title: Long-term rainfall variations and their impacts in the South West of Western Australia
Creators: name: Philip, Priya
Publisher: name: Griffith University
Year: 2020
Subjects: subject: rainfall
subject: groundwater

description: The South West of Western Australia (SWWA) has been of interest to research due to the decrease in rainfall over this region since the 1950s with a significant shift since the 1970s. SWWA depends on the winter rainfall for meeting its agricultural water requirements and town water supply. A decrease in rainfall has adverse impacts on these sectors. The decreasing trend has been attributed to the decrease in the magnitude and frequency of the westerlies which bring rainfall to this region and the major changes in the large-scale atmospheric phenomenon such as the Southern Annular Mode. The observed decline in water levels in the dams of this region indicates a decrease in runoff which is mostly a result of decrease in heavy rainfall. This suggests to the need to characterize the rainfall changes in SWWA both temporally and spatially. Total rainfall can be decomposed into light, medium and heavy rainfalls, and the land responds differently to these rainfall classes. The light and medium rainfall is crucial in replenishing the soil moisture which is beneficial to vegetation. While heavy rainfall contributes mostly to runoff flowing into dams, it also poses a higher risk of soil erosion compared to light and medium rainfall. This study aims to quantify contributions from these rainfall classes to the decrease and interannual variations in rainfall, and to relate the decrease to station characteristics, i.e., latitude, elevation and the mean annual rainfall, and to the largescale circulation pattern known as the Southern Annular Mode (SAM). The Southern Annular Mode (SAM) is the north south movement of the westerlies which are winds from west to east and occur in the midlatitudes between the 30˚ and 60˚ in the northern and southern hemispheres. 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) to the total rainfall of each station. It was observed that the decrease in heavy rainfall was mainly responsible for the decrease in total rainfall, followed by the medium and light rainfalls. Stations with a higher rainfall along the coast 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 and the SAM index explained 9% of the variation in heavy rainfall and 11% in total rainfall for the region. Rainfall volume is a crucial aspect because the precipitation over an area such as a catchment largely determines water resources availability for that area. Changes in the rainfall volume have considerable implications for regional water resources planning and management. As the rainfall volume is the product of the wet area, which is the area receiving rainfall and the rainfall depth, the change in rainfall volume is the result of change in rainfall depth or that in wet area or both. No study has yet been undertaken to examine the change in rainfall volume in SWWA. This study also aims to examine the spatial and temporal changes in rainfall volume and to attribute this change to that in the wet area and that in the average rainfall depth in SWWA. Gridded daily rainfall data at 0.05° resolution for the period from 1911 to 2018 were used for an area of 265,952 km2 in SWWA. The results showed that regions near the coast with mean annual rainfall ≥ 600 mm showed significant decreasing trends in rainfall volume, and 84% of which could be attributed to a decrease in the wet area, while the decrease in rainfall depth only played a minor role. The regions farther inland showed an increasing trend in rainfall volume although the trend was not statistically significant. The regions near the coast also showed a decreasing trend in wet area with a higher number of rain days while the regions farther inland showed an increasing trend in wet area with a lower number of rain days. In the coast, the rate of decrease in rainfall has been reduced, and heavy rainfall, in fact, has increased over past 30 years, although, there was no concurrent change in SAM. The runoff in SWWA has been steadily decreasing which has led to reduction in the water available for water supply and agriculture. The study aimed to understand the role of rainfall changes in the decrease of runoff in the study area. Daily rainfall, potential evapotranspiration and runoff data for 10 catchments in SWWA were used. It was observed that all catchments exhibited a decreasing trend in runoff while the rainfall showed an increasing trend in some catchments. Further, the rate of change in runoff was found to be 2.6 times the rate of change in rainfall. The AWBM model overpredicted the runoff suggesting the presence of factors besides the decreasing rainfall in the observed decrease in runoff. The results indicated that the decrease in rainfall has led to a persistent decrease in groundwater table, which accelerated the decrease in streamflow in the region. This study focused on the trend in rainfall of different intensities and their contribution to the total rainfall trend. It was found that the decreasing trend was mainly concentrated along the coastal region (high mean annual rainfall) while the inland region (low mean annual rainfall) experienced an increasing trend in rainfall. In other words, the wet regions were becoming drier and the dry regions were becoming wetter. The trend and variability in heavy rainfall was observed to be the major contributor to the variations in total rainfall. Rainfall volume along the coast also showed a significant decrease and this was mainly due to the decrease in wet area while the decrease in average daily rainfall depth only played a minor role. Although, in the long term, the rainfall shows a decreasing trend, in the recent period, the rate of decrease in the total rainfall has reduced and the heavy rainfall, in fact, has increased with no concurrent change in SAM. The decrease in runoff was not solely caused by the changes in rainfall, but the steady decline of groundwater levels in the region could also be contributing to the decreasing runoff. Although, in the recent years, the rate of decrease in rainfall has reduced, the groundwater in this region needs to first recover before we can observe any positive change in streamflow in this region.