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

Abstract

Abstract.A 133 yr data set from the 1055 km 2 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.

Investigation of hydrological time series using copulas for detecting catchment characteristics and anthropogenic impacts

“…These changes could correspond to the construction of flood retention basins started around 1982 (Lammersen et al, 2002) and ecological flooding strategy, which allowed small floods to happen for the rehabilitation of ecological systems in the floodplain, introduced in the upper Rhine from 1989 (Siepe, 2006).…”

“…Mann-Kendall's test was performed to confirm the existence of a trend in the annual discharge, precipitation and sediment loads, then the human intervention and climate impacts based on the available information of the catchments were discussed (Wu et al, 2012). Pettitt's method (Pettitt, 1979) can be used to detect the time point of trend alternation and analyze the impacts based on a double mass curve (Gao et al, 2013) or a hydrological model (Karlsson et al, 2014). These nonparametric methods for detecting the signal seem, however, not capable enough of explaining when and how much the system had changed, thus making it still difficult to relate the change to human activities.…”

An international comparison of rice consumption behaviours and greenhouse gas emissions from rice production

“…As a normal practice, the critical value of Z 1-a /2 for a p-value of 0.05 will correspond to a Z-score of 1.96 (or 95% confidence interval). If |Z| > Z 1-a /2 , then a statistically significant trend is unlikely to exist and the null hypothesis H o is rejected while the alternative hypothesis H 1 accepted (i.e., that there is a significant trend in the data series at a significance level of a (Hisdal et al, 2001;Karlsson et al, 2014).…”

Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and <sup>14</sup>C dating

“…Groundwater recharge, drain levels, well abstractions and sea levels were assumed constant over time for practical reasons and to reduce computational time. However, Karlsson et al (2014) showed that recharge has 5 changed significantly in Denmark during the last centuries. Changes in recharge could result in different age patterns (cf.…”