Hydrochemical and isotopic techniques have been widely applied in
hydrological sciences because isotopic tracers can identify water sources
and hydrochemical tracers can discern runoff flow paths. To better
understand the hydrological process, we combined hydrochemical and isotopic
techniques under controlled experimental conditions to investigate
hydrological process from rainfall to runoff in the Hydrohill experiment
catchment, a typical artificial catchment in Chuzhou, China. Hydrochemical
and isotopic data, i.e., pH, electric conductivity (EC), total dissolved
solids (TDS), anions (Cl-, NO3-, SO42- and
HCO3-), cations (K+, Na+, Ca2+ and Mg2+) and
dissolved Si, 18O and D in water samples were collected during a
rainfall event in 2016, and used to determine the hydrochemical and isotopic
characteristics of rainfall and runoff components. We applied EC, TDS,
SO42-, Ca2+, Mg2+, 18O and D as tracers to
investigate rainfall-runoff processes in the experimental catchment. Runoff
flow paths could be well identified by the relationship between 18O and
EC, TDS, SO42-, Ca2+ and Mg2+. The quantity of flow flux
and mass fluxes of main hydrochemical and isotopic tracers gauged at the
catchment outlet shows applicable tracers (Ca2+, Mg2+,
SO42-, and 18O) are mainly from deep groundwater runoff (from
soil layer of 60–100 cm beneath ground surface). Contributions of the event
water and pre-event water to the total runoff during the rainfall-runoff
process are different. The quantitative results were very encouraging as a
basis to develop hydrological models for further study.
Introduction
Runoff sources and their pathways play a significant part in runoff
generation and flow concentration, and thus have been widely studied during
the past decades (Zhao, 1989; Gu et al., 2018). Variations of temporal and
spatial dynamics inside the catchment indicate different flow paths and
water sources contributing to the total runoff outlet. Hydrochemical and
isotopic processes of total runoff outlet are mainly affected by the solutes
sources, hydrological pathways and the flow components contributions (Yang
et al., 2012). Hydrochemical tracers (Uhlenbrook et al., 2002; Tardy et al.,
2004) and stable isotopes (Carey et al., 2004) widely applied in
hydrological science have enhanced our understanding of runoff generation
processes. Environmental tracers are commonly used on a catchment scale
because it is possible to determine the source areas of runoff, flow
pathways, residence times and the hydraulic characteristics of flow systems.
Hydrochemical indexes such as pH, electrical conductivity (EC), total
dissolved solids (TDS) or the concentration of dissolved silicon and various
anions and cations (including K+, Na+, Ca2+, Mg2+,
Cl-, SO42- and HCO3-) have been used to separate
and quantify runoff components, so that the contributions of different
runoff components can be determined and the hydrological pathways and the
solutes sources can be investigated (Pilla et al., 2006; Mul et al., 2008).
Several methods, such as chemical/isotope (tracer) hydrograph separation,
rainfall-runoff models and base flow filters, have often been applied to
quantify runoff components (Gonzales et al., 2009; Cartwright et al., 2014).
Hydrograph separation combining hydrochemical and isotopic tracers is a
method commonly used to identify the origin and pathways of surface and
subsurface runoff and thus reveal the mechanisms of runoff generation
(Hooper et al., 1986; Klaus et al., 2013). For example, quick flow,
which contributes to the total outlet immediately after the rainfall event,
can include water from different sources (Hrachowitz et al., 2011;
Cartwright et al., 2014). More studies have used two-component mixing
models, three-component mixing models and multi-component mixing models to
distinguish pre-event water with event water (Huth et al., 2004;
Hugenschmidt et al., 2014) and identify water sources and flow paths, in
which isotope tracers (18O and 2H) are applied in conjunction with
geochemical tracers (Vincent et al., 2001; Brian et al., 2010; Stadler et
al., 2012). Mixing models can enhance the accuracy of research about
hydrological processes. The results obtained by separation techniques based
on the mass balance approach suggest a significant contribution of pre-event
water to catchment runoff, as documented in the majority of studies
performed worldwide (Klaus et al., 2013). For example, streamflow components
were determined by analyzing 18O, 2H, SO42-,
NO3-, Cl-, Na+, K+, Ca2+, Mg2+, EC and
other parameters mainly focusing on the temporal and spatial variations of
the sources which come from precipitation, stream waters, soil solution and
spring water. 18O and Si were finally used to evaluate contributing
sources by using mass balance equations and end-member mixing diagrams
(Ladouche et al., 2001). Based on hydrochemical tracers, a three-component
hydrograph separation was carried out in a steep, remote and
monsoon-dominated study site (7 km2) in northern Thailand (Hugenschmidt
et al., 2014). In the study, silica and EC, K+ and Ca2+ were used
as indicators for fractions of runoff components, surface runoff dynamics,
and to give a better understanding into groundwater behavior. In the study,
groundwater accounted for the largest contribution to stormflow (62 %–80 %)
throughout all events, followed by shallow subsurface flow (17 %–36 %) and
surface runoff (2 %–13 %). Thus, calculating the contributions of different
runoff components to the total runoff is quite important for accurate
predictions of water supply from mountain watersheds (Zhang et al., 2018).
