Elsevier

Journal of Hydrology

Volume 587, August 2020, 124942
Journal of Hydrology

Research papers
Mean transit time and subsurface flow paths in a humid temperate headwater catchment with granitic bedrock

https://doi.org/10.1016/j.jhydrol.2020.124942Get rights and content

Highlights

  • We delineate flow paths and mixing in soil water, groundwater, and stream water in a small crystalline catchment.

  • Distribution of mean transit time of the catchment was evaluated using time series of stable isotopes.

  • Mean transit time was less than two years for soil water and stream water, and more than five years for groundwater.

  • Lateral flow through the soil–bedrock interface significantly contribute to stream discharge.

  • Mean transit time of the first-order streams was controlled by hillslope gradient and soil depth.

Abstract

Headwater catchments make a considerable contribution to the discharge of whole stream networks and mean transit time (MTT) is a fundamental factor affecting flow and storage in the hydrologic system. We estimated the MTT along with the identification of subsurface water flow processes using time series measurements of stable isotopes and major solutes in the water of a small mountainous catchment with granitic bedrock under a humid temperate climate. The MTT was estimated to be 1.1–1.7 years, 0.5–0.8 years, and 1.5 years for stream water at the catchment outlet and first-order tributaries, soil water, and shallow groundwater, respectively, based on sinusoidal variation in deuterium excess in the hydrologic components and precipitation. The MTT of first-order streams was negatively correlated with the topographic gradient of the sub-catchments. The SiO2(aq) and Cl concentrations in first-order streams were used to discriminate dominant subsurface flow contributing to the streams, depending on the topographic gradient. Soil water had a higher seasonal variability in SiO2(aq) and Cl concentrations, which were controlled by rainfall infiltration, whereas shallow groundwater had significantly lower seasonal variability, with a depleted isotopic signature, suggesting the contribution of lateral water flows from the upgradient area. The increased seasonal variation in SiO2(aq) and Cl concentrations with increasing MTT in first-order streams indicated that the storage capacity and flow paths in the soil layer were determining factors for subsurface water flow in hillslope areas. Seasonal variation in the SiO2(aq) concentration at the catchment outlet revealed a major contribution of subsurface flow to stream discharge as lateral flow through the soil–bedrock interface and an interflow through the soil layer in the dry and rainy seasons, respectively. Through the integration of the MTT and seasonal variation in hydrogeochemical parameters in soil water, stream water, and groundwater, this study demonstrates that soil depth and the soil–bedrock interface were the main controls on streamflow generation in a headwater catchment with a crystalline bedrock.

Introduction

Understanding the timescales of water flow, such as the mean transit time (MTT), in a catchment provides insight into the hydrological processes related to water flow generation through various pathways (McGuire and McDonnell, 2006). The MTT in the catchment also provides an integrated tool for understanding the mixing processes of hydrological components (McDonnell et al., 2010, Tetzlaff et al., 2011). This implies that tracer inputs mix with stored water at the catchment scale to facilitate the damping and time lag between inputs and outputs (Kirchner et al., 2000, Soulsby et al., 2009). The estimation of MTT provides fundamental information about the storage, source, and flow path of water, with advection and dispersion at the catchment scale (McGuire and McDonnell, 2006, Spence, 2000). Because the distribution of MTT reflects how catchments store and release water and solutes, it can be used to evaluate the response of hydrological components to climate change and the release or persistence of contaminants (Kirchner et al., 2000, McDonnell et al., 2010).

Mountainous regions are recognized as water towers that significantly contribute to sustainable water resources in adjacent plain and downstream areas (Cantafio and Ryan, 2014, Gomi et al., 2002). Recent studies have reported that more than 50% of the world’s potential water supply is supplied from mountainous regions, although the water supply may vary depending on the climate (Alexander et al., 2007, Viviroli et al., 2007). Numerous studies have evaluated hydrological processes based on the MTT in small mountainous catchments, including headwater streams (Freeman et al., 2007, Hofmann et al., 2018, Kabeya et al., 2007, Katsuyama et al., 2009).

Previous studies have attempted to understand streamflow dynamics in headwater catchments by identifying relationships in MTT scaling for mountainous catchments (McGlynn et al., 2003, McGuire et al., 2005, Soulsby et al., 2006, Tetzlaff et al., 2009). Such studies have attempted to determine the spatial distribution of MTTs linked to the water flow system by comparing the MTT and topographic indices, such as catchment area and topographical gradient. Several recent studies have investigated the spatial variation in MTT based on the storage, mixing, and release of a catchment, suggesting that MTT is mainly influenced by flow paths and storage, and depends on bedrock permeability (Asano and Uchida, 2012, Hale and McDonnell, 2016, Katsuyama et al., 2010, Pfister et al., 2017).

