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.