Evaluation of rainfall trends in the South Island of New Zealand through the innovative trend analysis (ITA)

In this paper, an investigation of the temporal rainfall variability in the South Island of New Zealand has been carried out using a high-quality monthly dataset of 152 rain gages with more than 50 years of observation. Possible trends in seasonal and annual rainfall values have been detected by means of the Mann-Kendall test and of a new graphical technique, Sen's method, which allows the trend identification of the low, medium, and high values of a series. As a result, different behaviors emerged in the South Island, with a rainfall increase in the southwest regions and a decrease in northeast areas. Moreover, the comparison of the trend methodologies has revealed different trend results (increasing, decreasing, or trendless time series). In particular, this study shows that Sen's method could be successfully used in the evaluation of peak and low values of data for the trend analysis of seasonal and annual rainfall values.