Water delivery to the catchment outlet can be affected by variations of flow
paths connecting the residence time variations of water inside the catchment
(Hrachowitz et al., 2016). Surface water and subsurface water contributions
to the whole catchment and the inherent flow paths indicate the details of
hydrological processes. Hydrochemical data and stable water isotopes can be
used in mass balance to gain an understanding of source water and flow paths
in complex terrain (Robinson et al., 2009; Williams et al., 2011). End-Member
Mixing Analysis (EMMA) is a hydrological mixing model that can be used to
conceptualize runoff generation processes and has been successfully applied
to many catchments around the world (Maurya et al., 2011; Baraer et al.,
2015). For example, Zhang et al. (2018) utilized hydrochemical tracers from
2009 to 2011 to statistically model streamflow sources within a mesoscale
(264 km2) watershed. Snowmelt and rainwater from the subalpine zone and
groundwater sampled from the upper montane zone contributed 54 %, 22 %,
and 24 % of the annual streamflow, respectively, which contribute to a
better understanding of streamflow source waters in complex mountain
terrain.
More controlled experiments are needed to seek new fundamental understanding
through new mechanistic experimentations of how watershed systems work.
Field hydrological experiments at the Hydrohill experimental catchment at
the Chuzhou hydrological experiment base have improved the basic theory of
hydrology. For example, it has been confirmed that the runoff components
include surface runoff, interflow in the unsaturated zone and groundwater
runoff in the saturated zone and their runoff generation mechanisms (Liu et
al., 2018a, b). However, a further study examining the contributions and
water residence time of different runoff components to the total outlet of
the catchment is still required. Moreover, understanding of characteristics
of tracers in rainfall-runoff processes is also quite necessary and
important. Kendall et al. (2001) investigated the relations between Cl-,
Si, and δ18O of subsurface waters and rainfall at Hydrohill to
identify event water and pre-event water contributions to the outlet of the
catchment, and found the Cl- and Si in quick-flow runoff originated
from event water were not regarded as suitable conservative tracers for
either water sources or flow-paths in this catchment. However, due to the
advanced instruments in this research for testing more tracers, such as
conventional indexes, cations and anions, we have more choices.
Using data from a rainstorm in October 2016 at the Hydrohill experimentation
area in Eastern China, the paper aims to use the tracer analysis approach to
investigate the hydrological processes and their relationship with both
hydro-chemical and isotopic processes to get a better understanding of the
interaction mechanism between different fluxes in the critical zone. The
main objectives of this work are (i) to analyse the characteristics of
tracers and select the applicable ones that could be more effective in
describing hydrological processes, (ii) to test the effectiveness of
combining isotopic and hydrochemical indicators in identifying runoff
pathways, (iii) to investigate the mass fluxes of selected tracers in
different runoff components to the total outlet.
Materials and methodsStudy site
The study was performed in the 512 m2 Hydrohill artificial
experimental catchment, located in Chuzhou, southeastern China. The bottom
consists of two intersecting slopes that are inclined at 10∘ and
have an overall downslope gradient of 14∘ (Fig. 1c). Impermeable
concrete walls were set up on this aquiclude with the final objective to
enclose the catchment to prevent any lateral exchanges of underground flow.
The silt-loam soil removed before excavation was put back to a depth of
approximately 1 m. The surface of the basin was covered by grass and a
central drainage trench was constructed at the intersection of the two
slopes after three years of soil subsidence and a water-sampling instrument
was installed (Gu and Freer, 1995). The rainfall is unevenly distributed during
the year, with June, July and August accounting for 49 % of the annual
rainfall. The mean annual precipitation is 1060 mm (period 1952–2016), and
the average annual temperature is 14.9∘.
Aerial view of the Hydrohill experimental catchment and measurements
of runoff components, (a) aerial view of the Hydrohill catchment with
observation devices, (b) contour line on the Hydrohill catchment, (c) and
(d), runoff components measuring system at the outlet of the Hydrohill
catchment. SR: surface runoff; SSR30, SSR60, and SSR100: subsurface runoff
from soil layers of 0–30, 30–60, and 60–100 cm in depth.
This experimental catchment is equipped with a separate runoff observation
system, which can monitor surface runoff and subsurface runoff from soil
layers with depths of 0–30, 30–60,
60–100 cm (inferred as SSR30, SSR60, and SSR100) (Fig. 1b).
Each trough has a 20 cm aluminum lip that extends horizontally into the soil
layer to prevent leakage between layers. Hydrological cycling factors
observation devices including tipping bucket-type micro pluviometers, rain
cylinders, standard water collectors, soil moisture sensors, pressure water
level gauges were installed at the Hydrohill catchment (Fig. 1).
Data collection
Rainfall data was measured by the four rain gauges located in the lower and
upper parts of the catchment. The runoff data for surface water (SR), SSR30,
SSR60, and SSR100 and the total runoff (TR) can be measured by the five
90∘ sharp crested V-notch weirs and logarithmic weirs at the
outlet of Hydrohill (Fig. 1d). Continuous measurements were taken by using
pressure-type stage recorders.
Sample collection and analysis
During 25–29 October 2016, 125 samples were collected at the catchment.
