Land application equipment used on animal production farms must be field calibrated or evaluated in accordance with existing design charts and tables according to state rules that went into effect September 1, 1996. Technical Specialist certifying waste management plans after September 1, 1996, must also certify that operators have been provided calibration and adjustment guidance for all land application equipment. The rules apply to irrigation systems as well as all other types of liquid, slurry, or solid application equipment.

Information presented in manufacturers’ charts are based on average operating conditions for relatively new equipment. Discharge rates and application rates change over time as equipment ages and components wear. As a result, equipment should be field calibrated regularly to ensure that application rates and uniformity are consistent with values used during the system design and given in manufacturers’ specifications. Field calibration involves collection and measurement of the material being applied at several locations in the application area. This publication contains step-by-step guidelines for field calibration of hard hose and cable tow traveler irrigation systems.

Operating an irrigation system differently than assumed in the design will alter the application rate, uniformity of coverage, and subsequently the application uniformity. Operating with excessive pressure results in smaller droplets, greater potential for drift, and accelerates wear of the sprinkler nozzle. Pump wear tends to reduce operating pressure and flow.

With continued use, nozzle wear results in an increase in the nozzle opening, which will increase the discharge rate while decreasing the wetted diameter. Clogging of nozzles or crystallization of main lines can result in increased pump pressure but reduced flow at the gun. Plugged intakes will reduce operating pressure. An operating pressure below design pressure greatly reduces the coverage diameter and application uniformity. Field calibration helps ensure that nutrients from animal waste are applied uniformly and at proper rates.

The calibration of a hard hose or cable tow system involves setting out collection containers, operating the system, measuring the amount of wastewater collected in each container, and then computing the average application volume and application uniformity.

An in-line flow meter installed in the main irrigation line provides a good estimate of the total volume pumped from the lagoon during each irrigation cycle. The average application depth can be determined by dividing the pumped volume by the application area. The average application depth is computed from the formula:

\(Average\ application\ depth\ (inches)=\ \frac{Volume\ pumped\ (gallons)}{27,154\ (gal/ac-in)\ \times\ Application\ area\ (acres)}\)

The average application depth is the average amount applied throughout the field. Unfortunately, sprinklers do not apply the same depth of water throughout their wetted diameter. Under normal operating conditions, application depth decreases towards the outer perimeter of the wetted diameter. Big gun sprinkler systems typically have overlap based on a design sprinkler spacing of 70 to 80 percent of the wetted sprinkler diameter to compensate for the declining application along the outer perimeter. When operated at the design pressure, this overlap results in acceptable application uniformity.

When operated improperly, well-designed systems will not provide acceptable application uniformity. For example, if the pressure is too low, the application depth will be several times higher near the center of sprinkler and water will not be thrown as far from the sprinkler as indicated in manufacturers’ charts. Even through the average application depth may be acceptable, some areas receive excessively high application while others receive no application at all.

When applying wastewater high in nutrients, it is important to determine the application uniformity. Collection containers distributed throughout the application area must be used to evaluate application uniformity.

Many types of containers can be used to collect flow and determine the application uniformity. Standard rain gauges work best and are recommended because they already have a graduated scale from which to read the application depth.

Pans, plastic buckets, jars, or anything with a uniform opening and cross section can be used provided the container is deep enough (at least 4 inches deep) to prevent splash and excessive evaporation, and the liquid collected can be easily transferred to a scaled container for measuring. All containers should be the same size and shape to simplify application depth computations.

All collection containers should be set up at the same height relative to the height of the sprinkler nozzle (discharge elevation). Normally, the top of each container should be no more than 36 inches above the ground. Collectors should be located so that there is no interference from the crop. The crop canopy should be trimmed to preclude interference or splash into the collection container.

Calibration should be performed during periods of low evaporation. Best times are before 10 a.m. or after 4 p.m. on days with light wind (less than 5 miles per hour). On cool, cloudy days the calibration can be performed anytime when wind velocity is less than 5 mph.

The volume (depth) collected during calibration should be read soon after the sprinkler gun cart has moved one wetted radius past the collection gauges to minimize evaporation from the rain gauge. Where a procedure must be performed more than once, containers should be read and values recorded immediately after each setup.

Hard hose and cable tow traveling guns are calibrated by placing a row (transect) of collection containers or gauges perpendicular to the direction of travel, Figure 1. The outer gauge on each end of the row should extend past the furthest distance the gun will throw wastewater to ensure that the calibration is performed on the “full” wetted diameter of the gun sprinkler. Multiple rows increase the accuracy of the calibration.

Containers should be spaced no further apart than 1/16 of the wetted diameter of the gun sprinkler not to exceed 25 feet. At least 16 gauges should be used in the calibration. Sixteen gauges will be adequate except for large guns where the wetted diameter exceeds 400 feet.

(Maximum recommended spacing between gauges, 25 feet X 16 = 400 feet.) Gauges should be set at least one full wetted diameter of throw from either end of the travel lane, as shown in Figure 1. The system should be operated such that the minimum travel distance of the gun cart exceeds the wetted diameter of throw. Application volumes should be read as soon as the last gauges stop being wetted.

Figure 1. General layout and orientation of collection gauges for calibration of a hard hose and cable tow traveler irrigation systems.

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Figure 1. General layout and orientation of collection gauges for calibration of a hard hose and cable tow traveler irrigation systems.

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Date:_____________ Land Owner __________________ Farm No. ______________

*Absolute value; treat all values as positive. ↲

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