A stable isotope study of soil water: evidence for mixing and preferential flow paths

Oxygen isotope compositions of precipitation and soil water from profiles at six sites near Hanover, NH reveal information about rates and mechanisms of soil water movement. The O-18-enrichment I of shallow (< 20 cm depth) soil water, observed in most soil profiles, is probably due to evaporation. Isotopic fronts observed shortly after a relatively large, low-O-18 rainfall event suggest piston-type flow in the shallow soil accompanied by significant mixing with stationary soil water. In contrast, deep (>50 cm depth) soil water is only replaced during snowmelt or by substantial rainfall events and so residence times of this water can be several months or more. There is evidence that some of the water from smaller rainfall events bypasses the deep soil through preferential flow paths. A comparison of profiles from two sites on the same hill with different soil textures and moisture contents suggests that the proportion of immobile soil water and thus the amount of preferential flow can vary on a small scale depending on local soil characteristics. The observed soil water processes may have significant effects on plant water use, which is relevant for interpretations of isotopic tree-ring data as a record of paleoclimate. (C) 2003 Elsevier B.V. All rights reserved.