Use of CLIGEN to simulate climate change in southwest of Western Australia

Abstract

CLIGEN is a stochastic weather generator to reproduce, statistically, daily weather variables, and CLIGEN output has been used to simulate the impact of climate change on runoff and soil erosion in other parts of the world. There is little research on how to change parameter values for the stochastic weather generator based on observed weather data in regions where rainfall has significantly decreased. The aim of this paper is to develop a method based on available historical data to adjust CLIGEN parameter values when rainfall has gradually but significantly decreased. In southwest of Western Australia (SWWA), annual rainfall has shown a significant trend since the mid-1920s. Although the annual rainfall has clearly decreased during the last 90 years, the variation in monthly and daily rainfall is not as clear as annual rainfall. This study has focused on CLIGEN parameter values for the daily rainfall amount to investigate correlations between variation of daily and annual rainfall for SWWA. Long-term (90 years) daily rainfall data from Wilgarrup in this region were used to examine decadal variations in rainfall and to evaluate the changes to CLIGEN parameter values with a gradual decrease in annual rainfall. Average rainfall on wet days, standard deviations, skewness coefficients, and probabilities of a wet day following a wet day and a wet day following a dry day were analysed for each of 54 series of 30-year periods (starting from 1926 to 1979, finishing from 1955 to 2008) for Wilgarrup in SWWA. This paper shows that for the wet months in winter, there are similarities between changes in annual rainfall and changes in mean daily rainfall, standard deviation, the probability of a wet day following a dry day and mean monthly rainfall. There is little evidence to suggest the need to adjust skewness coefficients, wet day following a day wet probabilities or wet day following a dry day to simulate changes in mean monthly rainfall for this region. This paper also shows a relationship between trends in mean monthly rainfall and mean daily rainfall. These results indicate CLIGEN parameter values can be adjusted to reproduce the decline trend for the region in SWWA.