Mechanisms of consistently disconnected soil water pools over(pore)space and time

Abstract. Storage and release of water in the soils is critical for sustaining plant transpiration and groundwater recharge. However, the subsurface mixing of water available for plants or quickly flowing to streams and groundwater is not yet understood. Moreover, while water infiltrating into soils was shown to bypass older pore water, the mechanisms leading to a separation between water routed to the streams and water held tightly in smaller pores are unclear. Here we present an extensive data set, for which we sampled fortnightly the isotopic composition (2H and 18O) of mobile and bulk soil water in parallel with groundwater, stream water and rainfall in the Mediterranean long-term research catchment, Vallcebre, in Spain. The data revealed that mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix, but they constitute two separate subsurface water pools; despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentation results from rewetting of small soil pores with isotopically depleted winter/spring rain. Thus, stable isotopes, and therefore water residence times too, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing water uptake by plants or process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.