Managed Flow Regimes and Resource Values: Traditional versus Alternative Strategies

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hydrograph that mimics natural flows and provides a diversity of opportunities. The prob- lem is that information relating flows to resource values typically does not identify the percent of water that can be removed without impacting those values, but rather the static flow needed to provide them. One way to circumvent this problem is to develop percentage -based requests after ini- tially determining the total amount of water needed for a trigger request (there are other methods that could be used to develop a defensible percentage; exploring these is beyond the scope of the present paper). Using this method, the percentage -based request asks for the same total amount of water as the trigger request, but in an alternative way that is easier to administer and which provides greater diversity through the flow range. Trigger requests are different in years with different natural flows, so percentages based on trigger requests for different years (i.e., wet versus dry years) will vary. The natural flow regime used to develop the percentage could be based on an agreed -upon "typical" year, or an average of several years. For simplicity and the illustrative purposes of this paper, calcula- tions here were made for a single year (1995). Based on mean annual flows from the period of record, this was a wetter than average year. The trigger request for 1995 (Figure 3) asked for 73.5% of the water available in that year. Applying this percentage, we devel- oped the percentage -based request shown in Figure 4. On any given day, the flow request is for 73.5% of the available water. The resulting hydrograph mimics the natural hydrograph, but at a lower level. Advantages • Instream values are identified with flow ranges rather than specific thresholds. For many resources, a continuum of flows bet- ter protects resources than a single flow. • Allows protection of needed flows whenev- er they occur. • Requests higher flows during rare high flow events, protecting associated recre- ational opportunities such as challenge -ori- ented whitewater boating. • Recognizes that natural flow regimes may differ each year. Allows for lower requests in dry years and higher requests in wetter years, which is likely to better maintain bio- physical conditions (Hill et al. 1991). • Provides a more incremental regime, which mimics natural flows. • May allow easier administration of out -of- stream flows in some situations. Although percentage -based requests are probably more challenging to administer when flows are provided from an upstream dam (thus requiring daily adjustments), they may be easier to administer when there are a series of junior diversions involved. Junior users always have to adjust their diversions in times of shortage, and a percentage -based system actually simplifies the number of times that happens in comparison to trigger requests. For example, on rivers with "flashy" spring or winter high flow periods and a more predictable falling limb of the hydro - graph during the ' summer, it is the latter period when out -of- stream demand usually exceeds supply. In these cases, a percent- age -based request requires each junior user to sequentially stop diverting (when the dif- ference between natural flows and the per- centage- derived amount is equal to the sum of senior water rights). With a trigger system, they may have to cease and then begin diverting as flows pass through each threshold. • Allows for less precision in choosing thresh- olds for resource needs because a range of flows is provided. This fits with the consis- tent finding in recreation research that flow evaluations are incremental and have some variation (Shelby et al. 1992b; Whittaker et al. 1993). Disadvantages • The hydrograph resulting from the request changes each year, depending upon water availability. Junior out -of- stream users will not know ahead of time how much water is available. As previously noted, however, junior water users are not assured of water whenever a river is fully appropriated. • 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 the actual flows that will be provided. In this way, percentage requests also require the kind of planning needed for unregulated rivers. • There may be negative effects associated with percentage decreases that would not occur with trigger requests. For example, if 1,000 cfs is the precise flow required to pro- 240 Rivers • Volume 7, Number 3 2000 _ i