Analysis of rainfall trend in New Zealand

Climate variability and climate change have become key issues for the research community. In particular, precipitation plays a crucial role in the process because its variations in seasonal and monthly values, coupled with changes in extreme events, can impact on water resources, on natural environments and human activities, as well as on human health and safety. In this paper, an investigation of the precipitation variability in New Zealand has been carried out by means of a daily precipitation dataset of 294 rain gages with more than 50 years of observation. First, a statistical analysis was performed through the Mann-Kendall non-parametric test to detect possible trends at seasonal and monthly scale. Then, the daily rainfall series were studied by classifying the events into six different rainfall categories and by analyzing the presence of any significant trend through the non-parametric Mann-Kendall test. Results did not show a marked trend; however, a positive trend in winter and in spring and a negative trend in summer and autumn have been detected. The spatial analysis of the trend in the South Island for several time aggregations showed a negative trend in the eastern side and a positive trend in the southwestern side. Moreover, the trend analysis of the different rainfall categories showed a decreasing trend of the middle categories and an increasing trend of the weaker category.