4.3 Spatial variation characteristics of groundwater recharge
In this study, the groundwater recharge ratio by precipitation was
gradually decreased from upperstream to downstream, while the recharge
ratio by surface water increased gradually (Egusa
et
al,2016).
The
main reason was that the gully shape in the upperstream is narrow with a
larger uncut plateau area and relatively gentle geomorphic type, the
precipitation mostly enters the soil in the form of infiltration (Li, et
al,2008). Moreover, most of the terraces are located in the upstream,
which greatly increased the recharge capacity of soil water and
groundwater by precipitation (Arnáez et al,2015; Gates et al,2011). So a
larger groundwater recharge ratio by precipitation appeared in the
upperstream. There was little surface water in the upperstream due to
the smaller catchment area (Orlova et al.,2014; Egusa et al.,2016). The
groundwater recharge ratio by surface water was thus relatively smaller
in this area. In the downstream, because of the maturity gully shape
with increased gully proportion and seriously damaged the aquifer, and
the abundant surface water resources given for the relatively larger
catchment area, the groundwater could be recharged quickly by surface
water through the fractures of the aquifer (Sun et al.,2016). Therefore,
the groundwater recharge ratio by surface water was higher in
downstream. On the other hand, due to the most geomorphology had a
steeper slope in this area, the most precipitation lost in the form of
runoff and less infiltrated into the soil. Therefore, the groundwater
recharge ratio by precipitation was smaller in downstream. This result
is consistent with that reported by Zhang et al. (2016) in the Grand
River basin in Michigan, U.S. However, this process of groundwater
recharge by surface water occurred at the time of abundant surface water
resources
in
downstream. According to the analysis of the water transmission time,
the process of surface water recharged by groundwater was relatively
common in downstream (Santos et al.,2010; Zhang et al.,2016). This is
because there was a lower water level of surface water for the deeper
gully shape in the downstream. The groundwater of shallow aquifer, which
originated from vertical infiltration of precipitation, could
subsequently recharge surface water through the surface fractures.
The transmission time of groundwater recharge by precipitation was
longer than that by surface water. Due to the gully bottom area where
the surface water existed performed a lower elevation and cutting into
the bedrock, the groundwater could recharged with a short path through
passing through the silted soil layer at the bottom of the gully or
directly getting into the cracks of the rock layer (Zhi et al., 2017;
Broxton et al., 2009). Most of the precipitation nonetheless infiltrated
in the gully slope and the platform not cut by water flow as the gully
area was only about 9% of the watershed. Also, the water that
infiltrated into the soil could enter the underground aquifer with a
longer flow path through the thick loess aeration zone (Dewalle et al.,
1997).
Meanwhile, the recharge process of surface water to groundwater was
closely related to the topography and the geomorphology (Zhi et al.,
2017). The groundwater recharge ratio in the middlestream of the Bangou
watershed was much higher than the mean value of this watershed,
accounting for 93.2%. Since groundwater sampling well in the
middlestream is located at the bottom of the gully and is close to the
surface water, the groundwater was recharged by surface water. The
groundwater recharge ratio by precipitation in the Zhifanggou watershed
was much higher than the mean value of this watershed, accounting for
46.0%. Given the ideal condition of precipitation infiltration in the
soil caused by the good vegetation conditions in the middlestream (Gates
et al.,2011; Jayawickreme et al.,2008). There is a large distribution of
woodland with well soil pores in deep soil, making the precipitation
could recharge groundwater more quickly. Meanwhile, the surface water
was smaller, and the recharge capacity of surface water to groundwater
was consequently weakened, whereas, the recharge capacity by
precipitation was relatively enhanced.