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<br />ungaged site on a gaged stream is based on
<br />information from the Pearson Type-III estimate
<br />for the gaged site, and on information from
<br />either both flood-estimation methods, or from
<br />only one of the methods. At a gaged site, a
<br />weighted design-flood discharge is estimated
<br />from the Pearson Type-III estimate, and from
<br />either both flood-estimation methods, or from
<br />only one of the methods. Examples are provided
<br />for each of these procedures.
<br />
<br />The drainage-basin and channel-geometry
<br />flood-estimation methods presented in this
<br />report each measure characteristics that are
<br />presumed to be independent of each other. The
<br />drainage-basin flood-estimation method is
<br />based on measurements of morphologic and
<br />climatic characteristics that are related to how
<br />water flows off the land. The drainage-basin
<br />method measures the varying flood potential at
<br />stream sites as defined by differences in basin
<br />size, topographic relief, stream development,
<br />and precipitation. The ch.annel-geometry
<br />flood-estimation method, in contrast, is based on
<br />measurements of channel morphology that are
<br />assumed to be a function of streamflow
<br />discharges and sediment-load transport. The
<br />channel-geometry method measures the
<br />variability of floods that have adually occurred
<br />as defined by differences in channel width and
<br />depth.
<br />
<br />The drainage-basin flood-estimation method
<br />developed in this study is similar to the regional
<br />flood-estimation method developed in a previous
<br />study because both methods estimate flood
<br />discharges on the basis of morphologic relations.
<br />While the standard errors of estimate for the
<br />drainage-basin equations in this study appear
<br />to be higher, a direct comparison cannot be
<br />made because of the different methodologies
<br />used to develop the equations.
<br />
<br />The statewide drainage-basin and statewide
<br />channel-geometry regression equations
<br />presented in this report provide flood-
<br />estimation methods that mInImIZe the
<br />subjectivity in their application to the ability of
<br />the user to measure the characteristics.
<br />Although the user of the regional channel-
<br />geometry equations may still encounter a
<br />dilemma when a stream site is located within
<br />the transitional zone or when a stream crosses
<br />regional boundaries, application of the
<br />
<br />statewide channel-geometry equations may be
<br />utilized to preclude the regional subjectivity
<br />associated with estimating a design-flood
<br />discharge in this situation. Despite the greater
<br />variability in the error of measurement
<br />associated with the channel-geometry
<br />characteristics, the channel-geometry equations
<br />presented in this report are considered to be
<br />useful as a corroborative flood-estimation
<br />method with respect to the drainage-basin
<br />method.
<br />
<br />The estimation accuracy of the drainage-
<br />basin regression equations possibly could be
<br />improved if drainage-basin characteristics were
<br />quantified from larger scale data. The
<br />drainage-basin characteristics quantified by the
<br />GIS procedure were limited to the 1:250,000-
<br />and 1:100,000-scale digital cartographic data
<br />currently available for Iowa.
<br />
<br />REFERENCES
<br />
<br />Allen, D.M., 1971, Mean square error of
<br />prediction as a criterion for selecting
<br />prediction variables: Technometrics, v. 13,
<br />p.469-475.
<br />
<br />Baker, D.G., and Kuehnast, E.L., 1978, Climate
<br />of Minnesota, part X--Precipitation normals
<br />for Minnesota, 1941-1970: Minnesota
<br />Agricultural Experiment Station Technical
<br />Bulletin 314, 15 p.
<br />
<br />Choquette, A.F., 1988, Regionalization of peak
<br />discharges for streams in Kentucky: U.S.
<br />Geological Survey Water-Resources
<br />Investigations Report 87-4209, 105 p.
<br />
<br />Dempster, G.R., Jr., 1983, Instructions for
<br />streamflowlbasin characteristics file: U.S.
<br />Geological Survey National Water Data
<br />Storage and Retrieval System
<br />(WATSTORE), v. 4, chap. II, sec. A, 34 p.
<br />
<br />Eash, D.A.' 1993, A geographic information
<br />system procedure to quantify physical basin
<br />characteristics, in Harlin, J.M., and
<br />Lanfear, K.J., eds., Proceedings of the
<br />Symposium on Geographic Information
<br />Systems and Water Resources: American
<br />Water Resources Association Technical
<br />Publication Series TPS-93-1, p. 173-182.
<br />
<br />42 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA
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