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WEIGHTED USABLE AREA The weighted usable area is defined as the total surface area having a certain combination of hydraulic conditions, multiplied by the composite probability of use for that combination of conditions. This calculation is applied to each cell within the multidimensional matrix. This procedure roughly equates an area of marginal habitat to an equivalent area of optimal habitat. For example, if 1,000 square feet of surface area had the aforementioned combination of depth, velocity, and substrate it would have the approximate habitat value of only 270 square feet of optimum habitat. A worked example of an entire two-dimensional matrix (depth and velocity) is given in Table 6. In each cell of the matrix, the upper numeral refers to the surface area of stream having a certain depth- velocity combination. The numerals in parentheses refer to the weighted usable area. Due to the large number of calculations required for each reach matrix, a computer program has been developed by personnel of the Cooperative Instream Flow-Service Group to make all calculations for as many species as desired from a single set of hydraulic input data (Main 977). For each species and life stage, weighted usable area may be plotted against various monthly flow regimes, such as median monthly flows or 1- in-10 year monthly low flows (Figure 11) or against discharge (Figure 12). Such plots can show critical time periods for a given life stage, limit- ing habitat availability for each life stage (i.e., physical carrying capacity), and limiting habitat availability for different species. Since changes in hydraulic characteristics will initiate differential species reactions, the incremental method- may be particularly useful in predicting changes in species composition. Because the output from the incremental method is directly tied to the physical carrying capacity of the stream, it is possible to determine the approxiate change in standing crop of a given species at different instream flow regimes. In the absence of better information, this relationship is assumed to be 1 to 1. Therefore, it is possible to calibrate the output with a standing crop estimate taken after the limiting month for a species. If the calibration standing crop is, for example, 10 Kg/ha at the calibration discharge (i.e., the discharge yielding the minimum weighted usable area), a flow regime which would cause a 50% decrease in weighted usable would yield only 5 Kg/ha for that species. This capability should be extremely useful when negotiating for flow reservations or reservoir releases from water management agencies. 34