Measuring Water Flow and Rate on the Farm
Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona

written by
Edward C. Martin, Ph.D. Extension Irrigation Specialist

• Introduction
• Considerations in Selecting a Measuring Device
• Other Considerations
• References

Introduction

Proper water management involves two basic considerations: when and how much irrigation water to apply. The timing of an irrigation event (the when) involves utilizing information on plant needs and soil water conditions. How much depends mainly on the soil’s water holding capacity, the depletion level, and the rooting depth of the crop.

Once you have calculated how much water to apply, how can you be sure that you have accurately applied that amount? Or, if you miss your target amount, how do you determine how much water you actually applied?

The amount of water applied to a field is a function of time, flow, and area. The time of an irrigation is easily recorded. The amount of area irrigated is also easily calculated. Estimating flow rate in an open ditch is often guess work, at best. In this bulletin, we shall discuss ways to measure water flow in an open ditch.

Considerations in Selecting
a Measuring Device

Selecting the proper device for measuring water flow is often difficult due to several factors. Possibly the most limiting factor for most growers is cost. This includes the cost of the device itself as well as the cost for installation and maintenance. Quite often, the materials needed to construct the device are less expensive than the cost to install the device.

Accuracy is another important factor, though most may say that “some information is better than none.” Growers should be aware that many devices yield great accuracy in a laboratory setting, yet fail in the field. Be sure to ask about the field accuracy of the device.

The flow range of the device must also be taken into account. Devices such as sharp-crested weirs, short-throated flumes, or submerged orifices do not operate well in high flow situations.
Head loss is another consideration when choosing a water flow measurement device. Sharp-crested weirs usually require more head loss than do broad-crested weirs or acoustic flow meters (ultrasonic). However, sharp-crested weirs cost less and can measure much lower flows.
The condition of your site is also a factor. Is the canal or ditch that you are measuring lined or unlined? Is it concrete or plastic? Is the ditch geometry common or was it custom designed for your farm? Ultrasonic meters, for example, do not work well when the geometry of the ditch is irregular. Portable flumes can often be installed in an irregular earthen ditch with acceptable results.
Different devices yield different types of information. Decide whether you want rate or volume. Instruments such as weirs and flumes are used to measure the flow rate, but do not include volume. Most meters measure total volume. Some new ultrasonic devices give accurate measurements of both rate and volume.

The quality of the water needs to be considered when choosing a water measurement device. The device’s ability to pass sediment and debris can become critical when working with open channel flows. Although flumes and weirs pass debris easily, they often have difficultly with sediment. Sediment can build up in the flume or weir, giving a false reading.

Another component in measuring water flow is operation and maintenance. All too often, devices are installed properly — but poorly maintained. Poorly maintained equipment yields poor measurements and erroneous information. Factors to be considered include the need for electricity and the number of moving parts that may wear after time.

Construction and installation are always integral concerns, as equipment varies greatly. Installation of flumes does not require a high level of precision, but proper installation of an ultrasonic meter may take time and patience.

Having the ability to verify your device in the field can be very important. Slight differences in installation procedures can often cause these devices to misread and require some sort of field calibration. Also, be sure to check on the ease of troubleshooting and repair when choosing a measurement device

Finally, consider how many of these devices you need for your area and then decide which one will give you the best repeatability. It is difficult to obtain an instrument that gives you the same standard error from one device to another. Devices vary from one another, each requiring it’s own calibration. However, with proper installation and maintenance, most devices will yield acceptable standardization.

Other Considerations

There are other aspects that also need to be considered. Look at the potential for vandalism in your area. Devices such as ultrasonic meters have high replacement costs; you will want to protect these as well as possible. Flumes and weirs that are built into a ditch are less likely to be vandalized because there is little equipment to tamper with.

You may also want to weigh environmental concerns against the longevity, maintenance, construction, field verification, and standardization. For example, some devices require high maintenance, requiring frequent visits. If this site is located in a pristine wildlife area, you may opt for a device which will require fewer maintenance trips to reduce the disturbance to the surrounding area..

All of these factors have been consolidated into a table developed by the Bureau of Reclamation (Table 1). This table summarizes how various devices compare with each other in terms of cost, accuracy, flow, etc.

The table is presented on two pages. The first page lists the devices and some of the considerations previously mentioned. The second page is a continuation of the same table; the same devices are listed, but with additional information. To use the table effectively, first you need to know what flow rates you will have, what you want to measure, and whether you are measuring in an open ditch or closed conduit (pipe). For example, what would be a good device for measuring flow rate in a concrete ditch on a farm that usually carries about 5 cfs? Looking at Table 1–Part 1, first check the column marked “Flows < 10 cfs.” The sharp-crested and broad-crested weirs work well in this flow range, as well as the long-throated flumes and submerged orifices. The “differential head meters for pipe” also work well, but are intended for pipe, as stated.

Next, consider the canal type. The canal is concrete lined and both the broad-crested weir and the long-throated flumes perform best. Both devices have similar accuracy but the broad-crested weir is less expensive.

Table 1–Part 2 illustrates that both devices will measure rate but not volume. Both pass debris and having similar maintenance needs. In fact, these two devices are the same in Table 1–Part 2 except for construction. There, the broad-crested weir is considered to be easier to construct.
Now the decision will be left to you. If you want to save money, choose the broad-crested weir. If you want to save time and effort in construction, install the long-throated flume. Either way, you will be helping yourself by measuring the amount of water flowing into your fields. With this information, you will be able to determine how much water you have applied, or how long your irrigation set time should be. For information on these aspects of irrigation, contact your local Cooperative Extension office.

References

Clemmens, A.J. and J. A. Replogle. 1980. Constructing Simple Flumes for Irrigation Canals. U.S.D.A, Farmers’ Bulletin Number 2268.
Water Measurement Manual. 1997. A Water Resources Technical Publication. U.S. Dep. of the Interior, Bureau of Reclamation. Third edition.

Table 1 – Part1. Water measurement device guidelines from “Water Measurement Manual”. 1997. A Water Resources Technical Publication, U.S. Dep. of the Interior, Bureau of Reclamation, 3rd ed. Symbols +,0,- are used as relative indicators for comparison. A (+) symbol indicates positive features associated with the selection criteria. A (-) symbol indicates negative aspects. A (0) means there are no strong negative or positive attributes associated with the selection criteria. A “v” denotes device suitability varies widely, “na” denotes not applicable to criteria).

1 Venturi, orifice, pitot tube, shunt meters, etc.
2 Propeller meters, turbine meters, paddle wheel meters, etc.

Table 1 – Part 2. Water measurement device guidelines from “Water Measurement Manual”. 1997. A Water Resources Technical Publication, U.S. Dep. of the Interior, Bureau of Reclamation, 3rd ed. Symbols +,0,- are used as relative indicators for comparison. A (+) symbol indicates positive features associated with the selection criteria. A (-) symbol indicates negative aspects. A (0) means there are no strong negative or positive attributes associated with the selection criteria. A “v” denotes device suitability varies widely, “na” denotes not applicable to criteria).

1 Venturi, orifice, pitot tube, shunt meters, etc.
2 Propeller meters, turbine meters, paddle wheel meters, etc.

The University of Arizona is an Equal Opportunity/Affirmative Action Employer. Any products, services, or organizations that are mentioned, shown, or indirectly implied in this publication do not imply endorsement by the University of Arizona.
Document located http://ag.arizona.edu/pubs/crops/az1130.html
Published August 1999
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