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. I . O')i:1}?7 changed, and that merely diverting less water to carry out the same historical decreed purpose is not considered a change of water right. The holder of junior rights sees improved efficiency as a way to reduce losses so that more water can be consumed under the historical diversions available to him. This increases depletions (and reduces return flows) below the area. So long as this increased use is consistent with the original decreed water rights, there is no change of rights and no opportunity to apply the "no injury" rule. If the improver is not able to increase consumptive use it will then divert less water, consume the same amount, and return less water to the stream. In that case, and assuming upstream users already have an adequate supply, the reduced diversions will result in an increased supply below the headgate and a decreased late season supply in reaches below where return flows historically entered the stream. B. Water Quality Impacts There are two types of water quality impacts that result from efficiency changes: changes in the assimilative capacity of the stream and changes in the pollutant load entering the stream. Irrigation return flows may pick up sediment, dissolved minerals, or agricultural chemicals as they travel across the field and through the soiL Return flows could then add pollutants to the watercourse. The size of the pollutant load depends on farming methods, soil types, underlying geology, and distance from the stream, but in general, any reduction in return flows will result in a lower pollutant load entering the stream. The assimilative capacity of the stream measures its ability to absorb a given pollutant load without adversely impacting water quality. This process is more complex than simple dilution, because it depends on more than just the volume of stream flow (i.e., temperature, biological activity, chemical composition, etc.). However, in general terms, the ability of a stream to assimilate wastes does improve when the volume of stream flow increases. This tension between water quality and water quantity is already recognized in a number of ways, and may eventually limit diversion and use of surface water in Colorado. The precise effect of irrigation efficiency changes on the assimilative capacity of a stream will depend on downstream uses, types of pollution discharges entering lower reaches from other sources, timing of other discharges (i.e., seasonal irrigation return flows, intermittent industrial discharges, or year round municipal waste discharges), and the quantity of flows in the stream. For instance, the loss of return flows may reduce late fall and winter flows in smaller streams, making them less able to assimilate the discharges from municipal users. C. Groundwater Impacts Groundwater can be a renewable or a finite resource depending on whether overlying geology allows recharge of the aquifer. 20 ji L" ~. '~'-= ',--, ~<- '-