Ag Water Pumping

How is my irrigation water use measured?

1.) FLOW RATE (gallons per minute) and TIME (minutes) - or - 2.) TOTAL FLOW (gallons) are measured.

In the first method, the measured flow rate is multiplied by the time of flow to give total gallons applied. With your help, the irrigation system is set up and run under normal operating conditions. If an end-gun is used on your pivot, it is turned on as well. Our AWP team then measures flow with a sensitive and carefully calibrated strap-on flow meter. This is the FLOW RATE for that irrigation system. It is normally constant throughout the growing season.

To determine the time of irrigation, we check your pivot timer to be sure it is operating. If a new timer is needed or if no timer is present we will install one at no charge. Each month when we visit your irrigation system we will read and record that TIME. The difference in recorded time since the last visit multiplied by the flow rate is the total flow applied by your system that month.

In the second method, we use any operational, flow meter you may have installed. With your help, the irrigation system is set up and run under normal operating conditions. If an end-gun is used on your pivot, it is turned on as well. Our AWP team then measures flow with a sensitive and carefully calibrated strap-on flow meter. This flow rate is compared with the flow rate shown on the dial or indicator of your existing flow meter to confirm that your meter was installed correctly and is still operating near the manufacturers specifications. During monthly visits we record the totalized flow register. The difference in recorded total flow since the last visit is the total flow applied by your system that month.

For most irrigation systems in both AWP studies, the first method was used. We found few flow meters in place on farmers irrigation systems. For systems in the Automated AWP study, we used automated monitors to closely track time of operation, but we also visited these sites each month and read the pivot panel timers.

Why don't you install an in-line flow meter on my system?

Few farmers install these in their new pivots and other field irrigation systems because of initial meter cost. Debris in pumped pond and stream water, start-up surges in flow, and sand in well water can cause wear or damage the devices, adding to costs for maintaining the irrigation system. We found few flow meters in use on farms.

For similar reasons - costs of flow meters and time and cost for retro-fit installation - flow meters are prohibitive for this study. Like you, we have concerns about maintenance, but we feel confident that our periodic calibration with a strap-on meter and monthly visits would allow us to spot problems in a timely manner. Because of on-going interstate water talks and regional water permit freezes, EPD sought to begin monitoring on as large a sample as feasible and start as soon as possible. The Georgia General Assembly has allocated enough money for personnel and transportation to farms each month, but has not added enough for flow meter installation on farmers' systems. Timers are a less expensive, but still accurate, method of measuring flow on permanent irrigation systems.

Doesn't the timer method lead to under-reporting my water use?

No. Generally, the timer method will lead to over-reporting of water used.

For center pivots, this results from two causes. Timers are wired to the circuit that powers pivot motors. Whenever the pivot is moving, time is accumulating, even if you are "dry-walking" the system out of the way for field operations like harvest or plowing. If we assume that all operating time occurs with the calibrated flow rate we calculate a higher flow amount than you used. This is dry-walk time generally a small portion of the time accumulated each month because water application is done at a much slower rate of pivot movement. Even so, with your help we can identify and prefer to subtract time of dry walking so that we are only counting time of water application.

If you shutoff the end gun for part of the irrigation time, flow will sometimes drop below the flow rate we measured at the start of the study and during periodic checks. The time of operation remains the same, and since the flow rate we measured is higher than the current flow rate we calculate a higher flow amount.

The change in flow rate when the end gun is shut down is not easily predicted, but we have measured it for some systems. If your pump has a "steep" pressure-flow curve, it doesn't compensate much when an end gun sprinkler reduces outflow. Pressure increases on other sprinklers, and the total pivot discharge and flow remains about the same. If your pump has better pressure compensation or if individuals have pressure regulators, flow amounts will decrease when the end gun shuts down, and pressure on the remaining sprinklers will remain the same as before shut-down. Pressure regulators are always used with sprays located on drop tubes. Also for some systems with spray nozzles located on the boom, part or all of the nozzles may have pressure regulators. Flow decreases that occur with end-gun shutoff vary in importance. As pivot length increases, the amount of our over-estimate of flow diminishes. The end gun is only a small portion of flow in a large diameter pivot. The overestimate decreases too if the portion of the circle of rotation without the end gun is small.

Actual flow rate can decrease below measured flow rate for other reasons. As the growing season progresses, the depth from which your well pump must lift water may increase. This can lead to small reduction in pressure if the pump cannot compensate. Similarly, diesel engines for your generators may be operated at lower RPM at times than needed to keep power output at the correct amperage or power company brownouts may similarly prevent pump motors from delivering the full flow. These conditions that also damage your pumps should not occur, and we assume that they don't. In the few cases where AWP team observed that pumps could not be operated consistently because of old or faulty equipment or inadequate maintenance, we eliminated those sites from the metering program and selected another system or cooperator.

I set my pivot using the manufacturers percent timer chart to apply one inch with each application. Why is the flow amount and inches in your annual report of my field less than one inch each application?

In our experience in calibrating flow rates on pivots, we have found that they often deliver less than the original timer chart would suggest. Sometimes this is because changes you requested, like larger end guns or different spray nozzles than originally planned. Often it is because the pump is no longer maintaining the manufacturer's design pressure. This is one of the advantages of measuring flow rates with either the strap-on or in-line flow meters. You can use that information to correct your travel speed and increase each application to the desired application depth. Although we do not consider the efficiency of applications systems in determining the flow amounts, you should remember that only of 70 to 80% of the water you pump will reach the soil and replenish the root zone.