Changes in runoff time series in Thuringia, Germany – Mann-Kendall trend test and extreme value analysis

摘要

Qualified knowledge about the impacts of climate changeon hydrology is needed for the derivation of adaptation measures in the watersector. As temperature and precipitation time series in Thuringia, Germanyof the last 50 years reveal that the climate is becoming warmer and drier insummer and wetter in winter, the question of changes in runoff time seriesarises. In the presented study, simple robust analysis approaches to detectchanges in runoff characteristics are applied. A selection of 19anthropogenically undisturbed Thuringian catchments with daily runoff timeseries of up to 78 years without gaps, covering different landscapes andclimatic conditions in Thuringia, is made. Indicators of mean, high, and lowrunoff in the hydrological year, winter and summer are derived and testedfor trends, using the non-parametric Mann-Kendall trend test. To analyze theimpact of significant lag-1 autocorrelation (AR) in the series, a priorremoval of AR from the series before testing for trend(trend-free-pre-whitening) is performed. Results show that removal of AR hasonly minor influence on test results and is therefore considered as notnecessary. Mean flow and high flow indicators in annual and winter timeframe show increasing trends, escpecially in catchments in the higherregions of Thuringia like the Thuringian forest. In summer, all indicatorsshow decreasing trends, especially in the drier central and northernThuringian basin area. In order to assess changes in floods, 8 gauges,covering the 50-year time period 1949–1999, are selected. Annual maximumflow series are derived for the hydrological year, winter and summer. Afterfitting of 8 theoretical distributions to the samples by the method ofL-moments, 3 goodness-of-fit tests are applied. Flood quantiles for thereturn periods 2, 5, 10, 20, 50 and 100 years are calculated from means ofwell fitted distributions for all gauges. To analyze change in flood values,the relative difference of flood quantiles in 2 time periods, 1949–1979 (TP 1)and 1969–1999 (TP 2), with respect to the whole time period 1949–1999 arecalculated. Results show that flood values have increased in the later timeperiod in annual and winter time frame and have decreased in summer.