5 Conclusion

Stable isotopes (δD and δ18O) of precipitation, surface water, and groundwater were used to determine the groundwater recharge mechanism and the spatial-temporal variation pattern in the Loess hilly region. The hydrogen and oxygen isotopes of precipitation was most enriched, followed by surface water and then groundwater(this having the most depleted isotopes). The hydrogen and oxygen isotopes of the three water bodies were all gradually enriched from the gully head to the downstream in the small watershed. The transmission time of the precipitation and surface water to groundwater were 443.16 d and 64.58 d, respectively; and the corresponding groundwater recharge ratio were 29.22% and 70.78%, respectively. The main recharge source of groundwater in the Loess hilly region was surface water. The connection between precipitation and groundwater in the rainy season was stronger than that in the dry season, and the ratios of recharge were 32.83% and 25.60%, in the rainy and dry season, respectively. The groundwater recharge ratio by precipitation in upstream was larger than downstream. Meanwhile, the recharge ratio by surface water in upstream was smaller than the downstream. At the same time, the main recharge area for groundwater was located upstream in the small watershed of loess hilly region, while the main discharge area for groundwater was located downstream.