A LINEAR VARIABLE INTENSITY RAINFALL SIMULATOR FOR EROSION STUDIES. L. Darrell Norton, Soil Scientist, USDA-ARS National Soil Erosion Research Laboratory, Purdue University, West Lafayette, IN

The need for rainfall simulators became very evident in 1930's when erosion research stations were established as a result of the dustbowl to study soil erosion by water and it did not rain.

Runoff & Erosion Study 1931-33 Kansas Agriculture Experiment Station

Past Rainfall Simulator Technology Water Sprinkler Cans. Stationary Nozzles. Irrigation Sprinklers. Capillary Tube Rainfall Simulators. Rotating Discs. Rotating Booms. Oscillating Nozzles.

The need for a better method to study hydrologic processes within ARS became acutely evident when analyses began of the more that 10,000 man years of natural rainfall plot data used to develop the Universal Soil Loss Equation (USLE) which was first presented by Walter Wischmeier at the World Congress of Soil Science in Madison, Wisconsin in 1960 (Wischmeier, 1960) and still used in revised form to determine highly erodible lands in the USA.

In order to study more “process” based parameters based on the physics of rainfall, a “rainulator” was developed and first documented by Meyer and McCune (1958) using a pressure nozzle system studied by Meyer (Meyer, 1958).

Simulated Rainfall & Nozzles Meyer (1958) evaluated many nozzles and selected the Spraying Systems 80-100 VeeJet nozzle to test more extensively.

Exit Velocity Meyer & McCune determined that the Exit Velocity from the 80-100 nozzle was 8.8 meters/second, with drops smaller than 4 mm, still exceeding or slowing to terminal velocity after a 3.0 meter fall.

Kinetic Energy K.E. = ½ MV2 Meyer determined that the kinetic energy of the raindrops coming out of the 80-100 Veejet nozzle, was approximately 75% of what natural rainfall was determined to be.

Drop Size Distribution Drop Size Distribution was determined to be slightly smaller than natural rainfall.

Continuously Sprayed 80-100 Nozzle Meyer determined that the rainfall intensity of a continuously sprayed 80-100 nozzle was approximately 6250 mm/h. This intensity was determined to be excessive, so some method of reducing the intensity had to be chosen.

USDA-SEA Rainulator 1960-70’s Agronomy Research Center West Lafayette Indiana

The Swanson Type Rotating Boom Simulator in the WEPP Studies, Cottonwood, SD 1987

Late 1970’s ARS scientists at West Lafayette pushed for a new type of rainfall simulator to be developed that could aid them in studying erosion, runoff and infiltration. They worked to overcome the limitations of the rainulator on field plots. The result of their efforts brought a programmable rainfall simulator.

Desired Performance Improvements (Neibling et al., 1981) Reduce cycle time of rainfall application from 20 seconds to ½ second or less for application rates greater than 64 mm/h. Have the capability of easily varying intensities in time and space. Should work on slopes up to 3:1. Should have fewer water and electrical connections. Should use interchangeable components.

NSERL New Trough-Type Simulator

NSERL Trough Type Simulator Each trough simulator is 5.32 meters long. Five nozzles are mounted 1.10 meter apart on each trough simulator. Troughs are positioned vertically such that nozzles are 2.44 meters above soil surface. Water not sprayed to plot is re-circulated within the trough.

NSERL Trough Simulator The Trough Simulators developed in the late 70’s and early 80’s work very well in lab and not so well in field situations. However, these simulators were a dramatic improvement over the ‘Rainulator’. Utilizing the Trough-Type simulator had some important drawbacks though – water, manpower, truck, trailer and electrical requirements were high. Breakdowns were frequent and data were compromised. Nozzles often plugged.

Meyer (1994) Proposed Criteria: 1. Drop size distribution near that of natural rainstorms. 2. Drop impact velocity near those of natural raindrops. 3. Intensities in the range of storms for which results are of interest. 4. Research area sufficient size to represent treatment and conditions being evaluated. 5. Drop characteristics and intensity of application fairly uniform over the study area. 6. Raindrop application nearly continuous throughout the study area.

Cont'd 7. Angle of impact not greatly different from vertical for most drops. 8. Capability of applying the same simulated rainstorm repeatedly. 9. Satisfactory rainstorm characteristics when used during common field conditions, such as high temperatures and moderate winds. 10. Portability for movement from one research site to another.

So, Why Did We Develop Another Simulator? Studies had shown that not only the simulation of the energy of rainfall was important but so was the water quality. Natural rainfall in erosive storms is low in electrolytes and this must be controlled. We wanted to be able to conduct field experiments in remote locations, but large simulators were not practical. We wanted a simulator that could also continuously vary the intensity as a function of time.

Working Criteria We did not want to totally ‘reinvent the wheel’. The simulator must be able to be crated in a box no larger than 8’ (2.44 meters) for shipping purposes. Must be able to use a pickup truck to haul equipment to the field. Light enough that 2 people could move it. Must be easy and quick to set up in the field. Fabricated with components easily obtained.

1. The kinetic energy and drop size needed to be comparable with the previous simulators that collected the greatest volume of published erosion data. Namely, the Meyer-McCune, the Swanson and the Neibling type simulators and their variations. In essence the need to maintain the pressure nozzle V-jet system of Meyer (1958). 2. The intensity needed to be continuously variable so that any type of design storm could be applied by a preprogrammed rainfall/time intensity distribution. This meant the use of a portable laptop computer technology that was emerging at the time. 3. The water use requirements were low so that water of similar quality to that of natural rainfall for the area could be used to collect data.

4. The size of the plot had to be capable of collecting the three soil erodibility parameters for the WEPP Erosion Model (interrill erodibility, rill erodibility and critical hydraulic shear, Lane and Nearing, 1989). 5. The simulator had to be light weight and capable of being hauled in a small vehicle for use in all kinds of terrain and moved with few people. 6. The electrical requirements needed to be low to allow use without power lines of large generators. 7. The cost needed to be reasonable for wide use adoption.

Calibrating Nozzles by Comparing Flow Rate & Gauge Readings It is critical that an initial test be performed to check the accuracy of the simulator’s pressure gauges. This is done by measuring the flow rate through the nozzle at the desired pressure. A comparison should be made between the flow rate at various pressures, based on manufacturer’s specifications. The 80-100 VeeJet nozzle is manufactured to deliver 14.75 liters/minute at 6 p.s.i.g.

Norton Ladder Simulator – 2002 Brookside Labs Conference in Ohio

Ladder Rainfall Simulator Easy to transport. Quick to set up. Repeatable results. Lightweight. Made of mostly common materials. Reliable. Technology that has been tested.

Some considerations for the future are: The original 80-100 VeeJet nozzle is not perfect and has limits on the ability to conduct experiments with rainfall typical of different environments. Little work on kinetic energy of natural erosive storms has been conducted since the 1950’s and with the advance of technologies, there exists many possibilities to measure the kinetic energy and single drops as well as their distribution in space and time during an erosive storm.

Nozzles can now be designed to provide exactly the desired drop size characteristics. The nozzle deliver system chosen to more accurately simulate the physical characteristics of the rainfall needs to be carefully considered. Likewise, the water of a similar quality to natural rainfall must be used in order to simulate accurately the data observed in natural rainstorms.

Questions?