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OOLa Newsletter of the Colorado Water Resources Research Institute. Fort CoIlins, Colorado 80523 o (~ W (.;l -.:I ~ MARCH 1989 HOW BFFICIBNTLY IS WATER USED IN COLORADO? Neil S. Grigg a ratio of water used to water diverted 1s complicated by difference in uses; rather the term "water conservation'l 1s widely used to express the goal of using water wisely or lIeff1ciently." Also, utilities measure "unaccounted-for"waterll as a rough measure of water lost to sales, and observe the ratio of water returned as wastewater to the original raw water diverted. In that sense, without lawn watering, say in winter, this ratio is quite high. perhaps on the order of 90%, leading some to claim that water conservation in urban areas is not needed since most water ret~rns to the stream anyway. Water conservation in urban areas has two general goals: to reduce consumptive uses by lawns through xeriscape-type measures, and to cue back on in-house uses to reduce the need for storage reservoirs, treatment plants and pipelines. The river basin is the most complex unit in which to measure water use efficiency. The basic problem is that in Colorado an over-diversion by one water right owner is not necessarily a waste of water because the water not consumptively used will reappear downstream either as surface return flow, or later as groundwater water recharge to the stream or aquifer system. Such an overdiversion will, ,however, reduce the amount of water remaining for instream flows. In that sense, it is the same as the city situation: water not consumed reappears in the stream. Irrigation system improvements such as canal lining and trickle irrigation may reduce the amount of diverted water required to irrigate a given farm, but 1n Colorado's water law system the farmer is not allowed to use .the water saved so he has no incentive to reduce the amount of water diverted unless it reduces his operating costs. A highly controversial water issue in Colorado and the west is whether we use water efficiently. This is clearly seen 1n the debate over agricultural versus urban water uses. A good example of this debate was seen by many viewers of the Channel 6 presentation on February 15 of the Nebraska ETV water program. To some, agriculture was seen as a low-value use of water; they believe that all we need to do to provide urban and industrial water for the state is to divert irrigation water. Others point out that the value of agriculture is more than direct income from crops. The call to use water more efficiently comes up periodically in legislation. One of the obstacles to progress is a differing perception of what constitutes "water use efficiency" and how to achieve it. There 1s a lot of confusion about the term, as well as the related terms "conservationlt and "water salvaqe." According to or. Harvin Jensen, Director of CSU's colorado Institute for Irrigation Management, water use efficiency has a meaning to professionals in 1rrigation. but it is often misused. Those who call for increasing agricultural water use efficiency in colorado are generally seeking to introduce water-saving measures to raise the ratio of water used in crops to the total water diverted. However, irrigation water not used by crops returns to the hydrologic system either as surface or subsurface drainage, and such water-saving measures 00 not increase the total amount of water available to everyone in a basin. The term "water use effic1encytl is not used often 1n urban applications because calculating Co~~dO University