Abstract

Low-flow frequency statistics are needed by Federal, State, and local agencies for regulatory activities and water-use planning and management, as well as by commercial and hydroelectric facilities to determine the availability of water for supply and power generation. Logistic and multiple-linear regression analyses were used to develop zero-flow-probability and low-flow-estimating equations for eight separate geographic regions of Idaho and parts of adjacent States, using physical and climatic characteristics as the explanatory variables. As part of this process, the relevant statistics were updated for all gaging stations in Idaho, and some in adjacent States, that, in general, had 10 or more years of record through water year 2003 and were unaffected by regulation and(or) diversions.

Zero-flow-probability equations were developed for three of the eight geographic regions. These equations can be used to estimate the annual probability that the 1-, 7-, or 30-day low flows will equal zero. Using only data from the gaging stations used in the development process (sample data), these equations produced results with percent-correct values ranging from 74 to 100 percent. Any variability in the zero-flow characteristics within each region that was not represented by the data used to develop the equations may affect the predictive accuracy of the final equations.

Estimating equations were developed that can be used for estimating the 1Q₁₀, 7Q₂, 7Q₁₀, and 30Q₅ low-flow frequency statistics at ungaged, unregulated sites on streams in Idaho. The standard errors of prediction for these equations exhibited a minimum range of +26.9 to -21.2 percent and a maximum range of +311 to -75.7 percent. The 7Q₂ estimating equations generally exhibited the lowest standard errors and the 1Q₁₀ equations generally exhibited the largest standard errors. The equations may not yield reliable results for sites with basin-characteristic values that are outside of the range of values used to develop the equations. The equations also are not applicable for sites on regulated streams or on streams that are affected by significant gains and(or) losses resulting from factors including spring flow, channel seepage, diversions, and irrigation returns.

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