Monday, August 3, 2009

Water Use Efficiency? Maybe Not.

Most people and governments are supportive of and even promoting irrigation water use efficiency as a way to conserve water. Take an old gravity irrigation system applying 1000 gallons of water on an irrigated patch. This 65% efficient irrigation system will consumptively use (transpire and evaporate) only 650 gallons of the 1000 pumped and applied. The rest deep percolates below the root zone to rejoin the groundwater table (a non-consumptive use). So, 650 gallons are annually used consumptively (lost to the hydro system).

The irrigator is now paid a nice government incentive ($48K - $400 an acre on 120 acres - a typical Kansas incentive rate) to upgrade to a new, 90% efficient pivot so he can conserve water. He now pumps and applies only 850 gallons to the same patch - a 15% reduction (conservation). The new system now allows the crop to transpire and evaporate 90% of the pumped water while eliminating the inefficient water use (deep percolation) totally. This government incentive has just allowed the user to increase his consumptive draw on the aquifer from 650 gallons annually to 765 gallons each year (90% of 850 gallons). While inefficient water use is eliminated, and the pumped water is reduced 15%, the consumptive water use (CU) actually increases. Believe it or not, the change in the water table of a typical aquifer system changes as a result of consumptive water use, not gross pumpage. This conversion will actually increase the groundwater decline rate - all else being equal.

As if this were not bad enough, how many pivot conversions actually irrigated more acres than the original flood system? If land is available, we usually find a 15% increase in land when 15% more water becomes available due to efficiency improvements. And every newly irrigated acre increases consumptive use above the numbers discussed here.

Without clear and restrictive polices to control irrigated acres as system conversions are being made, consumptive use will tend strongly to increase as a result. And even if CU doesn't actually increase, it usually doesn't go down much at all. And even if it does happen to drop a bit, the system pays a premium for the 1-2% reduction which is easily offset by most of the other conversions that increase CU. This is a very inefficient way to seek water conservation.


  1. You say that you often see a 15% increase in irrigated land when there is a conversion from flood to pivot. Where does this land come from? Is it the edges, which previously had to be saved for pipe and such? I have read the opposite (pivots REDUCE the land irrigated because you may not irrigate the corners) but I don't believe it. Just wondering about the mechanism for this increase that you observe. Thanks.

  2. Lisa: In the old days a flood system here in Kansas would be on a quarter section - 160 acres - with a 900 GPM well. Many irrigators placed 2, 120 acre circles nozzeled for 450 GPM each on the single well - increasing 160 acres irrigated to 240 acres. We then limited such conversions to the acres officially irrigated before the change. While some pivots actually reduce acres on the conversion, others would find an adjacent 40 acres to water along with the new pivot. I really didn't intend the 15% statement as literally as you may have taken it. Most good business decisions will utilize the efficiency savings if at all possible - hence the 15% savings suggesting 15% more acres. Not all do, however.

    In a little more detail, the limiting of officially irrigated acres I just spoke of only applies to water right changes. If the old water right covered 160 acres but the operator has been irrigating only 100 acres due to well production drop offs, he can go to a full 120 acre pivot without triggering a change in place of use for the water right. There are many ways acres can be increased when new, higher efficient systems are put in. While not all can (or do) the clear trend is a net increase in irrigated acres over the universe of efficiency conversions.

    I hope this has been helpful. Wayne.

  3. Dear Wayne,
    Respectfully I wish to remind you that the purpose of irrigation is NOT replenish the aquifer but produce saleable products. Efficiency of irrigation is measured by dollars produced by cubic meter of water employed, and pressure irrigation is much more efficient in this sense.

  4. J: I can't argue with you that irrigation is to produce for an economic benefit, and the higher efficiency irrigation systems do this better. Irrigation is NOT done to recharge the aquifer.

    However, irrigation in most areas must be done under the conditions of a water right, and water rights in Kansas are capped to an annual maximum use. They are also in a priority system with every one else using water.

    Kansas State law prohibits any change in a water right from increasing its consumptive water use - with CU being the only factor causing changes in the aquifer storage volume (declining water levels). An existing water right that increases its CU is the same thing as allowing a new water appropriation to occur, and in an over-appropriated aquifer closed to new water rights, this unfortunately is to the detriment of all the senior water right holders in the area.

    There has been a series of new academic studies of late that are confirming the posit that CU increases as Ag installs new, highly efficient irrigation systems. This occurs by a number of ways. I must say that the irrigation systems are not at fault. It is the management institutions (our local GMD included) that cannot effectively regulate and control the conditions that cause increases in CU. So, in our world, where we're committed to reducing the groundwater decline rate, the only reasonable approach is to reduce CU. (We could import water from your area and meet our goals too, but imports are an unlikely solution as I'm sure you would quickly tell me)

    Anyone that is interested in or required to slow the aquifer depletion rates MUST reduce the CU. Just pumping less water does not always do the trick.

    Thanks for your comments. If other questions pop up, we can always dialog more.