These included rainwater, surface water and subsurface water. Rainfall
samples were collected through a rainfall water collector with a diameter of
40 cm located on the observation room roof. The rain immediately arrived at
the observation room through a pipeline. Rainfall samples were collected
every hour during the rainfall event. Section negative pressure sampling was
used to collect surface runoff and subsurface runoff samples. When there
were typical points like peak points and valley points on the hydrograph, we
started to collect water samples. Samples were stored in 50 mL high-density
polyethylene (HDPE) bottles. HDPE bottles were washed by deionized water and
rinsed with sample water three times at the time of collection. All samples
were filtered through a 0.45 µm filter and then sealed before
laboratory analysis. All the samples were transported to the laboratory in
Nanjing and preserved at 4∘ until analysis.
All samples were analyzed for general parameters (pH, EC), major ions
(K+, Na+, Ca2+, Mg2+, Si, Cl-, NO3-,
SO42- and HCO3-), and stable isotopes (18O and
2H). pH and EC were measured in the field using a portable EC digital
analyzer (HQ14d, Hach, USA) and a multi-parameter digital analyzer (HQ40d,
Hach, USA). The hydrochemistry of all water samples was analyzed within two
weeks of collection. Cations (K+, Na+, Ca2+ and Mg2+)
and dissolved Si were analyzed by Inductively Coupled Plasma Optical
Emission Spectrometry (ICP-OEC), and the anions (Cl-, NO3- and SO42-) were analyzed by Dionex ICS-2100 Ion Chromatography.
All samples were filtered through a 0.45 µm filter before laboratory
analysis. The concentration of HCO3- was determined by a titration
assay on-site. 18O and 2H analyses were carried out by the
4th generation Los Gatos Research Liquid Water Isotope Analyser and
expressed according to the Vienna Standard Mean Ocean Water (VSMOW).
Hydrographs of the separate runoff in the 2016 autumn rainfall event.
Results and discussionHydrological response in rainfall events
This rainfall event happened from 23:00 on 25 October to 12:00 on 28 October
and can be divided into three major rainfall periods (Fig. 2). Because the
whole depth of Hydrohill is 1m and the groundwater level is higher than 40 cm
during this rainfall, SSR100 is regarded as groundwater runoff. This event
was one of the biggest storms following a low-intensity rainfall event. The
general hydrological characteristics of these floods, including rain volume,
the maximum intensity in 1 h, mean intensity and peak discharge in separate
runoff and total runoff, are concluded in Table 1.
Principal hydrological characteristics of the flood event in 2016.
The first rainfall period occurred from 23:00 on 25 October to 23:00 on
26 October and the daily rainfall was 54 mm. Because there had been no recent
storms, and the soil before the event was dry, there was a minor flow
response. The discharge of the surface flow was nearly the same as the
discharge of SSR30 and SSR60, all of which were smaller than SSR100. This
indicates that the rainfall water infiltrated quickly and there was probably
25 % existing pre-event water. The second period was the largest and was a
complex process among the three periods which occurred between 07:00 and
17:00 on 27 October . The runoff response was so quick due to the high rain
volume. SR shows the quickest response and the volume of SSR100 was also the
highest in all runoff components, with a little longer residence time than
others. The third period which occurred during the whole day of October 28
was the shortest period. Though its process was also complex, the runoff
response was slow and the SSR100 remaining was the largest runoff volume.
The total runoff efficient (V/R ratio) in Table1 showed that more than
60 % of the rainfall had infiltrated the soil reaching the total outlet of
the catchment and the V/R ratio of total runoff in the third period was the
biggest (nearly 1.0) in all three rainfall periods, indicating that soil had
almost been saturated. In the three successive periods, the V/R ratio of SR,
SSR30 and SSR60 in the second period was bigger than that in the other two
periods. However, the V/R ratio of SSR100 in the three periods was higher
than that of SR, SSR30 and SSR60, indicating that other than the infiltrated
water, existing water before this rainfall also contributed to the total
runoff.
Hydrochemical Process
In this research, we analysed many more chemical parameters than previous
studies to find more suitable tracers. The statistical results, including
maximum values, minimum values and mean values of all indicators in this
rainfall event, are shown in Table 2.
Results of the hydrological and isotopic analysis for various
samples.
All three rainfall periods in this study showed vertical features of
chemical components. The pH values of most samples ranged from 7.06 to 8.92,
with an average value of 7.82, indicating an alkaline nature. The average pH
values were 8.44 in rainfall, 7.58 in surface runoff, 7.72 in SSR30, 8.07 in
SSR60, 7.95 in SSR100. The values of EC in water samples ranged from 9.81 to
394.00 µS cm-1 in this rainfall. Average EC values were
17.86 µS cm-1 in rainfall, 37.28, 51.42, 118.11 µS cm-1 in SSR30, SSR60 and SSR100, respectively. TDS is
often calculated by adding together all the major cations: Na++K++Ca2++Mg2+, and major anions: HCO3-+SO42-+Cl-+NO3- in mg L-1 (Walton,
1989). The lowest TDS value was recorded in the rainfall and the highest
ranged between 76.65 and 197.77 mg L-1 found in SSR100. The TDS was
13.63–28.64 mg L-1 for surface water, 19.72–41.22 mg L-1 for SSR30 and
27.22–117.51 mg L-1 for SSR60. Average TDS values were 6.72 mg L-1 in rainfall,
20.40 mg L-1 in surface runoff, 28.03, 53.59 and 123.10 mg L-1 in SSR30, SSR60
and SSR100, respectively. So we can find that when the flow path is longer,
both the TDS and EC values will show an increasing trend from rainfall to
SSR100.
