Managed Flow Regimes and Resource Values: Traditional versus Alternative Strategies

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olds to Represent Recreational Opportuni- ties" below). • Allows protection of needed flows whenev- er they occur. This is crucial for recreation because boaters use even infrequent high flows if they provide high quality opportu- nities, and this type of request protects these whenever they happen. • Recognizes that natural flow regimes may differ each year; allows for lower requests in dry years and higher requests in wetter years, which is more likely to maintain bio- physical cone !- (^ ,than a request that is the same r (Hill et al. 1991). • While still providing a stair -step request, trigger requests allow these steps to more closely align with the natural flow regime. Disadvantages • The hydrograph resulting from trigger request structures can change in each year, depending upon water availability. This is distinct from fixed -time requests. • Does not identify times when various flows will be available; recreation users have to plan their trips based on weather and run- off predictions, which determine when trig- gers will provide desired flows. In this way, trigger requests require the kind of plan - ning needed for unregulated rivers. • Administration of trigger requests is rela- tively more complicated. For example, as nat- ural flow availability decreases and approaches a trigger, there is less and less out -of- stream water available. At the trigger itself, no out -of- stream water is available and junior rights holders would have to stop diverting water. But once flow decreases below this trigger, requests are based on the next lower trigger, so that junior users would be allowed to resume diversions. Under this system, junior out -of- stream users may have to adjust their use more fre- quently through the flow range. Any appro- priation system requires these adjustments when water supply is constrained, but the adjustments are likely to be more frequent with trigger- requests. • Specific threshold flows are still required to represent instream flow values, even though some values may require greater diversity or be better represented by a con- tinuum of flows. • The request still results in a stair -step hydrograph that may not resemble natural flow regimes. Even if resource experts have correctly identified important threshold flows, quantum leaps between these levels may have unwanted consequences for resource values. Summary Trigger requests are most appropriate when a small number of instream resources are pro- tected, each with well- defined flow thresh- olds. Trigger requests can work well in river systems where water quantity is largely con- trolled by humans (systems with large dams, diversions, and storage capacity), but they are even more useful when the goal is to mimic large variation in flows from year to year. Trigger structures are better when requests are closer to what natural flows would pro- vide. Trigger structures are less ap}:ro) - )riate when there are multiple resource values with a variety of different threshold flows, or when resource values require a diversity or continu- um of flows (see below). Percentage -based Requests Percentage -based requests also specify amounts of water at a specific location, but define that amount in terms of a percentage of available flow. These requests focus on the per- centage needed to avoid effects on resources (Figure 4), and the percentage requested can range from 100% to some smaller number. A 100% request protects full diversity for a resource, but does not allow any out -of- stream use; if the goal is to substantially pro- tect the diversity of resource values but still allow some other use, some lower percentage makes sense. The chief advantage of percentage -based requests is the creation of an incremental 4,000 3,000 2,000 1,000 FLOW IN CFS 11995 natural hydrograph i Out -of- stream 0 kvs� 6PQ� j ai �o s sb y � s sh �o ^ �P ry9 FIGURE 4. Percentage -based request and natural hydrograph for 1995 on the Dolores River. L D_ Whittaker and B. Shelby 239 'L water availability \ Percentage -based request (73.5 %)