Numerous climatic studies have shown that weather patterns are changing in Wisconsin and other Midwestern States. Precipitation events are becoming more extreme in both volume and intensity and are occurring with larger variation on a state and regional basis. The timing and magnitude of these more extreme events plays a vital role in the potential for sediment and nutrient loss from agricultural land.
To assess the magnitude of a precipitation event, Depth-Duration-Frequency (DDF) charts are commonly used to evaluate rainfall depths (inches of rain) for different durations (e.g., 30 min, 1 h, 24 h). These values are then compared to statistical frequency of similar sized events to determine a ranking of a storm. A common example is the 25-year/24-hour event that is used as a design criteria in technical standards for sizing best management practices to be effective to a given storm size. An example in northeast Wisconsin is the value of 5.29 inches of precipitation received in a 24 hour period. This is the 25-year/24-hour storm event that should statistically occur once every 25 years.