Processes of EC, SO42-, Mg2+ and Ca2+ in
different runoff components. TR: total runoff.
During the observation periods, the main conventional physical and chemical
indicators (EC, TDS), and main ions (Mg2+, Ca2+ and
SO42-) showed similar characteristic patterns, i.e., with the
depth of the water flow path increasing, hydrochemical tracers increased
gradually (Fig. 3). Even though there are similar characteristics with
Mg2+, Ca2+ and SO42- at some stage, it is still not
certain whether K+,Na+, Cl-, and HCO3- are
applicable to hydrological process research. These indicators should be
studied further. In addition, there are no apparent rules in values of pH,
NO3- and dissolved Si, which cannot be regarded as applicable
tracers. According to the characteristics of all the chemical tracers, three
groups of the chemical behavior of the catchment discharge can be identified
at the catchment outlet:
Group I: EC, TDS, Ca2+, Mg2+ and SO42-, which can be
used as good tracers to identify runoff paths;
Group II: K+, Na+, Cl-, and HCO3-, which are less
applicable as tracers;
Group III: pH, NO3-, and dissolved Si, which are not suitable as
tracers.
The patterns of concentrations in all the runoff components reflect the
variable contributions of flow components to the catchment outlet according
to the main origin of the chemical elements. The measurement of conductivity
reflects the ion concentration in water (Hendrickx et al., 2002). In
reverse, the increase of conductivity in each separate runoff explained the
process of runoff components. During the migration process, the TDS of four
runoff components originating from gas precipitation, dissolved solid
precipitation, dissolved salts and ion exchange gradually rose (Gibbs,
1970).
SO42- originates essentially from the atmosphere but is
concentrated within the catchment by evaporation processes and storage in
soils (Liu, 1970). Ca2+ and Mg2+ are more affected by atmospheric
sources and mineral weathering within the soil (Mcdonnell et al., 1990).
Cl- would not form an insoluble salt with the main cations, so it is a
stable component and is suitable to be a tracer (Shen, 2010). During the
runoff process, Cl- will accumulate gradually with the growth of runoff
path (Ladouche et al., 2001); however, in this study, Cl- didn't
reflect the same pattern as other studies, and the change of Cl- is a
complex mixing process. This finding is consistent with early work at the
site by Kendall et al. (2001). The time-process changes of Ca2+,
Mg2+ and SO42- were consistent, indicating that the controls
on hydrochemical ions were uniform. Gu et al. (2007) previously found that
the time-process changes of hydrochemical ions in runoff were similar to
that in rainfall. This led to the conclusion that rainfall plays a leading
role in this change; however, it is not the only source because the ion
concentration gradually increases, which can also be affected by pre-event
water. In addition, hydrochemical ions can be the result of biological
processes and water-rock interaction in the soil, contributing to the
increase of hydrochemical ions. So, it is possible that there is a mixture
of pre-event water and event water in this rainfall, and a mixture of upper
layer runoff infiltration and lower layer runoff in space.
Isotopic compositions of waters
In this study area, there was a lack of hydrogen and oxygen data in
multi-year rainfall, but the data for Nanjing was adequate, allowing the
local meteoric water line in Nanjing to be used in this area. Rainfall, SR,
SSR30, SSR60 and SSR100 of this storm samples are distributed along the
local meteoric water line (Fig. 4) in a δ18O-δD
relation diagram (Tan et al., 2009):
δD=8.4δ18O+17.71
which means that interflow water and groundwater are not influenced by the
evaporation process (McDonnell et al., 1990). The rain seeps through the
soil and the unsaturated zone and mixes with non-evaporated water from
pre-event water (Ladouche et al., 2001).
Relationship between δ18O and δD in rainfall,
SR, SSR30, SSR60 and SSR100. GMWL: Global Meteoric Water Line (Craig, 1961);
LMWL: Local Meteoric Water Line (Tan et al., 2009).
During the above three rainfall periods, the sampling of SR, SSR30, SSR60
and groundwater runoff showed a common isotopic characteristic (Fig. 5): a
negative correlation between discharge and δ18O. The temporal
isotopic content of δ18O in all the runoff components had
tended to return to the initial value during the first recession stage,
during which the δ18O sampling of groundwater was the richest
and that of SR and SSR30 the most depleted, indicating that SR, SSR30, SSR60
may have been partially updated and diluted by the event water while there
was still a large part of the pre-event water in the groundwater runoff. At
the start of the rainfall, there was infiltration of depleted rainfall into
the soil layer with enriched 18O, and rainfall and soil water mixed.
