Historical trends in precipitation and stream discharge at the Skjern River catchment, Denmark

A 133 yr data set from the 1055 km2 Skjern River catchment in western Denmark has been analysed with respect to precipitation, temperature, evapotranspiration and discharge. The precipitation series have been tested and corrected using the standard normal homogeneity test and subsequently corrected for undercatch. The degree of change in the climatic variables is examined using the non-parametric Mann–Kendall test. During the last 133 yr the area has experienced a significant change in precipitation of 26% and a temperature change of 1.4°C, leading to increases in river discharge of 52% and groundwater recharge of 86%. A lumped conceptual hydrological model, NAM, was calibrated on the period 1951–1980 and showed generally an excellent match between simulated and observed discharge. The capability of the hydrological model to predict climate change impact was investigated by looking at performances outside the calibration period. The results showed a reduced model fit, especially for recent time periods (after the 1980s), and not all hydrological changes could be explained. This might indicate that hydrological models cannot be expected to predict climate change impacts on discharge as accurately in the future, compared to the performance under present conditions, where they can be calibrated. The (simulated) stream discharge was subsequently analysed using high flow and drought indices based on the threshold method. The extreme signal was found to depend highly on the period chosen as reference to normal. The analysis indicated that no significant amplitude increase of the hydrograph for both wet and dry extremes could be found superimposed on the overall discharge increase.

Evaluating the influence of long term historical climate change on catchment hydrology – using drought and flood indices

This study uses a 133 yr data set from the 1055 km2 Skjern River catchment in a western Danish catchment to evaluate: long-term past climate changes in the area; the capability of a conceptual hydrological model NAM to simulate climate change impacts on river discharge; and the occurrences of droughts and floods in a changing climate. The degree of change in the climatic variables is examined using the non-parametric Mann-Kendall test. During the last 133 yr the area has experienced a significant change in precipitation of 46% and a temperature change of 1.3 °C leading to (simulated) increases in discharge of 103% and groundwater recharge of 172%. Only a small part of the past climatic changes was found to be correlated to the climatic drivers: NAO, SCA and AMO. The NAM model was calibrated on the period 1961–1970 and showed generally an excellent match between simulated and observed discharge. The capability of the hydrological model to predict climate change impact was investigated by looking at performances outside the calibration period. The results showed a reduced model fit, especially for the modern time periods (after the 1970s), and not all hydrological changes could be explained. This might indicate that hydrological models cannot be expected to predict climate change impacts on discharge as accurately in the future, as they perform under present conditions, where they can be calibrated. The (simulated) stream discharge was subsequently analyzed using flood and drought indices based on the threshold method. The extreme signal was found to depend highly on the period chosen as reference to normal. The analysis, however, indicated enhanced amplitude of the hydrograph towards the drier extremes superimposed on the overall discharge increase leading to more relative drought periods.