadvantages for convenient and realistic use. The Type F Infiltrometer, for example, is bulky, due largely to the large water requirements and the need for protective tents when up-turned nozzles are used. On the other hand, the Type FA and Rocky Mountains Infiltrometers have a plot size of only 2-5 ft2, which restricts the development of surface runoff and soil erosion. Laboratory studies of the infiltration, runoff, and erosion processes generally utilize elaborate mechanical devices which are unsuitable for field use (cf. Turner 1964). The equipment described in this paper was developed progressively in the course of catchment studies in subalpine and alpine (Costin, Wimbush & Kerr 1960), forest (Gilmour 1965), and pasture (Costin & Gilmour, in preparation) areas in south-eastern Australia. Because the terrain is often rough and staff limited, the equipment had to be compact, robust and easily handled by one person. Other requirements were ease of assembly, operation and dismantling, and rapid movement from site to site enabling a large number of replicated plots (up to ten) to be examined before site conditions changed materially during any one day. A relatively large plot size (10 x 6 ft) helped to overcome problems of small scale soil variability and three-dimensional infiltration along the plot boundaries frequently encountered when small diameter infiltrometer rings are used, and to make the runoff and erosion effects more realistic. In erosion studies using simulated rainfall it is often desirable to reproduce the stress conditions most likely to be experienced in the area under investigation (Costin et al. 1960). A summer storm rainfall of about 80 points in 15 min occurs approximately every second year in the study area and was selected as the stress rain to be simulated.