Development of a New Storm Generator Model and Associated Precipitation Studies

Precipitation data for natural-resource models have historically used data collected over a 24-hr period. This could be midnight to midnight, 0800-0800, or some other observation time. When analyzing and using these data, the data are often assumed to have occurred from 2400-2400, even though they were not measured during this time. Furthermore, a often the time distribution of storms is needed for engineering design and other purposes, and a fixed precipitation pattern (?design storm?) is often used to distribute the ?daily? precipitation amounts. However, design storms do not capture the wide variability of rainfall intensities found in natural storms. Furthermore, storm rainfall spanning midnight is not considered when ?daily? totals are distributed. There are scanty short-time increment precipitation data (of the order of minutes) having fine depth resolution (.01 inch). Such data are needed for using advanced infiltration formulas and for short-time increment intensity-duration-frequency analysis. A storm generator has been developed at the ARS experimental watershed facility at Coshocton, Ohio that stochastically simulates storm occurrence, storm duration, storm depth, and within-storm precipitation intensities. The model is based on historic data, and does not include storm physics. The synthesized outputs have varying storm durations and depths (durations not constrained by artificial 24-hr times), and have intensities with a time resolution of the order of minutes. The storm generator synthesizes the month, day, year, hour, and minute of the beginning and end of individual storms (?storm? occurrence and duration), storm depth, and the within-storm intensities. The fundamental elements of the model are described. The model is undergoing testing and further development. Supporting studies of the storm generator are discussed. The model has potential utility for risk analysis for a variety of uses such as for crop pests and diseases, erosion modeling, intensity-duration-frequency analysis, flood analysis, drought studies, forecast aides, international agricultural trade, and engineering design.