However, due to the combination of event water and the water in the soil,
the values of δ18O in SR, SSR30 and SSR60 became similar, which
was apparent in the third period. As a result, the proportion of pre-event
water became smaller as event water infiltrated. From Fig. 5, we can find
that even though the rainfall volume of the second period was the largest,
the difference between all the runoff components was smaller than that in
the first stage, which indicated that the combination of rainfall and the
soil water was almost over. The contributions of event water (rainfall) and
pre-event water (soil water) during the rainfall event will continue to be
studied in the following research. In this study, due to the linear
relationship between δ18O and δD, δD shows
consistent feature with δ18O.
Processes of total discharge and δ18O in different
runoff components.
Identification of rainfall-runoff flow paths
From the relationship between δ18O and different hydrochemical
indexes (Fig. 6), we can find a linear relationship between δ18O
and EC, TDS, SO42-, Ca2+ in different runoff and can identify
the runoff path, including surface runoff and subsurface runoff. In detail,
δ18O and EC, TDS, SO42-, Ca2+ had a negative
relationship in surface runoff and SSR30 and showed a positive
correlation in SSR60 and SSR100, and the values of hydrochemical indicators
increased at depth.
Relationship between δ18O and different hydrochemical
indexes.
In previous studies, Gu (1992), Gu and Freer (1995), and Gu et al. (2018) concluded that there are
three runoff components including surface runoff, interflow in the
unsaturated zone and groundwater runoff in the saturated zone at the
Hydrohill catchment and obtained runoff production methods combined with
hydrochemistry and isotope tracing. The processes of hydrochemitry and
18O in rainfall and different runoff components are related to its
cause, flow paths, rainfall volume and intensity, etc. Hydrochemical ions
and isotopes can enter the catchment with precipitation and flow through
surface runoff and subsurface runoff to the total outlet. During this
process, biological interaction and water-rock interaction take place. The
bottom of Hydrohill is closed, and the final water source for runoff
generation is rainfall. Though rainfall plays a leading role in the
generation of runoff, it does not play a dominant role in hydro-geochemical
components (Gu et al., 2007). The composition of hydrochemical and isotopic
tracers in Hydrohill can vary due to the interaction between water and soil.
Thus, tracers can be effective tools to identify flow pathways in
rainfall-runoff processes.
Concentrations of K+, Na+, Cl-, HCO3- in
different runoff components for the rainfall-runoff events (The asterisk
represents an outlier).
Hydrological and isotopic flux
During the three consecutive rainfall periods, surface runoff and subsurface
runoff meet at the total outlet section. Meanwhile, the hydrochemical and
isotopic indicators in the surface runoff and subsurface runoff make
separate contributions to the total outlet output. Thus, the quantity of
flow flux and mass fluxes of hydrochemical and isotopic indicators outputted
from surface runoff, SSR30, SSR60, SSR100 and total runoff can be calculated
by Eqs. (2) and (3),
2R=∫t1t2Qdt3M=∫t1t2Qcdt
where R is the quantity of flow flux from runoff components and M is the
mass fluxes of hydrochemical and isotopic indicators outputted in the
runoff; t1 and t2 represent two moments, respectively, Q is the
quantity of flow at one sampling moment; c is the concentration of the
solutes. The results of their contribution ratios for the total runoff in
different runoff components in three rainfall events are shown in Table 3.
The contribution rate of the selected indexes during the
hydrological periods.
In this rainfall event, runoff can mainly generate from SSR100 (groundwater
runoff), contributing much more than SR, SSR30 and SSR60. Meanwhile, the
mass fluxes of Ca2+, Mg2+, SO42-, and δ18O
indicated that subsurface runoff contributed more to the total outlet,
especially from groundwater runoff. As a result, the quantity of flow flux
and mass fluxes of hydrochemical and isotopic indicators in groundwater
runoff showed a positive relationship. The SR, SSR30 and SSR60 also showed a
positive relationship, except that Ca2+ in SSR30 was smaller than
SSR60, having an opposite relationship with the quantity of flow flux in
SSR30 and SSR60. Even though different flow pathways existed in different
rainfall events, in recent years, the hydrological community has come to a
consensus that subsurface flow is generally dominated by the preferential
flow of various kinds (Jones, 2010; Lin, 2010; Uhlenbrook, 2010). Subsurface
preferential flow can route water through the subsurface, rather vertically
and laterally. In addition, the contributions of Ca2+, Mg2+,
SO42- and δ18O from SR, SSR30, SSR60 and SSR100 to
the outlet in the three events indicated that runoff components originate
from the mixing of event water and pre-event water, which is consistent with
results of Kendall's study (Kendall et al., 2001). During the
rainfall-runoff processes, different proportions of pre-event water have
been updated by a different percentage of event water, but the detailed
percentage of updated water was not calculated in this research.
Discussion
In this study, the characteristics of hydrochemical and isotopic factors in
one typical rainfall event were analyzed. Through the analysis, flow
pathways and main water sources reaching the total outlet could be
identified. In Fig. 7, there was an outlier among the values of almost all
the indicators, and the outlier of K+ in SSR60, Na+ in SR,
dissolved Si in SSR60 and Cl- in SSR30 exceeded much more than the
upper limit. The uncertainty of the values may be mainly due to a
measurement error or sampling error. Thus it is a little difficult to
identify whether they can be good tracers to analyze the hydrological
processes and they are classified in the second group and the third group.
