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ple, thresholds could be set at the optimum
<br />and at the low end of the range for each type
<br />of craft (at 300 cfs, 800 cfs, 1,300 cfs, and 2,500
<br />cfs) for a specified number of days when those
<br />flows are most likely to be available. Figure 2
<br />shows an example annual hydrograph (1995)
<br />and a fixed -time request using these thresh-
<br />olds in an "average" year.
<br />As shown, the request specifies 30 days of
<br />optimal whitewater rafting flows, 30 days of
<br />optimal whitewater canoeing flows, 10 days
<br />of scenic rafting flows, and 83 days of scenic
<br />canoeing flows. The total amount of water
<br />used in this request is similar to the amount
<br />that could be released from the reservoir in an
<br />average year (after water associated with
<br />senior rights has been diverted), and is the
<br />same amount that will be used with the alter-
<br />native flow management requests. Because any
<br />particular year may differ from an "average
<br />year," this example shows how a "mistimed"
<br />fixed time structure would not necessarily
<br />provide this many days at each threshold, a
<br />central disadvantage of the fixed -time
<br />request.
<br />4,000
<br />3,000
<br />2,000
<br />1,000
<br />0
<br />FLOW IN CFS
<br />1995 natural hydrograph
<br />Out-ot- stream
<br />Fixed -time water availability
<br />request
<br />Request >
<br />availability 4 n
<br />e s6 ,� �6 �,� s5 �� ,y �p ma p ry9
<br />FIGURE 2. Fixed -time request and natural hydro-
<br />graph for 1995 on the Dolores River.
<br />Advantages
<br />• Easily understandable, with only three vari-
<br />ables: flow, time, and location.
<br />• Parallel to requests made by out -of- stream
<br />users.
<br />• Request is the same in every year and can be
<br />easily portrayed in a single hydrograph.
<br />• Identifies timing of various flows, allowing
<br />recreation users to better plan their trips
<br />(assuming the timing of the request fits with
<br />natural flow availability).
<br />• Specific instream flow values are identified
<br />with specific threshold flows, which can be
<br />labeled and debated in a systematic fashion
<br />(see section on "Choosing Thresholds to
<br />D. Whittaker and B. Shelby
<br />Represent Recreational Oportunities"
<br />below). For simplicity, we chose minimum
<br />and optimum flows for each craft in this
<br />example.
<br />Disadvantages
<br />• Requires specific threshold flows to repre-
<br />sent instream flow values, but some values
<br />may require greater diversity or be better
<br />represented by a continuum of flows. Most
<br />studies show some variation among evalua-
<br />tions; not every boater agrees that a particu-
<br />lar flow is the best for a specific type of
<br />opportunity. In addition, many boaters enjoy
<br />a diversity of flow levels within a range.
<br />With a fixed -time request, it is more difficult
<br />to provide this diversity. If additional thresh-
<br />olds per opportunity are claimed, the com-
<br />plexity of the claim increases and timing is
<br />less likely to be in sync with natural water
<br />availability (see below).
<br />• The request must estimate when certain
<br />flows are available through a year, yet avail-
<br />ability often varies considerably in different
<br />years. In the 1995 example (Figure 2), there
<br />are periods in mid- and late May when the
<br />request exceeds available water (and none
<br />would be available for out -of- stream use),
<br />as well a period in mid- to late June where
<br />there is water available for higher flows but
<br />the fixed -time request cannot take advantage
<br />of it. In this example, the fixed- time request
<br />misses peak flows that do not arrive until
<br />June. In other years, however, peaks may
<br />occur in late April or May. In order to address
<br />this problem, requests could be made for
<br />the entire season when peaks can be expect-
<br />ed (e.g., requesting 2,500 cfs throughout
<br />April, May, and June), but this eliminates
<br />out -of- stream water use through the entire
<br />period.
<br />• The request typically results in a stair -step
<br />hydrograph that bears little resemblance to
<br />natural flow regimes. Even if resource experts
<br />have correctly identified important thresh-
<br />old flows, quantum leaps between these
<br />levels may have unwanted consequences
<br />for several resource values (e.g., beach ero-
<br />sion, fish stranding, changes in fishing suc-
<br />cess, or beach inundations).
<br />• Fixed -time requests do not account for
<br />annual differences in the volume of
<br />water available and fail to allow
<br />instream resources to benefit from higher
<br />flows in wet years. If requests are pur-
<br />237 1 K
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