The System at First Glance
Irrigation Flow Measurement with a New Metering Pipe System
The California Water Conservation Act (Senate Bill SBx7-7) was passed in 2009. The Farm Irrigation portion of the law was initially targeted to be implemented by July 31, 2012. At that time all applicable Water Districts are requested to supply the State with their long range plan to meter the volumetric flows into the Farm turn-outs and establish that they have implemented initial physical measures to record the volumes of water that they bill to the Farmers. The accuracy specifications of the measurements vary, with some applications acceptable if between ±5% to ±12%.
Value of water measurement knowledge: Metering the farm deliveries offers significant advantages to both the farm and delivery canal operations. Accurate flow rate indications help the canal operator to distribute the canal delivery among several users more competently. This stabilizes the canal and provides steady delivery to the farm operations, allowing the farm operator more control to optimize on-farm water-use. The value of flow measurement information to irrigated agriculture is briefly discussed in Appendix I (The Case for Flow Measurement).
Limitations of “Spot” Measurements: The discussion of spot measurements used in an attempt to achieve annual delivery information, and why we discourage the process, is discussed in Appendix II (Problems with “Point-in-Time” Spot Flow Measurements).
We examined many commercially offered systems in terms of (a) accuracy, (b) economics, and (c) convenience. Most systems, Ultrasonic, Venturi, Orifice, Weirs and Flumes, and intermittent insertion-sampling devices, had difficulty meeting all three criteria. We eventually selected for development a commercially available, reverse-propeller meter, inserted into a pipe, with added appurtenances, to prevent weed fouling, and to condition the flow profile. The system provided an electronic output that indicated flow rate and volume delivery for annual accounting, and can be upgraded to include supervisory control or automation.
Site conversion: The developed system adhered to the philosophy that often the most effective measuring technique is to convert the site to, as mathematicians would say, “a previously known solution.” A simple conversion starts with adding a pipe to the outlet end of the existing field structure. This addition usually does not require changing the irrigation delivery structure itself and is expected to cause minimal invasion of the farm field. Figure 1 illustrates one such solution by adding an HDPE pipe to the existing outlet that is turned to parallel the road-field boundary. An additional “known solution,” a reverse propeller meter, then detects the flow velocity in the pipe for determining volumetric delivery. The pipe is equipped with newly developed appurtenances to condition the flow from a gate jet or pipe elbow for accurate velocity detection with the time-tested, reverse-propeller meter, and to bypass weeds and grass.
Flow Conditioning and Weed Shedding: The conventional wisdom is that propeller meters, even reverse propeller meters, are severely challenged by the vegetative growth that occurs in open channel flow canal systems similar to that of the Imperial Irrigation System (IID). Many, if not most, of the farm-irrigation, water-delivery systems in California Irrigation Districts deliver water through a pipe placed through the canal bank, frequently under a canal service road. The pipes are usually 20-feet to 40-feet long. The opening and closing of a canal head gate, or sluice gate, usually controls the water delivery rate. This partly open sluice gate produces a strong jet into the pipe that can cause a distorted flow profile and flow spinning that greatly affects most efforts to sense an average velocity in the available length of pipe. To achieve both velocity-profile conditioning and control of flow spin, we introduced special flow conditioning measures between the entrance gate and the propeller meter that controlled both, and produced the desired uniform-flow profile for the propeller, or most any sensor, to detect average velocity. This system could be installed in most locations where the farm deliveries were through a pipe in the canal bank, or any location where a suitable section of pipe could be installed. This could even be placed in the beginning of a farm ditch and subsequently used as a culvert crossing by the farm operator. The ability to use the existing control gate at the delivery site is a low-cost measure and does not require changing the basic system to add any additional mechanized elements.
The hydraulic features included a weir-like blade about ¼ pipe diameter high placed a couple of diameters down the pipe to diffuse the jet energy across the pipe. Further downstream in the pipe was a large opening orifice (about 90% open area). That prevented jetting down the pipe wall and forced cross missing of the flow. A second orifice, this one preceded immediately with anti-spin vanes that resemble “shark-fins” protruding from the pipe walls, is positioned further along the pipe. Their purpose is to prevent flow spin and to further contribute to a uniform velocity profile. The weed and grass handling feature consists of a long vane attached to the inside pipe top that pushes weeds and grass down below the propeller blades. Smaller side fins push the weeds sideways around the meter. A weed-free zone thus exists for the operation of the reverse propeller meter.
The effectiveness of the weed shedding measures is illustrated in Figure 2, which shows the problem before we implemented the appurtenances in the system, and the subsequent successful performance for the remainder of the irrigation season.
Rice Culture: Most field-crop managers request a similar flow-rate delivery to their fields throughout the growing season. An exception is rice cultivation. Initially after planting, the grower requires a large flow rate to fill the rice field. Subsequently, the grower needs a much small maintenance flow rate for the remainder of the season. This maintenance flow rate amounts to about ¼ inch per day. The ratio of the large to small flow rate may be more than 20:1. To assure the delivery of large flow rates to quickly field large basins, we offer a dual metering system that can accurately measure a wide range in flow rates. This system is illustrated in Figure 3 with a typical 24-inch pipe and a parallel, but similarly equipped 6-inch pipe.
The system can be tailored to fit many field situations. It can measure large flow rates, small flow rates, and operate with small head differences between the delivery canal and the field water surface.
Table 1 lists the most commonly available pipe sizes in terms of inside pipe diameter. It also lists the estimated pipe system losses to expect. Thus, if the head difference is small and the desired delivery rate is high, the table will suggest a large pipe. Table 1 also suggests that the pipe velocity be maintained above about 3 feet per second to discourage sedimentation. This may not be practical for small head differences requiring large pipes. However, these situations frequently have water with low suspended sediment loads and can usually be made to work.
Appendix 1: The Case for Flow Measurement
In any case, to benefit from the computer models available that can improve uniformity, knowledge of the flow rate is an essential tool.
APPENDIX II: PROBLEMS WITH “POINT-IN-TIME” SPOT FLOW MEASUREMENTS
It has come to my personal attention that others are proposing to meet the requirements of the California Water Conservation Act (Senate Bill SBx7-7) as passed in 2009, by inserting a meter (propeller or other) into the discharge stream one or more times during the delivery for a point-in time value to meet the accuracy requirements. I strongly recommend that point-in-time methodology be rejected for the following reasons.For the required accuracy to happen, several conditions may be necessary. These are:
If both water-surface fluctuations occur simultaneously at inopportune times then the errors can approach 16%. If the water-surface level difference is smaller, or if the fluctuations in water levels are larger, then these errors will be further exaggerated.
To illustrate the errors that point-in-time measurements can generate, I am presenting some actual field meter recordings of complete irrigation deliveries. These will illustrate the potential billing errors that can result if point-in-time records are used. Referring to Figure A, the delivered volume is about 19.6 acre-feet. This calculates to require an average reading of about 10.2 cfs for 24 hours. It would simply be fortuitous for two, or even three, point-in-time readings to average 10.2 cfs.
Decades of experience in measuring flow rates from canals to farm fields lead to the following observations: