Changes in the precipitation regimes

In the literature, statistical trend studies of observed regional (or local) precipitation changes are numerous. For the Meuse basin and its vicinity, several researchers have recently examined global trends in the precipitation observations (using either station series or basin-average series) for different variables, e.g. WL ( 1994 ), KNMI ( 1999 ), Gellens ( 2000 ), Pfister et al. ( 2000 ), De Wit et al. ( 2001 ) and Vaes et al. ( 2002 ). De Wit et al. ( 2001 ) observed a small increase in the annual (November–October) and winter (November–April) precipitation depths from a basin-average precipitation record (1911–1998). Gellens ( 2000 ) found significant trends in extreme winter (October–March) k -day precipitation depths but none in extreme summer (April–September) k -day precipitation depths for the climatological network (165 stations) of Belgium (1951-1995). However, analysis of a few stations with long records starting from 1910 showed no significant trend. Vaes et al. ( 2002 ) examined the trends in the precipitation (1898–1997) at Uccle (Belgium) based on peaks-over-threshold (POT) values. For small aggregation levels (i.e. less than one day) there was a small decrease in extreme rainfall events over the century, while for large aggregation levels there was a more explicit increase in extreme rainfall events. However, the author could not conclude that the trends are significant. So far, a general picture of the main features of precipitation changes in the entire basin is still lacking. In this chapter, the available long records of daily areal precipitation for the Meuse basin and two selected (Rur and Geul) subcatchments are systematically analysed, intending to interpret the temporal changes identified for the streamflow variables in Chapter 3 . In addition to the analysis of changes in monthly, seasonal and annual precipitation totals ( section 4.2 ), this study also includes an analysis of changes in the annual and seasonal precipitation extremes ( section 4.3 ) and the frequency of precipitation events ( section 4.4 ). The statistical trend results are discussed ( section 4.5 ), followed by some concluding remarks ( section 4.6 ).