Measurement Resources

 

Challenges:

 

Agricultural water measurement is one of the more sensitive topics among water policy makers.  There are many factors that add to the complexity and accuracy of measuring agricultural water deliveries and water use.  For example, the Sacramento Valley is a closed basin and water is used twice.  Water that leaves one farm is frequently captured by a neighbor and used again.

 

There several fundamental differences that makes agricultural water measurement more challenging than urban water systems.  Differences that impact water measurement include the number of customers receiving water, volume of water delivered, water quality and the cost of measurement devices.

 

The most fundamental difference between agricultural and urban water systems is their patterns of use which is why they have different delivery systems.  Urban water is available to all customers on demand-when an urban water user turns on the tap, water comes out.  Because urban water users can take water many times a day at different flow rates, only a recording measurement device-such as a totalizing meter-can give accurate delivery data.

 

On the other hand, agricultural distribution systems are sized to deliver water to only a few customers at a time on delivery schedules that provide water to farms once every two to six weeks. Typical agricultural delivery systems are designed to provide water for periodic applications of large quantities of water to a field and the use the on-farm water storage properties of the soil root zone to provide water to cops between irrigations.  These systems must use either fixed rotational or arranged delivery schedules.  Over-delivery results in some customers not getting their optimal flow rate; under delivery results in canal spills (most agricultural water suppliers use open channel gravity-flow delivery systems).  Either of these conditions leads to low water use efficiency.  Water district operators usually measure water delivery flows during these delivery events to make sure that their canal system does not get out of balance.  As a result of these operational requirements, agricultural water suppliers typically have a record of the farm delivery flow rate and duration for each water use event.  This data can be used to estimate the volume of water delivered even without a recording water measurement device.

 

Agricultural water quality and the variability of agricultural deliveries also affect end user water measurement.  Farm size, crops, and irrigation methods are different from field to field.  Water delivery rates can even vary on a given field from one irrigation event to another because of plant maturity or cultural practices such as rice paddy flood-up.  Flow rate changes are even possible during an irrigation event due to irrigation management actions. 

Unlike urban water systems that deliver potable water, agricultural systems contain debris from the rivers and open canals.  Consequently, agricultural water measurement devices must handle a variety of flow rates under very difficult conditions.  For example, while a water meter may work adequately at the beginning of an irrigation season when flow rates are high and debris is low, later in the season they may not work at all because flow rates have been reduced below the operating range of the device or because aquatic weeds clog the impeller.  Because agricultural delivery flow rates, system configurations, and water quality varies so much, agricultural water end use measurement defies a "one size fits all" solution.

 

Finally the relative costs of measurement are very different in agricultural and urban settings.  For residential customers, the cost of implementing measurement (hardware, meter-reading, etc.) represents an increase in water rates of $5 to $20 per month.  On the other hand, agricultural farm-gate measurement represents an increase in farm costs for a single field $30 to $200 per month.  For most crops, this is a significant fraction of farm income-in some cases eliminating the ability of the farm to make a profit.  This high sensitivity to the cost of end use water measurement makes decisions about farm-gate measurement particularly significant. 

 

Methods of Implementation:

 

Most agricultural water measurement devices in service measure flow indirectly, either by water velocity, pressure, or head.  Determining flow is a key step in calculating delivery volumes. 

 

In agriculture, the most common method of determining flow is by measuring pressure or head.  Some devices that determine flow through the measurement of head or pressure are weirs, flumes, orifices and venturi meters.  These devices are usually the most cost effective method for wide-spread application in agricultural water distribution, and are proven to be durable enough to withstand the prolonged rigors of field work.

 

Weirs- Weirs are among the simplest of agricultural water measurement devices, plainly put, a weir allows the measurement of water by prescribing a specific flow over an in-stream barrier.

 

Flumes- Often called a flume box or flume gate, is a bypass waterway designed to force water to accelerate through a constrained end, thereby providing a preset capacity of flow, which allows the user to measure water by timing.

 

One of the most common categories of irrigation water meters is a group called convergence meters.  Convergence meters use the difference in pressure or head to measure flow rate. Types of convergence meters include venturi meters, nozzles and orifice plates. 

 

Venturi Meters- Venturi Meters also called venturi flow meters use the same principle as an orifice plate (Bernoulli) to provide measurable volumes of water through a specific choke point.  A venturi meter measures differences in pressure on each side of the choke point, and relates that to

 

Orifices/orifice plates- Orifices, often called orifice plates can be simply described as a metal plate or bulkhead with an aperture of specified flow capacity.

 

Another group or category of devices are also being used to measure water flow in agriculture. These devices determine flow by measuring water velocity, rather than by using pressure and head.  Common examples of these devices are propeller, turbine and magnetic flow meters

 

Propeller Meters- Propeller meters are the most commonly used type of water measurement devices in irrigation systems.  Propeller Meters use a multi-blade propeller, positioned within the waterway to determine the velocity of water flow passing through a known carrying capacity.

 

Turbine Meters- Turbine meters operate very similarly to propeller meters  in that they measure the flow of water by translating the number of revolutions into meaningful velocity numbers.

 

Magnetic Flow Meter- Magnetic Flow Meters rely on the principle of voltage induction in a conductor moving through a magnetic field that is proportional to the flow. The current is measured and mathematically related to flow.

 

More Resources:

 

 The Engineering Toolkit, Fluid Flow Meters 

 Cal Poly San Luis Obispo ITRC Flow Measurement Study 

 NRCS Irrigation Water Measurement Guide