Trends in moderate rainfall extremes: A regional monotone regression approach

Rainfall extremes are thought to have increased over recent years. Typically linear trends have been considered to describe the temporal evolution of high quantiles of the daily rainfall distribution. For long records it is important to allow more flexibility. Quantile regression methods are available to estimate monotone trends for single stations. Having multiple stations in a region, the significance of the trend at the regional scale is often of interest. From this perspective we propose a regression approach, that can be used to estimate a common monotone trend for the site-specific quantiles. Moreover, the method allows the construction of confidence bands and testing the hypothesis of an existing non-decreasing trend against the null hypothesis of no trend. The approach is applied to 102 series of daily rainfall over the Netherlands for the period 1910–2009. The results are compared with those from a (regional) Mann–Kendall test. Significantly increasing trends are found for the winter season and for the whole year. In the summer season trends are less consistent over the region, and are only significant in the western part of the Netherlands. For the summer season linearity of the trend seems less apparent than for winter and for the whole year. However, the deviation from linearity is not significant.