3 Results
3.1 Spatial and temporal variation characteristics of
hydrogen and oxygen isotopes in
water
The values of the hydrogen and oxygen isotope characterisation from June
2016 to October 2017 for different water bodies in the Zhifanggou
watershed are shown in Table 1. In different water bodies, the δ D
and δ 18O of groundwater was most depleted
(-64.95, -9.04 ‰),followed by surface water (-60.80, -8.20 ‰) and then
the precipitation was most enriched (-52.12, -7.74 ‰). The hydrogen and
oxygen isotope characterisation values from April to October 2017 for
the precipitation, the surface water and the groundwater in Bangou
watershed are shown in Table 2. The hydrogen and oxygen isotopes of each
water body in this watershed follow a similar lawpattern as in the
Zhifanggou watershed, i.e. the precipitation isotope was the most
abundant with the highest variation coefficient, while the groundwater
isotope was the most depleted, with a relatively stable isotope.
At the same time, the hydrogen and oxygen isotopes in the different
water bodies exhibited a certain spatial variability. In different parts
of the Zhifanggou watershed, The δ D of precipitation in the
upperstream was most depleted (-53.49‰), followed by
stream (-43.81‰), then the gully head (-43.12‰) , and the δ D of
precipitation in the downstream was most enriched(-36.11‰). Theδ 18O of precipitation in the upperstream was
most depleted(-7.72‰), followed by gully head (-6.55‰), then the
middlestream (-6.46‰), and theδ 18O of
precipitation in the downstream was most enriched(-5.43‰). The δ D
of surface water in the downstream was most depleted (-62.55‰), followed
by gully head (-61.01‰), then the upperstream (-60.95‰), and theδ D of surface water in the middlestream was most enriched (-57.55
‰). The δ 18O of surface water in the downstream
was most depleted (-8.46 ‰), followed by upper stream (-8.09 ‰), then
the gully head (-8.03 ‰), and the δ 18O of
surface water in the middlestream was most enriched (-7.64 ‰).
On the other hand, the δ D of precipitation in the downstream of
the Bangou watershed was most depleted (-39.85‰), followed by
upperstream (-39.77‰), then the middlestream (-39.50‰). Theδ 18O of precipitation in the middlestream was
most depleted (-6.07 ‰), followed by upperstream (-6.03 ‰), then the
downstream (-5.83 ‰). The δ D and δ 18O of
surface water in the upperstream of the Bangou watershed were most
depleted (60.13, 7.63 ‰), followed by middlestream (57.93, 7.64 ‰), then
the downstream (57.59, 7.16 ‰). The δ D andδ 18O of groundwater in the upperstream of the
Bangou watershed were most depleted (-57.55, -7.93‰), followed by
middlestream (-59.49, -7.44‰), then the downstream (-57.72, -7.39‰). In
summary, the three water bodies were gradually enriched from upperstream
to downstream in the watershed, but the difference was not significant.
However, due to the influence of the watershed environment, the spatial
characteristics of the hydrogen and oxygen isotopes in the surface water
of the Zhifanggou watershed and the precipitation of Bangou watershed
were not obvious.
The temporal variation characteristics of the hydrogen and oxygen
isotopes in different water bodies of the Zhifanggou watershed are shown
in Fig. 3. The hydrogen and oxygen isotopes of the precipitation
exhibited the largest variation over time, and their δ D andδ 18O were the most enriched in May (-19.45,
-3.11‰), and were more depleted in November and December (-94.65,
-14.26‰). The δ D and δ 18O of the surface
water were relatively stable from June to December 2016, and the
fluctuations mainly occurred within the period January - August 2017.
