2 Materials and methods
2.1 study area
The study was conducted in the Zhifanggou watershed
(36°43′-36°46′N,109°14′-109°16′E) and the Bangou watershed
(36°46′-36°44′N,109°24′-109°26′E), in Ansai county on the northern
Loess Plateau of China (Fig. 1). The Zhifanggou watershed is the first
branch of the lower reaches of the Xingzi River, a tributary of the
Yanhe River. The area of the watershed is 8.27 km2,
with a narrow and long north-south direction. The watershed has a
continental monsoon season climate. The mean elevation is 1425-1041 m,
the annual mean temperature is 8.8 °C, the annual mean evaporation is
1463 mm, the annual mean precipitation is approximately 549.1 mm, which
is unevenly distributed in the year and can vary considerably from one
year to another. The land consists of loessial soil, mainly consisting
of silt (0.002-0.05mm). The terrain is broken, and the density of the
gully is 8.07 km/km2. Groundwater mainly consists of
water flowing in the quaternary alluvial aquifer and the fissured
shallow weathering crust, with a mean depth of 13 m. The Bangou
watershed (36°46′-36°44′N,109°24′-109°26′E) is located at 14.60 km east
of the Zhifanggou watershed and belongs to the first-grade tributary of
the Yanhe River. The watershed area is 6.56 km2 with a
mean elevation of 1268-1055 m and annual mean precipitation of
approximately 534.7 mm. The soil type is mainly loessial, and the mean
depth of the phreatic surface is 12 m. The climate type and the
hydrogeological conditions of the Bangou watershed are similar to those
of the Zhifanggou watershed, but its gully is cut deeper, and the
aquifer is more fragmented.
2.2 sampling and analysis
The water sampling point of precipitation, surface water and groundwater
were set up in each part of the gully head, upperstream, middlestream
and downstream in the Zhifanggou watershed, respectively. Likewise,
water sampling point of precipitation, surface water and groundwater
were set up in each part of the upperstream, middlestream and downstream
in the Bangou watershed, respectively. The elevation, the longitude and
the latitude were recorded using a hand-held GPS.
The specific sampling points were
shown in Fig.1. Form June 2016 to October 2017, three water bodies in
the Zhifanggou watershed were continuously sampled at intervals of 15
days. Similarly, three water bodies in the Bangou watershed were sampled
from June April to October 2017 at similar intervals. Each sampling was
repeated twice, and the samples were sent to the laboratory of water
resources research institute, Xi’an University of Technology once the
sample collections were completed. All samples were tested for δD and
δ18O using DLI-100 liquid isotope laser analyser
LGR-LWIA (Los Gatos Research Inc., USA). Sampling methods for each water
body were as follows:
Precipitation sampling: an open area was selected near each sampling
point, and rain measuring cylinder with of type J16022 was set up to
collect precipitation samples. About 8 milliliters of liquid paraffin
oil was previously applied to the rainwater collector to prevent water
evaporation. During sampling, the water below the paraffin oil layer was
extracted with a 15 mL disposable syringe. The collected precipitation
samples were filled in 10 ml brown glass reagent bottles, which were
tightened and sealed with Membrane seal film. After the collection
completed, the water collector was cleaned up, and the liquid paraffin
oil reapplied. All the samples were refrigerated at 4 °C. The stable
isotope was after that tested to ensure the reliability of the test
results. According to statistics, 312 precipitation samples were
collected during the experiment.
Surface water sampling: surface water sampling points were set up at the
bottom of the gully, near the precipitation sample point. During the
collection, the sample bottle was inserted 20 cm below the surface of
the water to prevent the influence of evaporation fractionation. The
collected surface water samples were filled in 10 ml brown glass reagent
bottles, tightened and sealed with Membrane seal film. The samples were
after that refrigerated at 4 ℃, before the stable isotope was tested.
According to statistics, 312 surface water samples were collected during
the experiment.
Groundwater sampling: the sampling of groundwater consisted of
withdrawing water from the bottom of a well up to the surface using a
bucket, and quickly placing it in a cool place. The sampling and
refrigeration methods were similar to those applied in the case of the
surface water. Since there were only one groundwater well in the
Zhifanggou watershed laocated in the mouth part of this watershed, which
is used as a drinking water source, so as to only one groundwater
sampling point was set up in the mouth of this watershed. A total of 76
groundwater samples were collected. In the case of the Bangou watershed,
three groundwater sampling points were set up. The groundwater sample in
upperstream corresponded to seepage water from the rock formation, and
groundwater samples in the middlestream and downstream were from wells.
A total of 56 groundwater samples were collected.
2.3 Analysis methods
2.3.1 Estimation of water transmission time
A large number of studies showed that the annual (seasonal) isotope
characteristics of precipitation, surface water and groundwater followed
a certain periodicity, which can be characterised by sinusoidal
regression analysis of the following form:
δ=X+A*cos(ct-θ)
(1)
where δ corresponds to δ D, δ 18O
or d-excess; X is the annual mean of δ D,δ 18O or d-excess; C is the fluctuation
frequency of δ D, δ 18O or d-excess
(0.017214rad·d-1); T is the time, and θ is the lag
time.
The mean water transmission time (T) in the water circulation system is
as follows:
T=C-1[(Az2/Az1)-2-1]0.5(2)
Where Az1 is the amplitude of the input signal;
Az2 is the amplitude of the output signal; C is the
fluctuation frequency of δ D, δ 18O or
d-excess (0.017214rad·d-1).
2.3.2 Quantitative estimation of groundwater recharge
The MixSIAR statistical package (Stock and Semmens, 2013) is a general
Bayesian framework that was used to quantify the contributions of the
different flows relative to the total groundwater recharge. It has been
widely used in sourcing plant water, pollutants, soil carbon and in
food-web studies. In this study, only two end members were considered:
precipitation and surface water (as described in Section 3.1). As for
groundwater, the water isotopes of different samples were considered
independent consumers (input) as they were collected from different
sites. The error structure was set to default, “Resid*Process” and the
Markov Chain Monte Carlo length was set to “short” for better
predictions of the probability distributions of the sources (e.g.
precipitation and surface water). Because there was no prior
information, the “Uninformative”/Generalist was selected in this
framework. After running MixSIAR, a check diagnostic was conducted
according to Stock and Semmens (2013) to confirm the validity of the
output (contributions). Finally, the mean values of the runs were
considered as the most likely proportions of contributions, and the
standard deviation was considered as the uncertainty (Xiang et al.,
2019)(Fig. 2).