There is a need to collect more water samples from different rainfall events
to determine this.
Studying hydrological processes enables us to know more about the flow
paths, residence time distribution, and spatial distribution patterns of
soil (Keith et al., 2010; Spence et al., 2010). This also means that
hydrological processes are closely related to soil properties. For example,
soil pores can influence flow pathways and soil moisture can affect
hydrological processes, including solute transport and land-atmosphere
interactions, as well as a range of geographic and pedogenic processes
(Western et al., 2004). However, due to the lack of soil water data in this
study, such as the soil water content and the hydrochemical and isotopic
data of soil water, some further issues cannot be analyzed. Also, hydrograph
separation for the event water and pre-event water and residence time
distribution of runoff components in the Hydrohill catchment need to be
studied.
Conclusions
Hydrochemical and isotopic methods are effective in tracing water movement
from rainfall to runoff components at outlets of an artificial experimental
catchment with separate runoff processes. EC, TDS, SO42-,
Ca2+, Mg2+, 18O and D are more applicable to investigating
rainfall and runoff processes at the Hydrohill experimental catchment.
δ18O and EC, TDS, SO42-, Ca2+, Mg2+ had a
linear relationship and their correlations can be used to identify runoff
flow paths including surface runoff, interflow and groundwater runoff.
Flow flux and mass fluxes of main hydrochemical and isotopic tracers gauged
at the Hydrohill catchment outlet, Ca2+, Mg2+, SO42-,
and 18O in the total runoff are mainly from groundwater runoff
(SSR100). However, the quantitative results were very encouraging as a basis
to develop hydrological models for further study, including examination of
hydrograph separation for event water and pre-event water and residence time
of different runoff components.
Data availability
Data is available based on request to the corresponding authors.
Author contributions
NY contributed results analysis and drafting the manuscript, JZ and GW structured the manuscript and contributed results discussion, JL contributed methodology of the work, GL contributed analysis on results reasonability, and AL contributed data collection and water sampling analysis.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Hydrological processes and water security in a changing world”. It is a result of the 8th Global FRIEND–Water Conference: Hydrological Processes and Water Security in a Changing World, Beijing, China, 6–9 November 2018.
Acknowledgements
We
appreciate Weizu Gu for his valuable comments and suggestions,
Hao Zheng, Niu Wang for their assistance of data collection and
laboratory analysis and Chaoyu Zheng, Tongping Liu for their water
sampling.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 41830863, 51879162, 51609145, and 91647203), the Second Qinghai-Tibet Plateau Comprehensive Scientific Research Project (grant no. 2019QZKK0203), and the National Key Research and Development Programs of China (grant no. 2016YFA0601501).
ReferencesBrian, A. P., Wilfred, M. W., Feng, X. H., and Charles, J. V.: The
application of electrical conductivity as a tracer for hydrograph separation
in urban catchments, Hydrol. Process., 22, 1810–1818, 10.1002/hyp.6786, 2010.Baraer, M., McKenzie, J., Mark, B. G., Gordon, R., Bury, J., Condom, T., Gomez, J., Knox, S., and Fortner, S. K.: Contribution of groundwater to the outflow from ungauged glacierized catchments: A multi‐site study in the tropical Cordillera Blanca, Peru, Hydrol. Process., 29, 2561–2581, 10.1002/hyp.10386, 2015.Carey, G. and Feng, X. H.: A stable isotope study of soil water: evidence
for mixing and preferential flow paths, Geoderma, 119, 97–111, 10.1016/S0016-7061(03)00243-X, 2004.Cartwright, I., Gilfedder, B., and Hofmann, H.: Contrasts between estimates of baseflow help discern multiple sources of water contributing to rivers, Hydrol. Earth Syst. Sci., 18, 15–30, 10.5194/hess-18-15-2014, 2014.Craig, H.: Isotopic variations in meteoric waters, Science, 133,
1702–1703, 10.1029/93wr01684, 1961.Gibbs, R. J.: Mechanisms controlling world water chemistry, Science, 170,
1088–1090, 10.1126/science.172.3985.870,
1970.Gonzales, A. L., Nonner, J., Heijkers, J., and Uhlenbrook, S.: Comparison of different base flow separation methods in a lowland catchment, Hydrol. Earth Syst. Sci., 13, 2055–2068, 10.5194/hess-13-2055-2009, 2009.
Gu, W. Z.: Experimental Research on Catchment Runoff Responses Traced by Environmental Isotopes, Advances in Water Science, 3, 246–254, 1992 (in Chinese).
Gu, W. Z. and Freer, J.: Patterns of surface and subsurface runoff generation,
in: Tracer Technologies for Hydrological Systems, Proceedings of Symposium
Boulder, 229, 265–273, 1995.