Their δ D and δ 18O were the most enriched
in May (-19.45, -3.11‰) and were more depleted in February (-71.86,
-9.39). The hydrogen and oxygen isotopes of the groundwater were
relatively stable, with a δ D and δ 18O
the most abundant in August (-61.15, -8.16‰), and the most depleted in
March (-67.54‰) as well as in November (- 9.51‰). There was a lag time
between the depletion peak of groundwater and the peak of precipitation
and surface water, indicating that the groundwater recharge process by
precipitation and surface water was mainly through piston flow, with a
lower recharge rate and longer recharge period (Tan et al., 2016). The
temporal evolution of the hydrogen and oxygen isotopes in the three
water bodies of Bangou watershed is shown in Fig. 4. The depletion peaks
of the precipitation, the surface water and the groundwater appeared in
October in all cases. This indicates that there may be preferential flow
channels for water flow in this watershed. Hence, precipitation could
recharge groundwater faster, which induced a shorted recharge period.
Therefore, there was a possibility that the piston flow and the
preferential flow could jointly recharge groundwater in the Loess Hilly
region (Xiang et al., 2019; Tan et al., 2017).
At the same time, the d-excess of the precipitation in the two
watersheds was analysed. The results showed that the d-excess of the
precipitation from June to October remained smaller than 10‰, which
means that the precipitation water vapour mainly originated from marine
air mass. Also, the d-excess from November to May exceeded 10‰, which
indicated that the precipitation was mainly attributed to the
continental air mass.
3.2 Analysis of groundwater recharge
source
The relationship between the hydrogen and oxygen isotopes in the case of
the precipitation in the Zhifanggou watershed was fitted for two
periods, as shown in Fig. 5-a. The first period extended from November
to May and the second period extended from June to October, The equation
of the precipitation line wereδ D=7.50δ 18O+10.14,
R2=0.98 andδ D=7.84δ 18O+5.64,R 2=0.96, in the first period and in the second
period, respectively. The gradient of the precipitation line was smaller
than that of the global atmospheric precipitation line
(δ D=8δ 18O+10). Most of the groundwater
hydrogen and oxygen isotope points fell on the precipitation fitting
line from June to October. Away from the part of the surface water
points that fell in the region where the groundwater was located, the
rest of points were located at the lower right of the precipitation
fitting line. The results indicated that the precipitation from June to
October was the main recharge source for the surface water and the
groundwater, and there was a possibility that surface water and
groundwater could recharge each other. And the points of the surface
water and the groundwater were all still located at the lower right of
this line from November to May. This indicates that the surface water
and the groundwater could be recharged by precipitation in this period,
but its hydraulic connection with surface water and groundwater was
weaker than during the period from June to October.
The fitting equation of the hydrogen and oxygen isotopes for the
precipitation in Bangou watershed wereδD=7.42\(\delta^{18}O\)+8.11 (R2=0.99) andδD=6.72\(\delta^{18}O\)-2.62 (R2=0.96),
from April to May and from June to October, respectively. The values of
the gradient of these two lines were smaller than that of the global
precipitation line. The points of the groundwater and the surface water
were both located at the lower right of the precipitation fitting line
from June to October, which indicate that the precipitation in this
period was the main recharge source for the surface water and the
groundwater in Bangou watershed. The region where the locations of the
surface water and the groundwater points were coincident indicated that
the connection between groundwater and surface water was closer in this
period than during the period June-October. Also, this demonstrates that
the mutual recharge of surface water and groundwater was more frequent.
3.3 Water transmission time
Sinusoidal fitting was performed for the hydrogen and oxygen isotopes in
different water bodies of the Zhifanggou watershed in the period June
2016 - October 2017, as shown in Fig. 6. The results of δ D fitted
showed that amplitudes of the precipitation and the groundwater were
respectively 18.60‰ and 1.79‰, and the transmission time of the
precipitation to groundwater estimated by the model was 510.06 d. Based
on the results of δ 18O fitted, the transmission
time of the precipitation to groundwater was 376.25 d (Table 3).
