Deep groundwater – the most vulnerable part of the water cycle to climate change

Deep groundwater is the backbone of the regional water cycle, ensuring stream baseflow even after extended drought periods. The thicker the overlying vadose zone the more groundwater head dynamics is buffered against short-term fluctuations of groundwater recharge.  Thus deep groundwater is usually considered to be the least susceptible to climate change effects. However, the opposite is true. Long-term trends (20-41 years) of groundwater head in more than 200 wells in a 50,000 km² region in Northeast Germany have been analysed. Both increasing and decreasing trends were found, irrespective of land use, geology, etc. Factoring out local, mostly anthropogenic effects from the time series did hardly affect the results. In contrast, sign and size of long-term trends was very closely related to the degree of damping of the groundwater recharge signal. Damping was clearly related to mean depth to groundwater. The stronger the damping the more clearly groundwater head exhibited a 40-year decrease, whereas positive trends were found only for shallow groundwater sites.

A thorough analysis revealed a fundamental but widely ignored physical cause for these counter-intuitive results. From a thermodynamic perspective, seepage flux in the vadose zone can be described as dissipation of a hydrological input signal. This dissipation is subject to dispersion: The vadose zone acts as a low-pass filter due to preferential damping of the high frequency part of the input signal, recognizable by strong smoothing of soil hydrological time series at greater depth. Consequently, the smoother the time series the more probably a trend analysis would indicate a long-term monotonic increase or decrease even when no trend can be detected in the corresponding input signal. Note that in terms of spectrum analysis climate can be defined as a low-pass filtering of weather dynamics. Then it is only logical that deep groundwater as the part of the water cycle with the strongest low-pass filtering would be the first to clearly exhibit climate change signals.

This interpretation is consistent with an alternative perspective in terms of hydrological processes: The more shallow the groundwater, the more likely seepage flux will reach down to the groundwater table even after minor rain storms due to a smaller soil volume that needs to be refilled and to preferential flow. Consequently, groundwater recovers more rapidly from drought here. This holds especially true for groundwater in the riparian zone where most of the short-term response of stream discharge to rain storms is generated. In fact, none of these streams exhibited any clear trend. However, discharge in minor streams and water level in lakes at mid-slope positions already started to decline during the last decade. We would do well taking deep-groundwater dynamics as an early warning tool against climate change effects.