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

Qualified knowledge about the impacts of climate change on hydrology is needed for the derivation of adaptation measures in the water sector. As temperature and precipitation time series in Thuringia, Germany of the last 50 years reveal that the climate is becoming warmer and drier in summer and wetter in winter, the question of changes in runoff time series arises. In the presented study, simple robust analysis approaches to detect changes in runoff characteristics are applied. A selection of 19 anthropogenically undisturbed Thuringian catchments with daily runoff time series of up to 78 years without gaps, covering different landscapes and climatic conditions in Thuringia, is made. Indicators of mean, high, and low runoff in the hydrological year, winter and summer are derived and tested for trends, using the non-parametric Mann-Kendall trend test. To analyze the impact of significant lag-1 autocorrelation (AR) in the series, a prior removal of AR from the series before testing for trend (trend-free-pre-whitening) is performed. Results show that removal of AR has only minor influence on test results and is therefore considered as not necessary. Mean flow and high flow indicators in annual and winter time frame show increasing trends, escpecially in catchments in the higher regions of Thuringia like the Thuringian forest. In summer, all indicators show decreasing trends, especially in the drier central and northern Thuringian basin area. In order to assess changes in floods, 8 gauges, covering the 50-year time period 1949–1999, are selected. Annual maximum flow series are derived for the hydrological year, winter and summer. After fitting of 8 theoretical distributions to the samples by the method of L-moments, 3 goodness-of-fit tests are applied. Flood quantiles for the return periods 2, 5, 10, 20, 50 and 100 years are calculated from means of well 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 are calculated. Results show that flood values have increased in the later time period in annual and winter time frame and have decreased in summer.