1 Introduction
The surface water and groundwater resources are limited and unevenly
distributed in time and space in the Loess hilly region, where the
climate is predominantly arid and semi-arid. Even so, groundwater is, in
some regions, the only freshwater source for the production and life of
the inhabitants, such as drinking water
and irrigation
water (Xia et al., 2008). However, due to the insufficient precipitation
and the high evapotranspiration in this area, groundwater recharge rate
tends to be relatively low (Scanlon et al., 2010), while unreasonable
abstraction of groundwater continues to occur. The overexploitation of
groundwater compared to its renewal and recharge rate will cause the
fast depletion of groundwater, thereby threatening the ecological
restoration as well as the production and life of the residents. The
sustainable management of groundwater resources must, therefore,
requires an accurate quantitative analysis of groundwater recharge
mechanism (Shen et al., 2015). It is nonetheless particularly difficult
to provide a quantitative estimate of groundwater recharge given its
relatively low rate in the region.Various methods have been used in
previous studies to conduct an in-depth exploration of the groundwater
recharge mechanism in this region. Zhu et al. (2009a; 2010b)
quantitatively estimated the groundwater recharge ratio and the
residence time of Wuding River Basin in the Loess Plateau, based on the
water withdrawal analysis method. Gates et al. (2011) estimated the
impact of land use on groundwater recharge in the Loess Plateau using a
tracer method combining chloride ion, stable hydrogen and oxygen
isotopes. They concluded that concentrated water infiltration in gullies
and other low-lying areas was the main groundwater recharge mechanism in
this region. Hui et al. (2015) evaluated the effect of reforestation on
groundwater recharge in the Loess Plateau by applying the RHESSys model.
The hydrogen and oxygen isotope tracers have been widely used to examine
groundwater recharge in arid and semi-arid regions due to the numerous
advantages this technique presents (Emam et al., 2015; Qin et al.,2011).
Amongst these advantages, these tracers do not interact with other
substances in the water molecule migration process, they directly
participate in groundwater circulation, they do not pollute the
environment, and they can track long-term groundwater movements.
Regional water cycle and the mixing process of multi-terminal water body
could be traced by measuring δ D and δ 18O
in precipitation, soil water and groundwater in order for some of the
main hydrological processes to be revealed, which include the
precipitation composition, infiltration, evaporation, migration
(Kendall; Tan et al., 2016). Peng et al.(2005) investigated the effect
of vapour water from the continental evaporation on hydrogen and oxygen
isotopes composition in precipitation based on hydrogen and oxygen
isotope tracer method. Liu et al.(1995) explored the seasonal soil water
movement in the top meter of undisturbed desert soil in the southern
Arizona using δ D and δ 18O, the results
indicate repeated seasonal cyclic movement of soil water mainly occurred
in the top layer of 60–80 cm. Several previous studies have applied the
hydrogen and oxygen isotope tracer method in the groundwater research
field. For example, the seasonal recharge pattern of the Huanlia river
basin in Taiwan, China was quantitatively analysed by Yeh et al. (2014)
using the hydrogen and oxygen isotope tracer technique. Earman et al.
(2006) used these tracers to explore the groundwater recharge
characteristic caused by snowmelt in the southwest of the United States.
Thoma et al. (1979) calculated the amount of groundwater recharge by
atmospheric precipitation through the analysis of the seasonal variation
of δ 18O in the atmospheric precipitation in
Pelat dune, Israel. However, because of a lack of temporal water
monitoring data of hydrogen and oxygen isotope, The mechanisms governing
loess groundwater recharge in such a climatic and topographic
environment still remain unclear (Tan et al., 2016). Existing studies in
this field mainly focused on the analysis of groundwater source and
recharge mechanism (Xiang et al., 2019; Tan et al., 2017; Zhi et al.,
2017). Limited studies have focused on the spatial-temporal distribution
characteristics of groundwater recharge in a small watershed of this
region.
Water transmission time refers to the time-lapse that water molecules
have experienced from when they enter the small watershed system until
they leave it. The transmission time could not only reveal some
information about the water storage, the flow path and the water source
but is also closely related to some of the internal hydrological
processes in the watershed (Sivapalan, 2010; Mcguire et al., 2004).
Numerous studies have investigated water transmission time in small
watersheds. Asano et al. (2002) calculated the mean water transmission
time of precipitation to groundwater in Rachdani region using the index
of δ D, which was equivalent to a year. LEE et al. (2007)
evaluated the water residence time at different depths of soil layers in
Puji Island, South Korea, using deuterium surplus. Their results showed
that the mean residence time at 30cm and 60-80cm depths were about 74
and 198 days, respectively. However, due to the Loess Plateau, China is
predominantly covered by loess deposits ranging from 30 to 80 m in
thickness (Wang et al., 2010), there is a slower groundwater recharge
rate and the recharge period can be up to several decades, hundreds of
years or even longer in the process of groundwater recharged by rainfall
through unsaturated loess layers (Huang et al.,2013; Lin and Wei, 2006).
Few studies have explored water transmission time in the Loess Hilly
Region for relatively concealed water flow path and the insufficiency
temporal monitoring data of isotope.
In this study, over 17 months in the Zhifanggou watershed and over 7
months in the Bangou watershed of stable D and O isotope composition
monitoring were performed in precipitation, surface water and
groundwater in the Loess hilly region. The results could provide a basis
for the scientific research and rational development and utilisation of
groundwater resources in this region. Therefore, the overall objectives
of this study were
threefold:
(1) to analyze the spatial-temporal variation characteristics of
hydrogen and oxygen isotopes in different water bodies at small
watersheds in the Loess hilly region, and identify the main recharge
sources of groundwater; (2) to quantitatively estimate the recharge
ratio and the water transmission time of precipitation and surface water
to groundwater in the small watershed; (3) uncover the temporal and
spatial distribution model of groundwater recharge in small watershed of
the Loess hilly region.