The estimation of MTT based on an isotopic tracer technique provides general information on the hydrological cycle in the catchment, but the ability to determine the proportional contribution and mixing of hydrological components is limited. In this regard, hydrogeochemical tracers can be used to assess the contribution of water sources and flow characteristics, complementing the interpretation of transit time distribution (TTD) at the catchment scale (Banks et al., 2011, Frisbee et al., 2011). A small number of studies have attempted to assess water flow processes in the headwater catchment using both MTT and hydrogeochemical parameters (Asano and Uchida, 2012, Mueller et al., 2013, Soulsby et al., 2006).

We studied the processes of water flow in a headwater catchment with a crystalline bedrock and their responses to changes in hydrologic conditions. Our ultimate aim was to contribute to the establishment of a water management policy involving headwater catchments. The specific objectives were to determine the MTTs of various hydrologic components, including soil water, stream water, and groundwater in a small headwater catchment with a granitic basement, using a time series of stable isotopic composition, and to delineate the flow paths, mixing, and relationships between the hydrological components by integrating the spatial distribution of the MTTs, and the spatial and temporal variation in isotopic and hydrogeochemical parameters.

Section snippets

Study area

The Donghak catchment in Gyeryongsan National Park is located in the mid-western part of the Korea Peninsula. It is a small mountainous catchment of about 3.75 km2, with a gradient of 31.6°. The altitude of the watershed ranges from 175 to 827 m above mean sea level (Fig. 1). Yongsucheon, as one of the major tributaries of the Geum River, is a perennial stream that originates from the catchment. The stream consists of four headwater streams and discharges into the Yellow Sea via the Geum River.

Stable isotope composition of hydrological components

The measured δ18O and δ2H values of 195 precipitation events collected at the three stations with different altitudes could be grouped along the summer local meteoric water line (LMWL) and winter LMWL based on the differences in the d-excess (Fig. 2). The average δ18O and δ2H values of all precipitation events were −7.9‰ and −47.8‰, respectively, and had a higher standard deviation than the other hydrological components (Table 1). The weighted average values of δ18O and δ2H in all precipitation

Identification of water flow dynamics using MTT

The MTT of hydrological components in a catchment are controlled by various parameters, such as land use, soil type, vegetation, bedrock, and topography (McGuire et al., 2005, Rodhe et al., 1996). McGuire and McDonnell (2006) reported that soil depth, hydraulic conductivity, and immobile/mobile zones have most effect on transit time, depending on the catchment scale. The estimated MTT of stream water in this study was slightly lower than 2 years, which is the average MTT in crystalline

Conclusions

Time series measurements of stable isotope composition and conservative solutes (Cl and SiO2(aq)) in water samples were used to evaluate water flow processes in a small mountainous catchment with granitic bedrock based on the MTT and seasonal variation in hydrogeochemical parameters. The MTT of soil water ranged from 0.5 to 0.8 years, and increased with increasing soil depth. The Cl concentration in soil water was higher in the dry season due to evapotranspiration and lower in the rainy

CRediT authorship contribution statement

Youn-Young Jung: Investigation, Formal analysis, Data curation, Writing - original draft. Dong-Chan Koh: Conceptualization, Methodology, Writing - review & editing. Jeonghoon Lee: Conceptualization, Methodology, Writing - review & editing. Maki Tsujimura: Writing - review & editing. Seong-Taek Yun: Conceptualization. Kwang-Sik Lee: Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was supported by a National Research Council of Science & Technology (NST) grant from the Ministry of Science and ICT of the Republic of Korea (MSIT) (grant no. CAP-17-05-KIGAM), and the Basic Research Project (19-3411) of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and ICT. We thank Gyeryongsan National Park office of the Korea National Park Service (KNPS) for permission to install monitoring stations at the field site and

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      The cold spring water was expected to mainly pass through the weathered zone, which is the boundary between the soil layer and bedrock. In this regard, Jung et al. (2020) reported that shallow groundwater moving through the weathered zone has a short transit time of about 1.5 years in crystalline mountainous catchments in South Korea. In general, shallow groundwater is more vulnerable to human alterations than old, deep groundwater (Fan, 2016; Kim et al., 2019).

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