1982-03-18_REVISION - C1981013 (3)

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2.4 4. Construct the frequency line for the ungaged watershed (or any of its subdivisions) by entering the plot with drainage area, find- ing the magnitudes at each line of relationship, plotting the magni- tudes at their proper places on probability paper, and drawing the fre- quency line through the points. 5. Apply the frequency lines of step 4 in the procedure for present conditions. Discharges or volumes for with-project conditions are obtained by use of auxiliary relationships described in chapters 12 and 17. In practice the method is more complex but usually only in step 3: variables in addition to drainage area are related to the peaks or vol- umes. The variables are one or more of the following, alone or in com- binations, directly or by means of index numbers: type of climate, mean annual precipitation or rainfall or snowfall, mean seasonal precipitation or rainfall or snowfall, maximum or minimum average monthly rainfall, storm pattern, storm direction, x-year frequency y-hour duration rainfall, mean number of'days with rainfall greater than x inches, mean annual num- ber of thunderstorm days, mean annual or seasonal or monthly temperature, maximum or minimum average monthly temperature, orographic effects, as- pect, stream density, stream pattern, length of watershed, length to "center of gravity" of watershed, length of main channel, average water- shed width, altitude, watershed rise, main channel slope, land slope, depth or top width of main channel near outlet for x-year frequency dis- charge, time of concentration, lag, time to peak, percentage of area in lakes or ponds, extent or depth of shallow soils, extent of major cover, hydrologic soil-cover complex, geologic region, infiltration rate, mean base flow, mean annual runoff, and still others. Combinations of these ✓ variables are used as single variables in the analysis, one such combina- tion being the product of watershed length and length to center of grav- ity divided by the square root of the main channel slope. Index numbers (chap. 18) are used for variables (such as geologic region) not ordinarily defined by numerical values. The use of multiple regression methods (chap. 18) is a necessity if more than one variable appears in the relationship. There is only one adequate , measure of the accuracy of the relationship (therefore of the regional analysis) and this is the standard error of estimate in arithmetic units. Computation of the error is illustrated in chapter 18. CONCORDANT FLOW METHOD. This method can be applied only if storm rain- fall and high-water mark (HWM) data are available for a large general storm and flood over the watershed. In States where the method is regu- larly used the data are obtained after such a flood on any watershed with a potential for a project and stored until needed. When the project evaluation is to be made the stored data are supplemented by data from the usual field surveys (chap. 6).