Analysis of spatio-temporal trends using Standardised Precipitation Index (SPI)

The trend of Standardised Precipitation Index (SPI) was investigated for more than 100 years of data using nonparametric trend technique Mann Kendall (MK) to detect wet (increasing) and dry (decreasing) periods across Victoria, Australia. Preliminary trend analysis was carried out for five different stations out of the 70 stations selected for this study. Out of five stations, two stations showed significant decreasing, indicating an increase in magnitude of dry periods. While the other two stations exhibited significant increasing trends. The analysis was repeated using recent data from approximately half the data set between 1949 and 2011. Contrasting results from the original full data set analysis were revealed. Based on the conclusions drawn from the preliminary analysis, the trend analysis was applied using the MK test for the remaining 65 stations for full set of data. Overall, 29 stations exhibited a significant downward trend. These regions have experienced frequent drought events and evidenced by the presence of a negative trend in the SPI series obtained from the present study. On the other hand, 21 stations showed a significantly upward trend, indicating a shift towards wetter conditions. No significant trend was identified for the remaining 20 stations. In addition, this paper analysed the spatial pattern of the historical droughts using SPI. Conclusions drawn from this paper, point to the importance of selecting the time series data length in identifying trends. Due to the climate variability, trend testing results might be biased and strongly dependent on the data period selected. Therefore, the use of full data set available would be required in order to improve understanding on change or before to undertake any further studies.