Study to explore the "Mechanisms of consistently disconnected soil water pools over (pore)space and time" in discussion for Hydrology and Earth System Sciences

We present in HESSD an extensive stable isotope data set gathered by the Surface Hydrology and Erosion group at IDAEA-CSIC, Barcelona. Our study sheds light on the hypothesis of “ecohydrological separation” that was stated in the seminal paper by Brooks et al. (2010) almost ten years ago. Their publication initiated many stable isotope (2H and 18O) studies in ecohydrology, tree physiology, and vadose zone hydrology. However, the mechanism of how two subsurface water pools of different isotopic compositions evolve and persist are not yet understood.
This lack of understanding is partly due to insufficient sampling frequencies of isotope data in the field. We overcome this limitation by a unique sampling design gathering isotope data of mobile and bulk soil water, rainfall, groundwater, and stream water in a fortnightly frequency over 8 months. We further extended our study by four years using long-term stable isotope and soil moisture data to reveal that the seasonal dry down of the soil and the wetting up of the soil pores with isotopically distinct rainfall can explain the disjunct isotopic compositions of water that is either stored in the soil or routed quickly to the groundwater and streams.
Our findings provide the scientific basis to refute the often-made assumption in environmental modelling that water is well mixed in the subsurface. We further provide suggestions on how to implement our findings in future modelling frame works, by accounting for pore scale variability of water transport.