Ratio of True to Fixed-Interval Maximum Rainfall

Observations of precipitation amounts at specified times may fail to indicate the maximum amounts for a given interval because the period of precipitation may extend over more than one observational interval. Because the time of precipitation is a chance phenomenon, the amount of such overlap or straddle can be evaluated on the basis of probability. It is shown that a model based on simple assumptions yields values for the adjustment factors that are near those presently used, which are based on empirical averages. The empirical factors have been determined at stations equipped with recording gages as the ratio of average maximum amounts for true intervals and average maximum amounts recorded for the observational interval. For a single interval, the factor is approximately 114%.