YOUR BEST PRACTICE
specific capacity can be used as a general rating of the efficiency of one pumping water well to another, the W/We ratio is an actual value for a specific well. Well field managers often use this ratio to select which well to pump first or longer and which well to use only when needed.
From the time a new irrigation well has been properly constructed, fully developed and pump tested, the owner should know the most efficient pumping rate for that well. This information is then used to properly size the permanent pump for the aquifer and that well. From that date on, the maximum efficiency of the well will slowly decline due to declining water tables or aquifer head, microbial buildup in the openings of the well and on the pump inlet, mineral scale formation, and the wearing of pump impellers.
Slow declines in water production and well efficiency cannot be observed, especially where discharge piping begins at the pump base and goes directly out to the fields. From the start, the most productive aquifer zones provide the highest percentage of the initial flow rate. Microbial fouling and mineral plugging as the well is pumped will impact the most productive zones first. As the better zones begin to foul, other lesser productive zones begin to produce, and the well operator may not see slow declines in productivity. It isn’t until nearly all zones that contribute water flow into the well have
become fouled and plugged that a decline in performance is detectable. By that time, the well is already in a state where total well rehabilitation is needed. W/We ratio monitoring would have detected this slow loss of performance had it been monitored. Every time an existing well is rehabilitated, there will be a slightly lower set point of W/ We efficiency to begin remonitoring.
Well performance losses can develop so slowly that many farmers will not see the loss until it’s too late. Spending a couple hundred thousand dollars for an irrigation well and not spending just a little bit more for a water meter and a sounding tube does not calculate. It’s only through a coordinated program of collecting readings from water meters and measurements of water levels and tracking that the subtle losses in performance can be detected. If a decline in well efficiency can be seen in real time, a farmer can decide when to rehabilitate the well and/or to replace a pump.
Figure 1 is a composite display intended to illustrate the type of information that a well owner should be keeping for each well they manage. On the left is a Lithology log of the drill cuttings during drilling with depth and a general description of the aquifer with static water levels indicated by a light blue color. The second column is a visual record of the well design and pumping equipment installed.
Figure 1. A stylized graphical display of important water well information that could be used for better well management
irrigationtoday.org
The third column contains scales for four important measurements taken periodically over the past six years. In late 2017 the pumping water level dropped significantly, the gpm fell off steadily as the specific capacity fell below 10 gpm/feet drawdown and the W/We efficiency approached 50%. At this point it would become clear to the farmer that it was
time to pull the pump and rehabilitate the well. Doing a scheduled well rehabilitation or pump replacement in the offseason is preferable to performing one at a critical time in the growing season.
Maximizing the operating efficiency of pumping groundwater to grow crops or produce can save a farmer hundreds of dollars in operating costs yearly. Metering and monitoring an irrigation well can prevent costly failures of the pumping equipment, permanent damage to the well and perhaps even save a crop. The concept of metering irrigation wells for proper management should override the fear of oversight and taxation. That fear should be addressed through collective input to legislators through well owner associations.
In today’s high-tech world of irrigation technology with remote sensing and reporting of crop and soil conditions and water needs, remote performance of water well efficiencies should be incorporated into existing irrigation software programs. There are many large- scale farms supported by irrigation wells where well field management becomes a concern sooner than a small farmer with a single well. Monitoring the performance of all irrigation wells would allow bigger farm managers to stagger routine well maintenance operations.
Will the world of irrigation management see the benefits of this water well management concept as many municipal water providers do routinely? Total control and management of irrigation and crop production should include metering, monitoring and management of water well performance because it is so closely tied to the success of farming operations. The benefits of adding a flow meter and a sounding tube to an irrigation well and collecting and recording readings (if only manually) will pay for itself with reduced well maintenance costs and potential crop damage. Meter, monitor and manage for more efficient operation of your irrigation well or wells.
Gary L. Hix, RG, CWD/PI
Spring 2022 | Irrigation TODAY 29
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