Inconsistent hydrological trends do not necessarily imply spatially heterogeneous drivers

Abstract Trend analyses are widely used to check for climate change effects on hydrological systems. However, often inconsistent patterns have been found, that is, non-significant as well as significant but opposing trends in the same data set. These inconsistencies have often been ascribed to local, mostly anthropogenic effects like wetland draining or land use change. In this study local effects were subtracted from time series of lake water level and groundwater head covering a 28 years period in Northeast Germany. But this did hardly affect the observed inconsistent trends. In contrast, the apparent inconsistent behavior could be ascribed to different degrees of low-pass filtering of the groundwater recharge signal. Due to successively increasing attenuation of high frequency oscillations during the passage through the vadose zone minor long-term oscillations in the input signal became increasingly more visible, resulting in apparent monotonic trends for the 28 year period. There is strong evidence that this phenomenon could be ascribed to frequency-dependent damping of the input signal which has been found for a wide range of natural processes, including hydrological systems.