Managed Flow Regimes and Resource Values: Traditional versus Alternative Strategies

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V- IC1.1UdUU11 <e.g., rarnng in large rafts), a percentage -based structure wwould "waste" water when actual flows were above 1,000 cfs, and there would be lit - tle point to providing flows less than 1,000 cfs (e.g., 900 cfs) because they could not be used. Where thresholds are more absolute, trigger or fixed -time requests may be more appropriate if flows can actually be con- trolled via dams, diversions, and _ storage facilities. On largely unmodified rivers, per- centage -based requests make more sense. Summary This type of flow request is novel and may require some administrative or legal prece- dent, but existing mechanisms used to protect instream flows are themselves only a few decades old. In some situations, a percentage - based request offers a useful method for pro- tecting instream resources while providing for out -of- stream use. It is most appropriate on rivers that have many instream flow resources, lack significant human control of flows (i.e., those with relatively fewer diver- sions or dams), and which have considerable hydrologic variation from year -to -year. It is probably less appropriate on rivers where there are few instream flow resources, where flows are extensively controlled, and when protection can be achieved with substantially less than natural flows. Choosing Thresholds to Represent Recreational Opportunities Both fixed time and trigger request struc- tures require specific choices of threshold flows to represent recreational opportunities. Because a percentage -based request can be developed from the same thresholds as a trig- ger request (as discussed above), it may also be dependent upon threshold choices. Recreational opportunities can be dimin- ished either by lower flows or fewer days of a particular flow. Addressing this requires two specific decisions: choosi.np number of flows to represent each opportunity, and choosing particular threshold flows that will be requested. In a fixed time request, choos- ing a greater number of flows per opportunity increases complexity and makes timing more difficult; for trigger requests, only complexity is increased because timing follows the natur- al flow regime. In both cases, more thresholds D. Whittaker and B. Shelby 1 per opportunity provide greater recreational diversity, allo�dng for differences among boaters with different skill levels and craft types (Whittaker et al. 1993; Shelby and Whit- taker 1995; Shelby et al. 1998). More thresh- olds also ensure that opportunities are pro- vided with a frequency similar to a natural flow regime, as discussed below. After deciding the number of .flows per opportunity, the next decision is choosing which flow (or flows) to request. If only one threshold is requested per opportunity, and the goal is to provide a high quality version of that opportunity, the peak of a flow preference curve (the optimum flow) is the clear choice. This is the flow that the ereatest number of boaters consider best for a given type of expe- rience. The trade -off in using this single thresh- old as a trigger is that it may provide fewer days of that opportunity. With trigger requests, an opportunity is foregone as soon as the nat- ural flow drops below the lowest trigger for that opportunity. If the trigger is in the mid- dle of the range (e.g., the optimum flow), an opportunity may be forgone for a consider- able number of days. To solve this problem, it often makes sense to identify at least two thresholds per opportunity. One threshold at the optimum flow will pro- vide as many days as possible at a level that provides the highest quality for the greatest number of users. Another threshold at the low end of the range ensures a "floor" to the trigger claim and provides a minimally acceptable ver- sion of the opportunity for the number of days that the natural flow regime would provide it. Percentage -based requests avoid some of these complexities because variety is built into the request structure. But percentage -based requests (as developed in this paper) may also depend on the total amount of water request- ed through the associated trigger request, so the number of triggers is important. If more opportunities are identified, or more triggers are requested for each, a trigger claim and the corresponding percentage claim will ask for more water and appear more like the natural ` o -9i me. ;ewer opportunities or fewer L __.,_. triggers, a trigger request and the correspond- ing percentage -based request will ask for less water and appear less like the natural flow regime. In all cases, choices about the number of opportunities, the number of thresholds, and the choices between minimums and opti- mums are dictated by three considerations l