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

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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 18O 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 18O content in precipitation exhibits a distinct seasonal cycle with the highest (relatively enriched) values of c. −6% vs V-SMOW occurring in summer and the lowest (relatively depleted) values of c. −9% 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 18O 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.