Interannual variations in rainfall of different intensities in South West of Western Australia

Abstract

Significant decreases in rainfall in South West of Western Australia (SWWA) over recent decades are of concern for water supply and agricultural production in the region. Total rainfall can be decomposed into light, medium and heavy rainfalls, and the land responds differently to these rainfall classes. Little, however, is known about changes in different rainfall classes in SWWA. The objective of this study is to quantify contributions from these rainfall classes to the decrease and interannual variations in rainfall, and to relate the decreases to station characteristics, that is, latitude, elevation and the mean annual rainfall, and to the large scale circulation pattern known as the Southern Annular Mode (SAM). Long-term data for 30 stations were used, and daily rainfall was divided into three classes in such a way that they contributed equally (one third of the total) for each station. Trend and correlation analysis was applied to these classes. The decrease in heavy rainfall was mainly responsible for the decrease in total rainfall, followed by the medium and light rainfalls. The rate of decrease in heavy rainfall was 68% higher than, while that in light rainfall was about half, of the expected trend. Heavy rainfall exhibited a significant decreasing trend in winter and autumn but mildly increasing trend in summer. The number of heavy rain days also showed a significant decreasing trend. The rate of decrease relative to the mean was similar for both heavy rainfall and number of heavy rain days. Stations with a higher rainfall were more likely to experience a decrease in rainfall than those in the drier inland areas. Stations where rainfall was strongly correlated with SAM were mostly concentrated along the west coast of SWWA. The SAM index explained 9% of the variation in heavy rainfall and 11% in total rainfall for the region.