http://www.itrc.org/papers/wellrecords.htm ITRC Paper No. P 17-001

Irrigation Training & Research Center 1

IRRIGATION TRAINING & RESEARCH CENTER

California Polytechnic State University San Luis Obispo, CA 93407-0730

Phone: 805.756.2434 FAX: 805.756.2433 www.itrc.org

Using Electricity Consumption to Estimate Water Volumes Pumped from Wells1 by

Charles M. Burt, Ph.D., P.E. Chairman, ITRC

cburt@calpoly.edu cell: 805-748-3863

May 2017

The Sustainable Groundwater Management Act (SGMA) of California has led some organizations to want access to documentation of the volumes of groundwater pumped by individual wells. Many well pumps already have flow meters installed that provide an accurate measurement of both instantaneous flow rate and the cumulative volume pumped. For wells that do not have flow meters, it has been proposed for many years that pumped volumes can be accurately estimated by simply using the total power consumption, measured in kilowatt-hours (kWh).

This paper explains why power consumption measurements typically provide an inaccurate estimate of the volume of groundwater pumped.

The factors discussed in this paper include: 1. Average assumptions of pump efficiency are very inaccurate when applied to individual

wells. 2. Many well pumps use about the same kW per GPM, over a wide range of flows. Therefore,

they do not have a good relationship between kWh and acre-feet pumped. 3. Well pump conditions change over time due to:

a. wear b. changes in groundwater levels due to aquifer changes c. changes in flow rate as the discharge flow rate demand changes d. changes in flow rate as the discharge pressure requirements change

1 Development of this paper was funded by the California DWR Office of Water Conservation. Opinions in this paper are of the author, and not necessarily those of DWR.

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Assumptions of Pump Efficiency Perez and Burt (2012) examined the results of over 15,000 pump tests in California for trends by location, size, and other factors. The results of well pump tests are shown in Figure 1. Clearly there is a wide spread in overall pumping plant efficiency (OPPE) values. Furthermore, there are regional differences in well pump performance, as seen in Figure 2.

Figure 1. Well pump performance in California’s Central Valley (Perez and Burt, 2012)

Figure 2. Well Pumping Plant Efficiencies (Burt, 2011)

In summary, the overall efficiency of well pumps is highly variable. Using average values for estimating pumped volumes can be very misleading. One important implication from this information is that well pump efficiencies can degrade over time.

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Changes in Flow Rate Are Not Necessarily Accompanied by a Change in kW It is sometimes assumed that if the flow rate of a pump changes, the energy consumption will change significantly. In some cases this is true, but in other cases it is false. In some cases, the energy consumption will increase if the flow rate is reduced. To illustrate the concept, the curves of three typical well pumps are provided in this section. Pumps from three companies were selected, to show that this is not a manufacturer-specific characteristic. The pumps have flows that are commonly found in California agricultural wells. The first well pump curve, for an 11H Fairbanks Morse deep well pump, is shown in Figure 3. As will also be seen with the other two curves, only a curve for a single stage is shown (the three curves are for three different impeller diameters). For a deep well, there will be more than one stage to develop the required pressure to lift the water up to the ground surface. However, the impeller efficiency versus flow rate relationship remains almost constant, regardless of the number of stages. Figure 3 shows that with the 7.75” impeller, there is almost no change in BHP (brake horsepower) requirement between 1100 GPM and 1700 GPM. As the flow drops from 1700 GPM towards 1300 GPM, the power requirement actually increases.

Figure 3. Fairbanks Morse 11H well pump characteristics

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Figure 4 is the pump performance curve for a Peerless 12HXB well pump. It shows a fairly constant BHP requirement between 1100 GPM and 1500 GPM. Figure 5 is the pump performance curve for a Floway 12DKM well pump. Curve A shows a fairly constant Bowl HP requirement between 400 GPM and 1200 GPM.

Figure 4. Peerless 12HXB well pump performance curve

Figure 5. Pump performance curve for a Floway 12DKM well pump

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Pump Performance Changes over Time Proposals to use pump energy consumption to estimate pumped volumes (acre-feet) implicitly assume that conditions stay the same over time. This is definitely not the case. The pump efficiency can change due to wear, and the flow rate and pressure can change over time due to other factors (which also impact the pump efficiency as a related item). All of these change the relationship between volume pumped, and energy consumed. Aquifer Surface Elevation Changes Over Time In California, the tops of aquifers move up and down during the year, and between years. In some areas, the change is minor, but in many areas the fluctuation is quite large. Information of present and historic groundwater levels is available from California Department of Water Resources (DWR) and the US Geological Survey (USGS).

Figure 6. Example of changing water levels for one well in Madera County (top) and Fresno County

(bottom). https://nwis.waterdata.usgs.gov/ca/nwis/

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Surface Discharge Pressures/Flows Change over Time Some well pumps discharge directly into the air, with no valve on the discharge. These will not have any change in the discharge pressure over time. However, many well pumps have connections that will result in a change in discharge pressure over time. Examples are provided in Figures 7 through 9. The discharge into the concrete standpipe seen in Figure 7 is common; the standpipe will fill up to various heights depending upon how far away the water must travel through the concrete pipeline, as areas close to and far away from the pump are irrigated. This creates a different discharge pressure on the pump. Standpipes are sometimes three times as high as the one pictured.

Figure 7. Well pump discharge into the side of a concrete standpipe. No flow meter.

Figure 8 shows a well pump discharge that can go in two directions. There is no flow meter on this well pump. The long, straight pipe goes to a drip system, with an entirely different flow rate requirement than the concrete pipe system. As with Figure 7, the concrete system will create a different discharge pressure, depending upon where the water is used in the concrete/PVC pipe system.

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Figure 8. Well pump that discharges into the side of a concrete stand or into a drip system in the foreground

(not seen).

Figure 9 shows a well pump that discharges into a drip system. Every time the system irrigates a different field block, the flow rate and pressures change. The pressure also changes as the filters change from being clean to being dirty, and during filter backflushing.

Figure 9. Well pump that discharges into a drip system

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Pump Wear over Time Pump hardware changes over time. This includes the bowls, the impellers, and the lineshaft bearings. The net result is a change in both flow rate and efficiency, which both impact the relationship between energy used and the volume of water pumped. Burt and Feist (2012) reported on the effect of sand wear on pump performance. Figure 10 illustrates the impact of sand wear on well pump flow rate over time. The impact depends upon the impeller design and materials, as well as the concentration of sand.

Figure 10. Sand wear impact on well pump flow rate (Burt and Feist, 2012)

Conclusion It is not recommended to estimate deep well pumping volumes based on electricity consumption records. The inaccuracies can be great. A good flow meter, properly sized and positioned, is the best solution.

References Burt, C.M. 2011. Characteristics of Irrigation Pump Performance in Major Irrigated Areas of

California. ITRC Report R 11-004. Prepared for the California Energy Commission. www.itrc.org/reports/characteristics.htm

Burt, C.M. and K. Feist. 2012. Pump Performance with Sand Wear. Presented at Managing Irrigation Systems in Today's Environment. USCID Water Management Conference. Reno, Nevada. November 13-16, 2012. United States Committee on Irrigation and Drainage. 8 pp.

Perez Urrestarazu, L. and C. M. Burt. 2012. Characterization of Pumps for Irrigation in Central California: Potential Energy Savings. ASCE Journal of Irrigation and Drainage Engineering 138(9):815-822.