Irrigation System

The Irrigation System

Despite what the irrigation manager may know about his/her soil type, the crop’s water requirements, the weather, or any other associated factor, irrigations cannot be scheduled appropriately without knowing the output and efficiency of the irrigation system. In sprinkler systems, if the operating pressure and sprinkler nozzle sizes are known, the gross output (or flow rate) in gallons per minute (gpm) can be estimated using sprinkler manufacturer charts (Rain Bird, Nelson, Weathertec, etc.) or can be calculated using Equation 1 below. A pitot gauge (photo 1) can be used to measure the water discharge pressures of individual nozzles, and a drill bit set (incremented in 64th inch) can be used to measure the diameter, and evaluate wear, of nozzle orifices. Total system output will be the sum of the output from all nozzles in the system not including leaks. To optimize system efficiency, leaks should be fixed as soon as possible. Water waste from leaks in side-roll systems can sometimes be measured using a bucket and stopwatch.

Calculating flow rate of individual sprinkler nozzles:

Nozzle Discharge (gpm) = 29 x √P x D² [Equation 1]                           

Where;

√P = square root of the nozzle pressure (psi)
D = nozzle orifice diameter (inch)

For irrigation scheduling, the system flow rate must with converted to depth of water applied per unit time (precipitation rate).  If all sprinklers are working properly and have the same nozzles sizes, and if the irrigated area is known, gross precipitation rate can be estimated from available charts (ie. Rain Bird) or can be determined using Equation 2 below.

Calculating precipitation rate:

            Precipitation Rate (inches/hour) =  96.3 x Q         [Equation 2]

                                                                 S_p x S_l

Where;
Q = sprinkler flow rate (gpm)
S_p = sprinkler spacing (feet)
S_l = lateral spacing (feet)

Properly designed and maintained sprinkler systems generally provide good water application uniformity when operated during low wind conditions at the specified design flows and pressures. Oftentimes, due to poor maintenance, mismatched or worn sprinkler heads and nozzles, improper operating pressures or flows, leaks, clogged nozzles, improper lateral/sprinkler spacings and/or other problems, system efficiencies are compromised and water application uniformities are low.

By conducting an irrigation system audit, the actual precipitation rate and the application uniformity (or evenness of that precipitation over the irrigated area) can be more precisely determined. The first steps in the audit include replacement or repair of faulty components. This includes broken or malfunctioning sprinklers (http://www.rainbird.com/pdf/ag/imp.pdf), worn nozzles, leaking sprinkler seals, pipe-joint gaskets, hoses, and mainlines, etc.  Additionally, all sprinklers should be of the same type and have the same output (nozzle sizes) and water-throw distance. Sprinkler lines should be as straight as possible and all sprinklers should be oriented upward (90 degrees with the ground surface), be free-flowing (no clogs), and be uniformly spaced.

After taking these preliminary steps, catch-cans or rain gauges can be used to measure precipitation rate and uniformity (Rain Bird: Distribution Uniformity for Sprinkler Irrigation). With side-roll (wheel-move), hand-move, or solid set systems, catch-cans should be set in rows perpendicular to the irrigation laterals on one side of each lateral or between laterals in solid-set systems. With hand move or wheel-move systems, where only one lateral is operated per set and then moved, cans should be placed on the ‘direction-of-next-move’ side of the lateral (Photo 2). As a general rule, can spacing should be about 10% of the sprinkler application radius and should extend out to the sprinklers throw distance (or radius). For example, if the distance of water-throw from a sprinkler line is 50 feet, cans should be set about 5 feet apart at distances of 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 feet away from the line. While the number of can rows is limited only by the number of cans available, it is advisable to set out at least three rows spaced at about 1/3 the sprinkler-throw distance (i.e. about 16 feet apart with a 50 foot throw). Straight-sided cans such as soup or coffee cans must be used if water depth will be measured with a ruler (see Note 1 below). Cans should be oriented straight up and the can opening should be level and at least 2 feet lower than the base of the sprinklers. To help identify particular areas where uniformity problems may exist, draw a diagram showing the location of each can with reference to the sprinkler line and sprinklers. Avoid placing cans too close to a sprinkler as water may stream directly into the can as the system is pressuring up.

Note 1: If the water will be measured volumetrically with a graduated cylinder, cup showing milliliters, etc. then the catch cans do not need to be straight-sided. However, the area (A) of the catch-can openings must be ascertained (A = diameter² x 0.7854) so it can be multiplied by the measured water volume to derive the depth. For example: Assume you’re using plastic cups that have circular, 3 inch diameter openings. The area of each cup’s opening would be 7.07 sq. inches (3 x 3 x 0.7854). Since there are 16.39 cubic centimeters (or milliliters) in one cubic inch, each volumetrically measured 116 ml (16.39 x 7.07) would equal 1 inch depth.

