Determination of groundwater recharge mechanisms using stable isotopes in small watersheds of the Loess Plateau, China

Water sustainability is a major challenge on the Loess Plateau of China, since the drying of soil and loss of surface water is threatening regional water security. Fundamental to effective water management is an understanding of groundwater recharge mechanisms. Based on a time series of stable isotopes data for precipitation, surface water and groundwater, the groundwater recharge ratios and water transmission times were quantitatively identified for the studied region. The results showed that groundwater discharge to surface water was a common phenomenon during the dry and wet seasons. However, groundwater could also be recharged by precipitation and surface water during specific months when experiencing large precipitation events. Over shorter time scales (<1 year), groundwater was mainly recharged by surface water, while the groundwater recharge ratio from rainfall during the wet season was higher than that from melting snow during the dry season. Over longer time scales (>1 year), precipitation was the primary recharge source of groundwater in small watersheds due to the general flow direction of groundwater to surface water. Groundwater recharge by precipitation mostly occurred through a combination of piston flow and preferential flow, where preferential flow was the primary recharge mechanism for groundwater replenished by precipitation in this region. Surface water could quickly recharge groundwater by lateral flow through fractures in the aquifer and vertical piston flow. These findings could, therefore, be used to provide a reference for the utilization and protection of groundwater resources in the small watersheds of the loess hilly regions of the Loess Plateau.