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 ā€Vā€ 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 ā€Uā€ 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).