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Estimation of mean residence times of subsurface waters using seasonal variation in deuterium excess in a small headwater catchment in Japan
Authors:Naoki Kabeya  Masanori Katsuyama  Masatoshi Kawasaki  Nobuhito Ohte  Atsuko Sugimoto
Institution:1. Department of Soil and Water Conservation, Forestry and Forest Products Research Institute, PO Box 16 Tsukuba, Ibaraki 305‐8687, Japan;2. Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake‐cho, Sakyo‐ku, Kyoto 606‐8502, Japan;3. Graduate School of Environmental Earth Science, Hokkaido University, N10 W5 Sapporo, Hokkaido 060‐0810, Japan
Abstract:We measured deuterium excess (d = δD ? 8δ18O) in throughfall, groundwater, soil water, spring water, and stream water for 3 years in a small headwater catchment (Matsuzawa, 0·68 ha) in the Kiryu Experimental Watershed in Japan. The d value represents a kinetic effect produced when water evaporates. The d value of the throughfall showed a sinusoidal change (amplitude: 6·9‰ relative to Vienna standard mean ocean water (V‐SMOW)) derived from seasonal changes in the source of water vapour. The amplitude of this sinusoidal change was attenuated to 1·3–6·9‰ V‐SMOW in soil water, groundwater, spring water, and stream water. It is thought that these attenuations derive from hydrodynamic transport processes in the subsurface and mixing processes at an outflow point (stream or spring) or a well. The mean residence time (MRT) of water was estimated from d value variations using an exponential‐piston flow model and a dispersion model. MRTs for soil water were 0–5 months and were not necessarily proportional to the depth. This may imply the existence of bypass flow in the soil. Groundwater in the hillslope zone had short residence times, similar to those of the soil water. For groundwater in the saturated zone near the spring outflow point, the MRTs differed between shallow and deeper groundwater; shallow groundwater had a shorter residence time (5–8 months) than deeper groundwater (more than 9 months). The MRT of stream water (8–9 months) was between that of shallow groundwater near the spring and deeper groundwater near the spring. The seasonal variation in the d value of precipitation arises from changes in isotopic water vapour composition associated with seasonal activity of the Asian monsoon mechanism. The d value is probably an effective tracer for estimating the MRT of subsurface water not only in Japan, but also in other East Asian countries influenced by the Asian monsoon. Copyright © 2006 John Wiley & Sons, Ltd.
Keywords:deuterium excess  residence time  seasonal variation  stable isotope of water  hillslope hydrology  headwater
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