The gamma probability distribution, with a shape parameter that varies linearly with number of days in a wet period and a scale parameter independent of length of a wet period, is shown to provide an adequate model to describe variation in precipitation amounts over an i-day wet period. Abstract The gamma probability distribution, with a shape parameter that varies linearly with number of days in a wet period and a scale parameter independent of length of a wet period, is shown to provide an adequate model to describe variation in precipitation amounts over an i-day wet period.