In farming there is no replacement for the skills of observation, understanding of your system, or historical knowledge of your farm. But so often we irrigate based on duration and run time – we run the water for two or three hours and see how our plants respond. Water metering gives us a solid sense of how much water we are using.
Positive Displacement Meters
Positive displacement flow meters operate by repeatedly filling and emptying a tiny compartment with a known volume. The flow rate is calculated based on the number of times these compartments are filled and emptied. As an analogy, if you fill a 5-gallon bucket and pour it into the bathtub four times, you know you’ve poured 20 gallons of water into the tub. This is the fundamental concept of positive displacement flow meters. Examples of positive displacement meters include oval gear, piston, and nutating disc.
Velocity Meter
There are several types of velocity meters, some examples of which are paddle wheel, turbine, ultrasonic, and electromagnetic. Some meters are mechanical and read water velocity by turning some sort of wheel. For example paddle wheel meters have a series of blades that spin with the passage of water (think old-timey steam boat) or turbine meters that have a pinwheel like corkscrew in-line with the pipe (think air through a jet engine).
In contrast ultrasonic meters use acoustic waves injected into the pipe. Depending upon the method used, the sound will either be reflected off of the pipe wall (transit time), or off of suspended particulates (Doppler). The flow rate is calculated based on differential time.
Ultrasonic meters are commonly used in agriculture. Since there are no moving parts required to detect flow, they continue to measure accurately in low-flow situations. In addition, there are ultrasonic meters that can be clamped directly onto the pipe, requiring no re-plumbing of existing infrastructure.
Yet another type of velocity meter is electromagnetic flow meters. Electromagnetic flow meters can measure the flow of electrically conductive liquids, such as water. In these meters, sensor rods are mounted within the pipe, and a magnetic field is generated by energized coils outside of the pipe. The conductive liquid (water) moves through the meter generating a voltage reading that is interpreted to flow rates. Like ultrasonic meters, electromagnetic meters have the benefit that they do not have moving parts which can wear.
Wattage-based Meter
These meters calculate daily water usage by tracking the electrical power consumption (wattage) of the water pump. By combining a short pump test with electrical-use data, these systems can accurately estimate water volume pumped without needing to cut into the pipes to install physical inline flow meters. In some circumstances these types of electrical meters also enable the farmer to turn on and off the pump remotely.
This article was compiled with funding from the Department of Water Resources Underrepresented Communities, California Tribes, and Small Farmers Groundwater Technical Assistance Program.