Description

Our rainfall simulator is designed for use over 50 × 50 cm plots and consists essentially of a drop-forming chamber which is supported over a metal structure a certain height above the soil surface. The drop-former chamber is connected to a water reservoir and the water is supplied by a pump at an adjustable constant rate. The drop-forming mechanism is the heart of any rainfall simulator, as it produces water drops simulating natural rainfall. Overall dimensions are 58 × 58 × 2 cm, and the drop formers cover a 50 × 50 cm² area. The drop formers are made of pieces of plastic capillary tube with internal and external diameters of 1 and 3 mm, respectively, into which a double nylon thread with diameter of 0.4 mm is inserted to increase the hydraulic charge and prevent clogging by air bubbles. The drop-formers are distributed in a hexagonal pattern with a density of 4,900 per square meter fixed with rubber in the holes of the lower part of the chamber. The drop-formation rate and drop diameter are controlled by the length of capillary tubes, the diameter of the capillary tubes and the inside threads, and the water pressure inside the camber. The chamber has a water inlet on the lower side and an outlet of air on the upper side; when the chamber is full of water, the outlet is closed and the pressure into the camber is controlled by the water flow. Silicone sealant is used to waterproof joints.

Structure

The rain chamber rests on top of a telescopic prism structure of galvanised iron. The upper telescoping part can be elevated through the lower part, and the rain chamber can rise up to 1.5 m above the soil surface. The entire structure is held up by a triangular base with three spikes at each corner, which can be hammered into the soil to allow easily levelling in a rugged terrain, adapting to slopes even of 45°.

Wind Protector and Breaking Mesh

The side sides of the structure are enclosed with transparent polyethylene sheets fixed on the frame except for the frontal and the upper part. This protection prevents possible evaporation of drops, prevent drops from being blown off the plot and the distortion of the simulated raindrops by the wind. A small portion of the upper part is open to the air because randomization of the drop pattern depends on air turbulence. Optionally and especially recommended for bare soil, the rainfall simulator may incorporate a 3 mm spacing stainless steel mesh beneath the drop-formers. The mesh serves to break down the drops into a suitable size distribution and prevent them from falling continuously in the same place.

Pumping System

A peristaltic pump controlled by a microprocessor supplies a volume of water that can be adjusted continuously. The electricity is supplied by a portable 45 kW generator driven by a 98 hp gasoline engine.

Water Reservoir

A water reservoir of 25 l capacity or larger is provided to store and supply water for the production of the simulated raindrops. The principle of Mariotte’s bottle is used to maintain a constant head in the reservoir in order to maintain the flow of water constant though the system. The water reservoir can be filled with deionized water or with normal water. A glass pipe together with a flexible polyethylene tube connects the water reservoir to the rain chamber through the peristaltic pump. Where the water enters the chamber a thermometer has been installed in the flexible tube to control the temperature of liquid inside the chamber.

Runoff Collector

Runoff and sediment transported across the lower edge of the experimental plot is collected by a triangular aluminium plate and funnelled into bottles placed at the outlet of the trough. The plate is fixed to the soil with Saran (Trademark of The Dow Chemical Company, http://www.dow.com/saran/resins/index.htm) resin dissolved in ethyl-methyl-ketone. A perimeter edging can be erected in the plot using the same principle with minimum disturbance of the soil to assure that all runoff is channelled to the plate. Saran resin forms a thin, flexible and waterproof film easily brushed in soils.

Characteristic of Simulated Rain

Rainfall Intensities

The rainfall simulator described in this paper allows for a wide range of rainfall intensities. The drop-forming chamber provides a random homogeneous distribution of the simulated rain over the experimental plot. The peristaltic pump permits intensities of simulated rain to be varied from almost zero to more than 120 mm/h. Generally, low intensities are of interest for solute-leaching studies and high ones are frequently used for erosion studies. Intensities of about 20 to 120 mm/h are commonly used for a variety of applications. In the middle of this range, natural rainfall events of about 50 mm/h have return periods of 5–10 years along the Mediterranean basin, and in southern Spain thunderstorms of this intensity are quite common (Elías Castillo and Ruíz Beltrán 1979).

Drop Size

Natural rainfall consists of a wide distribution of drop sizes that may vary from near zero, low-intensity rain, to about 7 mm in diameter for high-intensity rain (Meyer 1979). However, unlike natural rain, the simulated rain has a very limited range of drop sizes, about 3.5–4.0 mm in diameter and they do not vary with intensity. Varying the drop-size distribution is possible by changing capillary tube diameter and/or by using the metal mesh to break down the drops into a suitable size distribution.

Drop Velocity

The simulated drops have lower velocities than the natural drops at similar rain intensities. Drop velocity depends on the fall height of the drops because of the limited structure of the rainfall simulator. The fall height can vary between 1 and 1.75 m by means of the telescopic structure but a fall height of 1.5 m is normally used with a velocity at impact time of near 5 m/s. Drops falling from 1.5 m height cannot reach their terminal velocity, as they would need a fall height of more than 10 m. Moreover, all drops have null initial velocity so that final velocities are the same regardless of the rainfall intensity.