Measuring and Modeling Stable Isotopes of Mobile and Bulk Soil Water

Abstract

Recent findings from stable isotope studies have opened up new questions about differences in the isotopic composition (d 2 H and d 18 O) of mobile (MW) and bulk water (BW) in soils. We sampled the isotopic compositions of MW using suction lysimeters and BW with the direct-equilibration method. The study was conducted at two landscape units in each of three catchments: the Bruntland Burn (Scotland), Dorset (Canada), and Krycklan (Sweden). We further used the numerical one-dimensional flow model SWIS (Soil Water Isotope Simulator) to simulate the hydrometric and isotopic dynamics. The model included evaporation fractionation, allowed differentiation between a fast and a slow flow domain, and included isotopic exchange via water vapor. Our measurements showed that MW plots along the local meteoric water lines, whereas BW plots below, which is indicative of evaporation fractionation. We suggest that the relative volume of MW to BW is relevant for explaining these isotopic differences because MW volumes are usually relatively low during periods of high evaporation. Under this condition, differences between MW and plant water isotopes are not paradoxical but rather related to the water that cannot be sampled with suction lysimeters but is still available for plant water uptake. The simulations accounting for fast and slow flow supported the conceptualization of the two soil pore domains with isotopic exchange via vapor exchange because this model setup resulted in the best model performance. Overall, these findings are of high relevance for current understanding related to the source and isotopic composition of water taken up by plants.Abbreviations: BW, bulk soil water; LMWL, local meteoric water line; MAE, mean absolute error; MW, mobile soil water; OPD, one-pore domain; PET, potential evapotranspiration; TPD, two-pore domain with isotopic exchange via vapor exchange; TPD_noex, twopore domain without isotopic exchange via vapor exchange; TWW, two water world.

Using StorAge Selection functions to quantify ecohydrological controls on the time-variant age of evapotranspiration, soil water, and recharge

“…These fluxes are dominated by vertical flows ( ) determined by the soil structure, and may contain diffusion ( ) between fast and 15 slow flow domains (Gerke and van Genuchten, 1993;Vogel et al, 2010;Sprenger et al, 2018), evaporation ( ), and rootuptake ( ). In many circumstances, unsaturated soil is assumed to have negligible lateral fluxes (e.g.…”

“…Modelling water and conservative tracer fluxes in soils is challenging due to subsurface heterogeneity, complex mixing, and diffusive exchange between faster and slower flow domains in respective larger and smaller soil pores (Gerke and van Genuchten, 1993;Vogel et al, 2010;Sprenger et al, 2018). The fast flow domain provides more rapid vertical fluxes 30 (downward) while the slow flow domain sustains storage and can exchange water with the fast flow domain in wet soils but Hydrol.…”

“…Improved characterization of soil fluxes, including the combination of more physically based modelling (e.g. Sprenger et al, 2018) may aid in understanding and properly explaining apparent ecohydrological separation, particularly with the observations in enrichment. Further aspects that require attention for soil SAS functions are the wettest soil moisture periods.…”

Water ages in the critical zone of long-term experimental sites in northern latitudes

“…Further, an assessment of the variability of water ages within different pore spaces (e.g. mobile versus more tightly bound water - Brooks et al, 2010;Good et al, 2015;Sprenger et al, 2018b;Smith et al, 2018) and with soil depth is still needed; this would also provide a test of the common assumption of a well-mixed system in tracer-aided modelling (van der Velde et al, 2015). Also, it has not yet been established how the ages of soil water storage and evaporation and transpiration fluxes are related to the variability of soil storage volumes.…”

Effects of climatic seasonality on the isotopic composition of evaporating soil waters

“…and Ehleringer, 1998;Rothfuss et al, 2015;Sprenger et al, 2017), quantifying groundwater recharge rates and mechanisms (e.g. Healy and Scanlon, 2010;McGuire and McDonnell, 2006;Sakakibara et al, 2017), quantifying root water uptake (e.g.…”