A Protocol for Conducting Rainfall Simulation to Study Soil Runoff

The overall goal of this procedure is to simulate rainfall, having a standard drop size, intensity, and uniformity across the target area to study soil runoff. This is accomplished by first adjusting rainfall simulator controls to achieve the approximate pressure and flow rate for the selected nozzle. The second step is to calibrate the rainfall simulator to achieve the exact flow rate for the selected nozzle and a uniform distribution of raindrops across the target area.

Next, properly packed soil boxes are positioned in the target area on a platform adjusted to a uniform slope. The final step is to conduct a rainfall simulation and collect soil runoff for analysis. Ultimately, rainfall simulation is used to study the effects of soil properties, soil amendments, antecedent, soil moisture, topography, and rainfall intensity on soil runoff under standard conditions that approximate natural rainfall.

This video depicts a rain simulation protocol that's been used around the world to consistently evaluate runoff from soils. So demonstration of this method is important because rainfall simulator operation and calibration involve many steps. Also, there are many variables which can influence the results.

Peter will be demonstrating the procedure for packing the soil boxes. Peter's a student at the University of Maryland Eastern Shore, and he's worked in my lab for the last four years. Begin the protocol by procuring boxes of identical dimensions.

These boxes are 100 centimeters long, 20 centimeters wide, and 7.5 centimeters deep, and have nine five millimeter drain holes. They also have a five centimeter lip and a collection gutter at one end. For each box, line the bottom with four ply cheesecloth to retain the soil and allow water flow.

Once a box is lined, weigh the box and cheesecloth and record the measurement for later use. Next, obtain prepared soil to fill the boxes. Work with the first box and scoop enough soil to half fill the box.

When smoothed out about 3.5 centimeters, spread the soil evenly and pack it with a flat brick. The soil should not compact under the pressure of the brick. Next, add another two centimeters of soil.

Then level it out with a leveling gauge to a packed depth of five centimeters. The height of the box lip. Weigh the packed soil box to determine the amount of soil that was added to the box.

You use the same weight of soil to fill the remaining boxes. Pack each box to a depth of five centimeters and a uniform density. Vacuum the gutter to remove any soil that spilled into the gutter.

During the packing process, the rainfall simulator consists of a frame to support several soil boxes under a nozzle. Begin operating it by closing the single lever ball valve before the main water supply to the simulator is pressurized. Turn the set screw on top of the pressure regulator valve counterclockwise to reduce the pressure.

Then open the next in line flow control valve, completely return to the single lever ball valve and open it completely. Now, adjust the pressure regulator valve by turning it clockwise to about eight PSI. Check the pressure at the gauge near the top at the rainfall simulator.

Next, partially close the inline flow control valve while monitoring the flow meter and pressure gauge. Stop when the flow meter reads the approximate flow rate of the nozzle in use here, 1.5 gallons per minute, and the pressure gauge reads the approximate PSI for the nozzle six PSI. In this case, close the single lever ball valve to stop flow without changing the flow rate and pressure settings.

Obtain six empty soil boxes for use in determining rainfall uniformity. Prepare them by covering the drain holes with duct tape to prevent water from leaking out. Place the empty boxes on a level frame so they are evenly spaced and none is directly under the nozzle.

Mark the positions of the boxes and always use the same positions. Make use of a 10 foot long, two inch diameter PVC pipe to divert flow from the nozzle. The pipe should have a 45 degree elbow attached to its end.

Also position a large graduated cylinder to catch flow from the pipe. To calibrate the nozzle, position the pipe over the nozzle and hold it there. Open the single lever ball valve and collect the discharge from the pipe in the cylinder for 10 seconds.

When done, keep the pipe in place and determine the water volume in the cylinder, which should match the expected value for the nozzle. With the nozzle calibrated. Remove the PVC pipe to allow rainfall to wet the box area.

Let the water fall for 10 minutes. After exactly 10 minutes abruptly stop the rainfall by positioning the 10 foot PVC pipe over the nozzle to the flow. Then close the single lever ball valve to measure the volume of water collected in each box.

Pour it into a graduated cylinder. Use this data to determine rainfall uniformity across the boxes. If the coefficient of variation is greater than 0.05, turn the nozzle one quarter turn and repeat the calibration process.

Before placing packed soil boxes in the rainfall simulator, prepare to incline the frame to the desired slope. First, position the frame at a height to allow placement of collection bottles and if necessary, funnels beneath the box gutters. Next place aboard at least a meter in length along the length of a soil box mounted on the frame.

Place a carpenter's level on the board for reference. During the process, start elevating the back of the frame using bricks and shims to achieve a 3%slope. Stop when the front of the box is three centimeters below the level board.

Also, check that the front and back of the frame are level from side to side. Now place six packed soil boxes in the previously marked positions on the inclined frame for the simulation position runoff collection bottles below the drain spouts. In addition, use paperclips to attach shields over the gutters to prevent rain from directly entering the gutter or the collection bottle.

Place the 10 foot PVC pipe over the nozzle and start water flow. Collect the discharge for 10 seconds and recalibrate the flow rate as before. When done, remove the pipe from over the nozzle to start rainfall simulation.

Monitor the water draining from the drain spout of each box. Note when the draining water turns from a slow drip to a continuous stream. Record this as the runoff initiation time to collect runoff samples.

Assign one assistant to each box. Switch the collection bottles at prescribed times at runoff initiation. Terminate a rainfall event by positioning the 10 foot PVC pipe over the nozzle and closing the ball valve.

This plot of runoff in liters versus soil initial moisture shows the wetter soils had less capacity to store water, along with lower infiltration rates resulting in greater runoff volumes. The soils were prepared with varying soil moistures using the manuscript protocol. These were subjected to simulated rainfall with an intensity of 3.2 centimeters per hour For 40 minutes.

Here, the time to run off initiation on the vertical axis is seen to be negatively correlated with the initial soil moisture water infiltrated into drier soils longer before wetting the soil surface and causing runoff. The data show a positive correlation between urea and concentration and antecedent soil moisture Content. Drier soils allow infiltration that leches urea n into the soil and away from the soil surface.

When runoff does occur less urea N is available at the surface for movement. In runoff. This plot shows cumulative urea n loads as a function of time.

Each curve represents one replicate of a soil box with one of the antecedent moisture conditions. The plot once again demonstrates that drier soils have a longer time to run off initiation and lower cumulative loads. So after watching this video, you should have a good understanding of the operation and calibration of a rainfall simulator and know how to control for variables such as an acet and moisture content that can affect the results.The.