Gu, W. Z., Lu, J. J., Zhao, X., and Norman, E. P.: Responses of
hydrochemical inorganic ions in the rainfall-runoff processes of the
experimental catchments and its significance for tracing, Adv. Water Res.,
18, 1–7, 2007 (in Chinese).Gu, W. Z., Liu J. F., Lin, H., Lin, J., Liu, H. W., Liao, A. M., Wang, N.,
Wang, W. Z., Ma, T., Yang, N., Li, X. G., Zhuo, P., and Cai, Z.: Why
Hydrological Maze: The Hydropedological Trigger? Review of Experiments at
Chuzhou Hydrology Laboratory. Frontiers in Hydropedology: Interdisciplinary
Research from Soil Architecture to the Critical Zone, Vadose Zone J., 17, 1–27,
10.2136/vzj2017.09.0174, 2018.
Hendrickx, J. M. H., Das, B., Corwin, D. L., Wraith, J. M., and Kachanoski,
R. G.: Relationship between soil water solute concentration and apparent
soil electrical conductivity, Anal. Methods, 4, 1275–1282, 2002.Hooper, R. P. and Shoemaker, C. A.: A comparison of chemical and isotopic
hydrograph separation, Water Resour. Res., 22, 1444–1454, 10.1029/WR022i010p01444, 1986.Hrachowitz, M., Bohte, R., Mul, M. L., Bogaard, T. A., Savenije, H. H. G.,
and Uhlenbrook, S.: On the value of combined event runoff and tracer
analysis to improve understanding of catchment functioning in a data-scarce
semi-arid area, Hydrol. Earth Syst. Sci., 15, 2007–2024, 10.5194/hess-15-2007-2011, 2011.Hrachowitz, M., Benettin, P., Van Breukelen, B. M., Fovet, O., Howden, N. J., Ruiz, L., and Wade, A. J.: Transit times-the link between hydrology and water quality at the catchment scale, Wiley Interdisciplinary Reviews, Water, 3, 629–657, 10.1002/wat2.1155, 2016.Hugenschmidt, C., Ingwersen, J., Sangchan, W., Sukvanachaikul, Y., Duffner,
A., Uhlenbrook, S., and Streck, T.: A three-component hydrograph separation
based on geochemical tracers in a tropical mountainous headwater catchment
in northern Thailand, Hydrol. Earth Syst. Sci., 18, 525–537, 10.5194/hess-18-525-2014, 2014.Huth, A. K., Leydecker, A., and Sickman, J. O.: A two-component hydrograph
separation for three high-elevation catchments in the Sierra Nevada,
California, Hydrol. Process., 18, 1721–1733, 10.1002/hyp.1414, 2004.Jones, J. A.: Soil piping and catchment response, Hydrol. Process., 24,
1548–1566, 10.1002/hyp.7634, 2010.Keith, D. M., Johnson, E. A., and Valeo, C.: A hillslope forest floor (duff)
water budget and the transition to local control, Hydrol. Process., 24,
2738–2751, 10.1002/hyp.7697, 2010.Kendall, C., McDonnell, J. J., and Gu, W. Z.: A look inside “black box”
hydrograph separation models: a study at the hydrohill catchment, Hydrol.
Process., 15, 1877–1902, 10.1002/hyp.245,
2001.Klaus, J. and Mcdonnell, J. J.: Hydrograph separation using stable isotopes:
review and evaluation, J. Hydrol., 505, 47–64, 10.1016/j.jhydrol.2013.09.006, 2013.Ladouche, B., Probst, A., Viville, D., Idir, S., Baqué, D., Loubet, M.,
Probst, J. L., and Bariac, T.: Hydrograph separation using isotopic,
chemical and hydrological approaches (Strengbach catchment, France), J.
Hydrol., 242, 255–274, 10.1016/S0022-1694(00)00391-7, 2001.Lin, H.: Linking principles of soil formation and flow regimes, J. Hydrol.,
393, 3–19, 10.1016/j.jhydrol.2010.02.013,
2010.
Liu, C. Q.: Biogeochemical processes and cycling of nutrients in the earth's
surface: cycling of nutrients in soil–plant systems of karstic
environments, Southwest China, Science Press, Beijing, China, 1970 (in
Chinese).Liu, J. F., Gu, W. Z., Liao, A. M., Wang, N., Lu, J. J., Lin, J., Liu, H.
W., Wang, W. Z., Ma, T., Cai, Z., Liao, M. H., Li, X. G., Zhuo, P., and
Yang, N.: Hydrology of artificial and controlled experiments, Practice on the Watershed Hydrological Experimental System
Reconciling Deterministic and Stochastic Subjects Based on the System
Complexity: 1, Theoretical Study (chap. 11), 227–251, 10.5772/intechopen.78721, 2018a.Liu, J. F., Liao, A. M., Wang, N., Lin, J., Liu, H. W., Wang, W. Z., Ma, T.,
Cai, Z., Liao, M. H., Li, X. G., Zhuo, P., Yang, N., Lu, J. J., and Gu, W.
Z.: Hydrology of artificial and controlled experiments,
Practice on the Watershed Hydrological Experimental System Reconciling
Deterministic and Stochastic Subjects Based on the System Complexity: 2,
Practice and Test (chap. 12), 253–281, 10.5772/intechopen.79357, 2018b.Maurya, A. S., Shah, M., Deshpande, R. D., Bhardwaj, R. M., Prasad, A., and
Gupta, S. K.: Hydrograph separation and precipitation source identification
using stable water isotopes and conductivity: River Ganga at Himalayan
foothills, Hydrol. Process., 25, 1521–1530, 10.1002/hyp.7912, 2011.