Therefore, the mean transmission time of the precipitation to
groundwater was 443.16 d. Table 4 shows the transformation time between
surface water and groundwater in different parts of the watershed. The
fitting results of δ D provide evidence that the transformation
time of surface water to groundwater was 77.14 d, 55.37 d and 65.53 d,
in the gully head, upperstream and middlestream, respectively. Also, the
estimated result for the transformation time based on \(\delta^{18}O\)fitted was 64.68 d, 60.13 d and 63.88 d, in the gully head, upperstream
and middlestream, respectively. Threrfore, the mean transmission time of
surface water to groundwater was 70.91, 57.75 and 64.71 d in the gully
head, the upperstream and the middlestream, respectively. The mean
transmission time of surface water to groundwater in this watershed was
64.58 d, which was about 15% of the transmission time of the
precipitation recharge to groundwater. In the downstream, the surface
water was recharged by the groundwater. Based on the fitting results ofδ D and δ D18O, the transmission time of
surface water recharged by groundwater were 55.26 and 49.53 d,
respectively. The mean transmission time value was 51.10 d. This show
that the downstream was the main groundwater discharge area in the small
watershed. At the same time, the time of groundwater discharge to the
surface water was similar to the time of surface water recharge to
groundwater. This indicates that there might be a similar channel
through which the groundwater and the surface water could recharge each
other simultaneously.
3.4 recharge ratio of
groundwater
Based on the Bayesian model, the mean groundwater recharge ratio by
precipitation and surface water in Zhifanggou watershed were
respectively 34.7% and 65.3%,
from
June 2016 to October 2017. From April to October 2017, the groundwater
recharge ratio by precipitation and surface water in the Bangou
watershed were 17.2% and 82.8%, respectively. In the Bangou watershed,
the groundwater recharge ratio by precipitation was 22.5%, 6.8% and
14.1%, in the upperstream, middlestream and downstream, respectively
(Fig. 7a). The groundwater recharge ratios by surface water were 77.5%,
93.2% and 85.9% in the upperstream, middlestream and downstream,
respectively (Fig. 7b). The groundwater recharge ratio by precipitation
in the gully head, upperstream, middlestream and downstream of the
Zhifanggou watershed were 33.3%、32.7%、46.0% and 10.6%,
respectively (Fig. 7a), and the recharge ratio by surface water were
66.7%、67.3%、54.0% and 89.4%, respectively (Fig. 7b). The
groundwater ratio by precipitation from the upstream to the downstream
in the small watershed gradually decreased, while the recharge ratio by
surface water increased. These results show that the upperstream of the
small watershed was the main recharge area of groundwater by
perciputation. But there were two phenomenons that the groundwater ratio
by precipitation in the middlestream of Bangou watershed decreased
sharply compared with oter parts, and the ratio increased sharply in the
middlestream of Zhifanggou watershed compared with other parts of the
watershed. This phenomenon might be related to the regional situation
such as topography and vegetation, and the location of the sampling
points in the middlestream.
The study period from June to October was classified as rainy season
since the d-excess value of precipitation was less than 10‰, and the
period from January to May was classified as a dry season. The
groundwater recharge ratio by precipitation and surface water in
Zhifanggou watershed were estimated in the periods June-October and
November- May(Fig. 8). Similarly, the groundwater recharge ratio by
precipitation and surface in the Bangou watershed were estimated in the
periods June-October and April-May, as shown in Fig. 8. In the rainy
season (June-October), the values of the groundwater recharge ratio by
precipitation were 37.65% and 28.00% in the Zhifanggou watershed, and
the Bangou watershed, respectively (Fig. 8a), and the corresponding
ratios by surface water were 62.35% and 72.00%, respectively (Fig.
8b). In the dry season (January-May), the values of the groundwater
recharge ratio by precipitation were 24.60% and 8.30% in the
Zhifanggou watershed, and Bangou watershed, respectively (Fig. 8a), and
the corresponding ratios by surface water were 75.40% and 91.70%,
respectively (Fig. 8b). In the rainy
season, the values of the groundwater recharge ratio by precipitation
were 1.53 and 3.37 times greater than in the dry season, for the
Zhifanggou watershed and the Bangou watershed, respectively. These
results evidence that the rainy season was the main season in which
groundwater was recharged through precipitation. Also, the groundwater
in the dry season was mainly recharged by surface water, where the ratio
was about 1.21 and 1.27 times greater than in the rainy season, in the
Zhifanggou watershed and the Bangou watershed, respectively.