4.2 Temporal characteristics of groundwater recharge
The
groundwater recharge by precipitation was affected by the stratigraphic
structure of the study area (Ndlovu et al., 2016; Adrian et al., 1989).
Tang et al. (2016) found that the rock formations in the Ansai area of
China were mainly composed of clastic feldspathic sandstones, and the
rock layers were filled with hydrophilic substances with high viscosity
such as chlorite. The rock formation in this region was relatively
broken due to the strong brittleness of feldspar sandstone. The thinner
the rock formation, the more the fracture develops (George et al.,
2009). The rock formations in the Zhifanggou watershed were mostly
thin-layered (Fig. 1f). The rock fissures were relatively developed, and
the soil layer in some areas was relatively shallow. In case of large
precipitations, the water could easily penetrate the loess layer and
enter the groundwater aquifer along the rock fissures (Zhu et al.,
2010), which provides a possibility for groundwater recharge by
precipitation. According to the studies of Xiaolu et al. (2018) and Zhu
et al. (2010), the infiltration depth of precipitation and the
groundwater recharge ratio by precipitation were allpositively
correlated with the precipitation. Precipitation from June to October
accounted for 88% of the total annual precipitation in the Loess
Plateau, which was the time when most moderate and heavy rains occurred.
In this study, precipitation and groundwater were more closely related
in the rainy season, and the groundwater ratio was 7.23% higher than
that in the dry season (Dvory et al., 2016). Groundwater recharge season
occurred mainly during the rainy season (Machiwal et al., 2012). The
precipitation was scarce from November to May, and the groundwater was
mainly recharged by snow melting in some areas (Wang
et
al.,2017; Diodato et al.,2013). Groundwater recharge by precipitation in
this season was smaller, and the connection between groundwater and
precipitation was weaker. These findings are similar to those reported
by Yeh et al. (2014) in the Hualien River watershed, Taiwan, China.
Gullies in the small watershed of the loess plateau were formed under
the long-term undercut erosion and scour of the flowing water
(Descloitres et al.,2003). The gully cuts through the loess and
penetrates the bedrock weathering zone (Guyassa et al.,2017; Orazulike
et al.,1988). Local gullies even cut through weathering zones into fresh
rocks to form deep grooves. Undercutting of rock strata destroyed the
underground aquifers in some areas, causing groundwater to leak out and
recharge surface water (Unland et al.,2014). When the surface water
resource was abundant with a high water level, it could enter the
underground aquifer through the fault zones, so that surface water and
groundwater could recharge each other (Borman et al.,2014; Almanaseer et
al.,2012). The response mechanism of groundwater to precipitation and
surface water was also closely related to the shape of the gully
(Demisachew et al., 2018). The gully shape of Bangou watershed was a
narrow and deep with steep slopes on both sides, which was approximately
a āVā the type of shape. Most of the precipitation was concentrated into
the gully in the form of runoff in a short time. The surface water could
then recharge groundwater through the fracture surface of the aquifer.
The gully shape of the Zhifanggou watershed was wide and shallow with
gentle slopes on both sides, which was approximately a āUā type of
shape. The contact time between the runoff and the surface soil was
longer, which made water to be stored in the soil and recharge
groundwater through vertical infiltration. Due to the sedimentation at
the bottom of the gully, surface water needs to penetrate the alluvium
to recharge groundwater through the aquifer fracture zones (King et al.,
2015).