After running the system at normal operating pressure for a timed period when winds are low, measure the depth of water in each can and record the measurements on the diagram. Keep in mind that most sprinklers apply a decreasing amount of water to the field with increasing distance away from the sprinkler. Consequently, sprinkler lines should be spaced to provide sufficient overlapping water application patterns to get an even or uniform application of water between the lines. Under normal operating conditions, this spacing is about equal to the water throw distance, or application radius, of the sprinkler. If a particular sprinkler provides a 100 foot diameter water application pattern, for example, sprinkler lines should be spaced 50 feet apart (the application radius). While the amount of water measured in the cans may decrease with distance away from a single line, the depth of water measured in cans placed an equal distance away from the line should be about equal if all sprinklers are working properly. When the side-roll or single hand move lateral is moved to the next set (ie. 50 feet away from the previous set), and is operated for the same time period, the water in all cans, ideally, if left undisturbed, should be about equal.

If the cans were placed between two laterals that were operated simultaneously (i.e. solid-set system), the measurements can be used directly to calculate application rate and uniformity. If the measurements represent applied water from one side of a single lateral or wheel-move, two options are available:

1) If the hand-move line or side-roll is not going to be moved to the next set and operated promptly then record the initial measurements on the diagram and dump the water but place the cans back in the same spot where they’ll be ready for the next set when it does occur, or;

2) If the hand-move line or side-roll is going to be moved immediately and operated, then leave the cans undisturbed, move the hand pipe or side-roll to next set and run for an equal time period as the first set.

Note 2: Sprinklers are designed to operate most effectively within specific pressure ranges. When a sprinkler is operated below this range, most water is deposited in large drops or streams in a doughnut pattern around the sprinkler at a distance less than the optimum water throw range for the sprinkler. Operating a sprinkler at pressures above the design range results in excessive misting (small droplet size) and water is more easily blown away or evaporated or it may accumulate too close to the sprinkler. In either case, water application uniformity is compromised and efficiency is decreased. Sprinklers and nozzles should be chosen based on the available head (pressure), desired water flow rates, and the irrigated area (to be discussed in another tip sheet). Reputable sprinkler manufacturers/dealers provide design specifications for their sprinklers and in many cases they can be found on the internet
(ie. Rainbird - http://www.rainbird.com/ag/products/impacts/index.htm,
Nelson - http://www.nelsonirrigation.com/products/,
Weather-Tec - http://www.weathertec.com/ag_products.htm.

Once the catch-can data are collected, one of two methods can be used to quantify the variability of water distribution over the irrigated area; the coefficient of uniformity (CU) method, or the distribution uniformity (DU) method.

CU Method

In the coefficient of uniformity (CU) method, the calculated average (mean) catch-can measurement is compared with the deviation of each measurement from the mean using Equation 3. If all measurements were exactly the same, application uniformity would be perfect and CU would equal 100%. Generally, a CU of 80% or greater is considered acceptable for agricultural systems.

            CU = 100 (1.0 - ∑x/mn)     [Equation 3]

Where;

            CU = Uniformity Coefficient (expressed as a percentage)

            x = the deviation of each measurement from the mean

            m = the average or mean measurement

            n = total number of measurements

In this example, the CU is very good at 85%.

DU Method

The Distribution Uniformity (DU) method (Equation 4) is usually used to measure water application variability when there are differences in pressures and nozzle sizes between sprinklers or in systems that have been poorly maintained. 

DU = 100 x (MQ1/M)                  [Equation 4]
               

Where;

DU = Distribution uniformity (%)

MQ1 = mean of observations in lowest 25% of the measurements

M = mean of all measurements

Note that the DU is somewhat lower than the CU (79% vs. 85%) and indicates that about 20% of the field receives less water than the average application depth.

Further Thoughts:

Keep in mind that the catch-can technique demonstrated above quantifies the water application uniformity for a small section of lateral only. In long laterals (i.e. ¼ mile side-rolls), there may be significant changes in pressure along the line due to slope, friction, etc. and these will cause variations in sprinkler output, even if all sprinklers are exactly the same.  Therefore, it’s advisable to conduct several audits along the line to evaluate this variability and calculate a mean CU or DU for the whole system.

If CU or DU appears to be low because of dry areas mid-way between sets or sprinklers, overall uniformity can usually be improved by either decreasing the set distance or staggering the sets on alternate irrigation events.

While knowing the output and application uniformity of your irrigation system is essential for efficient irrigation management, it is not the only consideration. For optimum irrigation scheduling, you must know something about your soil and the crop’s water requirements throughout the season. These topics will be covered in upcoming tip sheets. Stay tuned. 

DS