Mcdonnell, J. J., Bonell, M., Stewart, M. K., and Pearce, A.: Implications
for Stream Hydrograph Separation, Water Resour. Res., 26, 455–458, 1990.Mul, M. L., Mutiibwa, R. K., Uhlenbrook, S., and Savenije, H. H.: Hydrograph
separation using hydrochemical tracers in the Makanya Catchment, Tanzania,
Phys. Chem. Earth, 33, 151–156, 10.1016/j.pce.2007.04.015, 2008.Pilla, G., Sacchi, E., Zuppi, G., Braga, G., and Ciancetti, G.:
Hydrochemistry and isotope geochemistry as tools for groundwater
hydrodynamic investigation in multilayer aquifers: a case study from
Lomellina, Po plain, South-Western Lombardy, Italy, Hydrogeol. J., 14,
795–808, 10.1007/s10040-005-0465-2, 2006.
Robinson, Z. P., Fairchild, I. J., and Arrowsmith, C.: Stable isotope
tracers of shallow groundwater recharge dynamics and mixing within an
Icelandic sandur, Skeiđarársandur, Hydrology in mountain regions:
Observations, processes and dynamics, 119–125, 2009.
Shen, X.: Natural Hydrochemistry, China Environmental Science Press, 102 pp., 2010
(in Chinese).Spence, C., Guan, X. J., Phillips, R., Hedstrom, N., Granger, R., and Reid,
B.: Storage dynamics and streamflow in a catchment with a variable
contributing area, Hydrol. Process., 24, 2209–2221, 10.1002/hyp.7492, 2010.
Stadler, S., Sültenfuß, J., Holländer, H. M., Bohn, A., Jahnke,
C., and Suckow, A.: Isotopic and geochemical indicators for groundwater flow
and multi-component mixing near disturbed salt anticlines, Chem. Geol., 294,
226–242, 10.1016/j.chemgeo.2011.12.006, 2012.
Tan Z. C., Lu, B. H., Wang, J., and Sun, Y. Y.: Characteristics of stable
hydrogen and oxygen isotopes of precipitation and runoff in Wudaogou
Hydrological Experimental Catchment, J. Hohai Univ., 37, 650–654, 2009 (in
Chinese).Tardy, Y., Bustillo, V., and Boeglin, J. L.: Geochemistry applied to the
watershed survey: hydrograph separation, erosion and soil dynamics. A case
study: the basin of the Niger River, Africa, Appl. Geochem., 19,
469–518, 10.1016/j.apgeochem.2003.07.003, 2004.Uhlenbrook, S.: Catchment hydrology-a science in which all processes are
preferential, Hydrol. Process., 20, 3581–3585, 10.1002/hyp.6564, 2010.Uhlenbrook, S., Frey, M., Leibundgut, C., and Maloszewski, P.: Hydrograph
separations in a mesoscale mountainous basin at event and seasonal
timescales, Water Resour. Res., 38, 1–14, 10.1029/2001WR000938, 2002.Vincent, M., Jean-Francois, D. L., and Couren, M.: Investigation of the
hydrological processes using chemical and isotopic tracers in a small
Mediterranean forested catchment during autumn recharge, J. Hydrol., 247,
215–229, 10.1016/s0022-1694(01)00386-9, 2001.Walton, N. R. G.: Electrical Conductivity and Total Dissolved Solids-What is
Their Precise Relationship?, Desalination, 72, 275–292, 10.1016/0011-9164(89)80012-8, 1989.Western, A. W., Zhou, S. L., Grayson, R. B., McMahon, T. A., Blöschl,
G., and Wilson, D. J.: Spatial correlation of soil moisture in small
catchments and its relationship to dominant spatial hydrological
processes, J. Hydrol., 286, 113–134, 10.1016/j.jhydrol.2003.09.014, 2004.Williams, M. W., Barnes, R. T., Parman, J. N., Freppaz, M., and Hood, E.:
Stream Water Chemistry along an Elevational Gradient from the Continental
Divide to the Foothills of the Rocky Mountains, Vadose Zone J., 10, 900–914,
10.2136/vzj2010.0131, 2011.Yang, Y. G., Xiao, H. L., Zou, S. B., Zhao, L. J., Zhou, M. X., Hou, L. G.,
and Wang, F.: Hydrochemical and hydrological processes in the different
landscape zones of alpine cold region in China, Environ. Earth Sci., 65,
609–620, 10.1007/s12665-011-1108-7, 2012.Zhang, Q., Knowles, J. F., Barnes, R. T., Cowie, R. M., Rock, N., and
Williams, M. W.: Surface and subsurface water contributions to streamflow
from a mesoscale watershed in complex mountain terrain, Hydrol. Process.,
32, 954–967, 10.1002/hyp.11469, 2018.
Zhao, R. J.: Catchment Hydrological Modelling, Water Conservancy Press,
Beijing, 76–79, 1989 (in Chinese).