STOCHASTIC SIMULATION OF STORM OCCURRENCE, DEPTH, DURATION, AND WITHIN-STORM INTENSITIES
Newer watershed models require detailed continuous temporal precipitation to drive modeled hydrologic processes. The variability of precipitation inputs to models is a major source of variability in watershed flows that is often evaluated in the context of risks. However, available short-term-increment rainfall data are not adequate. A storm-generator model (StormGen) that synthesizes storms directly (several storms per day to several days per storm) was developed and tested using precipitation data from Coshocton, Ohio. The concepts for modeling the four elements of actual storms (storm occurrence, storm duration and depth, and within-storm intensities) and storm-model characterization and parameterization using an empirical and statistical approach are presented. Supporting studies have been conducted to help with practical parameterization in ungauged areas. Times between storms (TBS) are represented and simulated well by exponential distributions. Storm durations are characterized by empirical distributions of durations in a month. Storm depths require conditional simulation given storm durations. Within-storm intensities utilize probabilistic information contained in Huff curves. Initial performance evaluation of three of the elements of the model for a 200-year simulation shows that the model works well. TBS was modeled best (monthly average deviation of -1.3%, with values ranging from -3.2% to 3.0%). Monthly average storm duration deviations ranged from -6.2% to +1.6% with an average of -1.3%. Monthly average storm depth deviations ranged from -17.1% to +0.1% with an average of -9.2%, although actual magnitudes ranged only from -2.2 to 0 mm. Average deviations between simulated and measured average monthly precipitation was +1.7%, ranging from -8.6% to +12.6%. Corresponding depth differences ranged from -10.0 to 10.8 mm, with an average of +0.5 mm. Total simulated precipitation for the entire period deviated from measured precipitation by +0.6%, corresponding to +0.5 mm. The Poisson assumption for statistical independence of storms in the model is validated using Coshocton data, with the ratio of average storm duration to average TBS of only 0.107. The results of this study suggest that the StormGen model is promising and that modeling concepts and characterization deserve further investigation.