Our study showing that subsurface water of the studied silty-loam soil in the Vallcebre research catchment did not mix well over the studied 8 month period is now published in Hydrology and Earth System Sciences.
The SURFACE HYDROLOGY AND EROSION GROUP at IDAEA-CSIC gathered an extensive data set of water in the soil (both mobile and bulk soil water), in a piezometer, rainfall, and streamflow. The fortnightly sampling frequency revealed that despite pronounced changes in the soil wetness, the water in smaller pores were of different isotopic composition than the water moving in bigger pores. Mobile water sampled with suction lysimeter (circles and blue frame in Figure) was more similar to summer rainfall, while bulk soil water (stars and green frame in Figure) was more similar to winter rainfall. Thus, there is water located in small soil pores, which does not mix well with newly infiltrating, indicating non-uniform water transport across the studied soil profile. We could explain the isotope pattern relating rainfall stable isotopes and soil moisture, which showed that smaller pores (soil is dry) get refilled with more isotopically depleted rainfall. These findings provide a mechanistic explanation for subsurface heterogeneity in stable isotope values, which is important for improving model realism of hydrological simulations or ecohydrological applications of isotopes.
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