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lJ ., 'j' r. n II ".i. ,0 5 The mean annual precipitation increases progressively from 16 inches in the southwest to 42 inches in the southeast (fig. 2). On the average about 75 percent of the precipitation occurs in the period April through September, with June receiving the greatest amount. The greater part of the precipitation results from summer thunderstorms which may be very severe at times. Downpours in excess of 5 inches in 24 hours have been recorded in practically every part of the State but more fre- quently in the eastern part. Most of the floods in Kansas result from this type of storm. As much as 12.59 inches of rainfall in 24 consecutive hours has been recorded in the State. However, this is much less than the maximum possible 24-hour rainfall which is estimated to range from about 28 inches in the north- west to about 35 inches in the southeast for areas of 10 square miles (U. S. Weather Bureau - Corps of Engineers, 1947). Although rainfall is the primary cause of floods, there is no exact cor- relation between the total amount and the resulting flood discharge. In addition to the relatively stable physiography of a specific river basin, many variable factors have an important influence on the magnitude of a flood. Some of the more important ones are: (1) antecedent moisture conditions, (2) direction of the storm movement, (3) variation of rainfall intensity, (4) variation of infil- tration rate, and (5) type, growth, and seasonal variation of vegetal cover. Because the interrelation between these variables is not defined, it can only be predicted that a great storm is likely to cause a great flood of uncertain discharge. Figure 3 presents a map of Kansas showing the range of daily rainfall, in inches, that may be equaled or exceeded on the average of once in 50 years. A rainfall-frequency analysis based on all rainfall records in Kansas is not with- in the scope of this report, and the map was prepared by the following ap- proximate method. The 50-year values of the 24-hour intensities for seven first-order recording stations were obtained directly from the U. S. Weather Bureau (1955). For 26 additional second-order nonrecording stations the Weather Bureau supplied the maximum daily rainfall reported for each year of record. These daily amounts, however, observed at regular reporting hours, do not represent the maximum 24-hour rainfall for storms that were continuous beyond the time of observation. For each station, the annual maxima were arrayed in descending order and plotted on frequency graph paper. A smooth Curve was drawn to average the plotted points giving lesser weight to the ex- treme values. The 50-year values defined by the curves were plotted on the map at the station locations and isohyetals were constructed. It is expected that somewhat greater amounts of rainfall would have been defined if true 24- hour rainfalls were available for the non recording stations. However, the values for the recording and nearby nonrecording stations were fairly con- sistent and the difference is believed to be small. The number of stations in- cluded in the analysis is sufficient to define only the general trend of the ex- pected rainfalls.