[1] To investigate the effects of bedrock permeability on the linkage between hillslope and riparian groundwater in a weathered granite headwater catchment, the groundwater dynamics were studied using intensive hydrometric and tracer observations. Water flow from the hillslope, through the hillslope/riparian interface, and into the riparian zone consists of two components: saturated through flow on the soil-bedrock interface during storms and groundwater flow within the permeable bedrock occurring year-round, except during the driest season. Most of the water, which will contribute to the stream, infiltrates the largest part of the catchment and the hillslope area and recharges the deeper groundwater body. Therefore bedrock permeability is an important factor in determining the hillslope-riparian linkage.Citation: Katsuyama, M., N. Ohte, and N. Kabeya (2005), Effects of bedrock permeability on hillslope and riparian groundwater dynamics in a weathered granite catchment, Water Resour. Res., 41, W01010,

Abstract:We measured deuterium excess (d D υD 8υ18 O) in throughfall, groundwater, soil water, spring water, and stream water for 3 years in a small headwater catchment (Matsuzawa, 0Ð68 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: 6Ð9‰ 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 1Ð3-6Ð9‰ 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.

We used stable isotope techniques to investigate water utilization of two native trees, Sabina vulgaris Ant. and Artemisia ordosica Krasch., and one introduced tree, Salix matsudana Koidz., in the semiarid Mu‐Us desert, Inner Mongolia, China. The study site was in a region where there has been a decline in agricultural productivity, caused by severe desertification over the past several decades. S. matsudana is used extensively for reforestation to protect farms and cultivated lands from shifting sand dunes. We identified water sources for each tree species by comparing the stable isotopes δD and δ18O in water in stems, soil, and groundwater. We also measured δ13C levels in leaves to evaluate the intrinsic water‐use efficiency (WUE) of each plant. Comparison of isotopes showed that S. vulgaris and S. matsudana consume relatively deep soil water as well as groundwater, whereas A. ordosica uses only shallow soil water. The δ13C measurements indicated that S. vulgaris has exclusively high WUE, whereas that of the other species was typical of temperate‐region C3 plants. The water source data plus WUE data suggest that planted S. matsudana uses groundwater freely, whereas native plants conserve water. Thus, reforestation with S. matsudana might cause irreversible groundwater shortages. Corresponding Editor: E. A. Holland.

[1] Tracer approaches have been used worldwide to clarify time and geographic sources of stream water in catchments, although the relationship between these two sources is poorly discussed. We considered the mean residence time (MRT) and its spatial distribution to determine the relationship between geographic source components and time source components. There were clear differences in solute concentrations and MRT among shallow, middle, and bottom layers along the vertical profile of the riparian groundwater body. Those in the stream water were intermediate compared to those in the shallow and middle layers; thus, the consistent geographic sources were groundwater in these layers. In the context of end-members mixing analysis (EMMA), however, the end-members were rainfall, hillslope groundwater, and riparian groundwater in the bottom layer. The other riparian groundwaters were a mixture of end-members. The discrepancy between the geographic sources and the end-members was resolved by considering the MRT, the time required to move the end-member or geographic source within the catchment. Our approach clarified the relationships among the geographic sources, time sources, and hydrological pathways, which are the essential factors of runoff generation processes and hydrochemical processes. Therefore, to go beyond previous applications of EMMA on the basis of systematic learning from observed data, it is insightful to combine the common approaches to analyze landscape heterogeneity and process complexity and to reconsider the framework of the hydrobiogeochemical models in various regions and at multiple spatial scales.Citation: Katsuyama, M., N. Kabeya, and N. Ohte (2009), Elucidation of the relationship between geographic and time sources of stream water using a tracer approach in a headwater catchment, Water Resour. Res., 45, W06414,

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