From the Grand Junction Free Press (Hannah Holm):
Drying up farm and ranch land is widely recognized as undesirable, but there’s a common belief that small improvements in irrigation efficiency could yield big benefits to other water users. Could it really? Animated discussions at recent water meetings, including the Gunnison Basin Roundtable, indicate that the answer is complicated.
First of all, many people assume that efficiency must be the same as conservation, which means using less water, which should mean more available for other users and/ or the environment. Right? Well, in the case of agricultural water use, not really.
When water is diverted from a stream and put onto the land, part of that water is taken up by plants, part of it evaporates, and part of it makes its way back to the stream. With flood irrigation, a lot of the water diverted from a stream is simply used to push water to the end of the ditch, after which it makes its way back to the stream. Seepage will also eventually return to a stream, in some cases sustaining late season flows. Increasing efficiency through a sprinkler or drip system may require less diversion of water out of the stream to transport water to the plants, but the plants will consume just as much as before.
To actually “save water” that can then be available to other uses, you have to reduce the amount of water that’s actually consumed, either by plants or through evaporation. That means changing to a less thirsty crop, reducing your acreage, or giving your plants less water than they really want — which is likely to lead to lower crop yields. Apart from measures to reduce evaporation and weed growth, there’s not really any way to reduce actual water use and keep getting the same production as before.
That doesn’t mean that irrigation efficiency improvements have no value. For the stretch of stream between the headgate and return flow, smaller diversions as a result of increased efficiency could mean the difference between a stream with fish and one without, one you can float and one you can’t.
More conservation coverage here.