EVALUATION OF PROBABILITY DENSITY-FUNCTIONS IN PRECIPITATION MODELS FOR THE PACIFIC-NORTHWEST

Recent work has found that a one-parameter Weibull model of wet day pr ecipitation amount based on the Weibull distribution provides a better fit to historical daily precipitation data for eastern U.S. sites tha n other one-parameter models. The general two-parameter Weibull distri bution was compared in this study to other widely used distributions f or describing the distribution of daily precipitation event sizes at 9 9 sites from the U.S. Pacific Northwest. Surprisingly little performan ce was sacrificed by reducing the two-parameter Weibull to a single-pa rameter distribution. Advantages of the single-parameter model include d requiring only the mean wet day precipitation amount for calibration , invertibility for simulation purposes, and ease of analytical manipu lation. The fit of the single-parameter Weibull to the 99 stations inc luded in this study was significantly better than other single-paramet er models tested, and performed as well as the widely endorsed, more c umbersome, two-parameter gamma model. Both the one-and two-parameter W eibull distributions are shown to have L-moments that are consistent w ith historical precipitation data, while the ratio of L-skew and L-var iance in the gamma model is inconsistent with the historical record by this measure. In addition, it was found that the two-parameter gamma distribution was better fit using the method of moments estimators tha n maximum likelihood estimates. These findings suggested that the dist ribution in precipitation among sites in the Pacific Northwest with dr amatically different settings are nearly identical if expressed in pro portion to the mean site event size.