Generalized skew coefficients for use in defining flood-tYequency curves that tbllow log Pearson type 3 distributions are shown by isopleths on a map of conterminous United States. The generalized logarithmic skew coefficients range from 0.6 along the eastern seaboard to -0.5 in Indiana and Illinois.West of the one-hundredth meridian the coefficients range from -0.3 to 0.2 except for a small area in Nebraska where the generalized skew goes as high as 0.4. The validity of the map values is verified by a split-sampling procedure. In the west the discharge of 50-and 100-yr peaks. computed by using map values of skew is more accurate than that computed by using the observed skew of a sample of 30 annual peaks. East of the Mississippi River the accuracy is even higher and approaches the equivalent of 60 annual peaks along the east coast. An equation gives the adjustment by which a T-yr peak computed by using map skew would have to be increased to give a discharge that has an average exceedance probability equal to I/T. Flood-frequency curves are commonly computed by fitting a Pearson type 3 distribution to the logarithms of annual peak discharges observed over a period of years. Such a curvedepends on the mean, the standard deviation, and the skew coefficient computed from the logarithms of the annual peaks and is subject to the error inherent in estimating the population parameters from the sample statistics. The Water Resources Council [1967] recognized that the skew coefficient has greater variability between samples than the mean and the standard deviation do and suggested the possibility of using a regional value of skew coefficient in place of that based on a short record of annual peaks.

Storage requirements for various levels of streamflow regulation in the 22 major regions of the contiguous United States are presented to supersede those given in Committee Print 32 of the Select Committee on National Water Resources, U. S. Senate. At high levels of development, the storage required to provide the desired flow in 95 and 98% of the years is substantially larger than that previously given. Carry‐over storage requirements based on probability routing of annual discharge are combined with seasonal storage requirements to give the revised storage requirements, which are then used to compute revised evaporation losses and revised cost estimates by the methods previously used. The maximum net flow that could be made available for the 22 regions for 98% of the years is shown to be 965 billion gallons per day (bgd) for which a storage capacity of 3.6 billion acre‐feet would be required. A net flow of 922 bgd could be made available with 2 billion acre‐feet of storage. Any increment of flow above the 922 bgd figure would have storage costs exceeding 10 cents per thousand gallons delivered.

The double mass curve is used to check the consistency of many kinds of Jiydrologic data by comparing data for a single station with that of a pattern composed of the data from several other stations in the area. The double-mass curve can be used to adjust inconsistent precipitation data.The graph of the cumulative data of one variable versus the cumulative data of a related variable is a straight line so long as the relation between the variables is a fixed ratio. Breaks in the double-mass curve of such variables are caused by changes in the relation between the variables. These changes may be due to changes in the method of data collection or to physical changes that affect the relation.Applications of the double-mass curve to precipitation, streamflow, and sediment data, and to precipitation-runoff relations are described. A statistical test for significance of an apparent break in the slope of the double-mass curve is described by an example. Poor correlation between the variables can prevent detection of inconsistencies in a record, but an increase in the length of record tends to offset the effect of poor correlation.The residual-mass curve, which is a modification of the double-mass curve, magnifies imperceptible breaks in the double-mass curve for detailed study.Of the several methods of fitting a smooth curve to cyclic or periodic data, the moving-arc method and the double-integration method deserve greater use in hydrology. Both methods are described in this manual. The moving-arc method has general applicability, and the double integration method is useful in fitting a curve to cycles of sinusoidal form.

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