The mechanism of soil water movement as inferred from ¹⁸O stable isotope studies

Abstract

The seasonal movement of soil water through a deep vadose zone of a sandy hill was studied in the central part of The Netherlands. The behaviour of soil water was tracked by monitoring soil water fluctuations and changes in the environmental tracer lS 0 in vertical profiles at test sites with grassland, heathland and forest. In the temperate climate of the study area a soil moisture deficit develops during summer and a precipitation surplus prevails during winter. The 18 0 content in precipitation exhibits a distinct seasonal cycle with the highest (relatively enriched) values of c. -6%o vs V-SMOW occurring in summer and the lowest (relatively depleted) values of c. -9%o vs V-SMOW in winter. The influence of this cycle can be traced back in the subsurface to a depth of sometimes more than 6 m. This seasonal fluctuation moves around the average annual 18 0 content in rainfall, tending closer to the average as depth increases. In the saturated zone the same average value is observed. This suggests that, although the precipitation surplus is limited to the winter period, part of the summer precipitation also infiltrates to below the root zone and contributes to the groundwater recharge. The results of this study indicate that preferential flow dominates in the root zone. Below the root zone of low homogeneous vegetation covers, the soil water movement is dominated by diffuse flow. In forests, a more heterogeneous root pattern causes a more preferential type of flow, also at greater depths below the root zone.Mécanisme du mouvement des eaux souterraines d'après les conclusions des études de l'isotope stable ls O Résumé Nous avons réalisé une étude du mouvement saisonnier des eaux souterraines à travers une zone vadose profonde d'une colline sableuse, au centre des Pays-Bas. Le comportement des eaux souterraines a été suivi en contrôlant les modifications du traceur environnemental 18 0 selon des profils verticaux sur des sites d'essai situées dans des prairies, des landes et des forêts. La teneur en 1S 0 des précipitations présente un cycle saisonnier propre, avec des valeurs maximales (relativement enrichies) d'environ -6%o par rapport au V-SMOW l'été, et minimales (relativement appauvries) de -9%o par rapport au V-SMOW l'hiver. L'influence de ce cycle peut être retrouvée dans le sous-sol jusqu'à une profondeur parfois supérieure à 6 m. Cette fluctuation saisonnière est proche du pourcentage annuel moyen de 18 0 dans les précipitations, tendant à se rapprocher davantage de la moyenne lorsque la profondeur augmente. Dans la zone saturée, on observe la même valeur moyenne ce qui suggère qu'une partie des pluies estivales s'infiltre sous la couche d'enracinement et contribue à la réalimentation des nappes phréatiques, bien que l'excédent de précipitations se limite à la période hivernale. Il ressort des résultats de cette étude que l'écoulement préférentiel est dominant dans la couche

A porewater-based stable isotope approach for the investigation of subsurface hydrological processes

“…This is an indication of a weak isotopic enrichment of some pore water samples. Since soil evaporation is dependent on atmospheric boundary conditions and soil characteristics (Gonfiantini et al, 1998) the ob- served enrichment of heavy isotopes may have occurred by evaporation of soil water in the uppermost soil layers. Most care was taken to avoid kinetic fractionation during sampling and transport, still a weak secondary enrichment cannot be strictly excluded.…”

“…In the absence of kinetic fractionation this variability determines the atmospheric boundary condition for hydrological stable isotope studies at the plot scale. The propagation and attenuation of the seasonal isotope signal with increasing soil depth allows for an investigation of vertical water movement in the unsaturated zone at the plot scale (Zimmermann et al, 1967;Maloszewski et al, 1983;Stewart and McDonnell, 1991;Gehrels et al, 1998). Vertical stable isotope pore water profiles in the unsaturated zone have also been used to investigate recharge mechanisms and recharge rates in the vadose zone (e.g.…”

Temporal dynamics of soil water balance components in a karst range in southeastern Spain: estimation of potential recharge

“…In nearby areas with similar climate, the maximum E occurs in May (Domingo et al, 1999). The seasonal pattern of variation in y, high in spring and late autumn and characteristically low in summer when plants actively take up soil water, is well known (Henninger et al, 1976;Ehrenfeld et al, 1997;Gehrels et al, 1998).…”

Partitioning of evaporation into transpiration, soil evaporation and interception: a comparison between isotope measurements and a HYDRUS-1D model

“…We assume that the net precipitation that infiltrates into the soil is not affected by isotope fractionation. A study from Gehrels et al (1998) also showed that interception will not play a significant role in isotope fractionation for lower vegetation types. Thus, the net precipitation has the same isotope signature as precipitation.…”