Catchment-scale surface water-groundwater connectivity on China's Loess Plateau

- The surface water-groundwater connectivity in a watershed with deep depositional environment is determined by geochemistry;
- EC and Cl concentrations are unsuitable for flow path analysis due to the impacts of chemical weathering;
- Stable isotope is a good indicator for water transformation analysis;
- A gaining surface water system was detected for the watershed.

Thick loess deposits (up to 200 m deep) and a large unsaturated zone (up to 100 m below the soil surface) complicate the interactions between surface water and groundwater in China's Loess Plateau, especially in catchments characterized by tableland-gully topography in this region. In the Heihe watershed, electrical conductivity (EC), chloride (Cl) concentrations, and stable water isotope data (δD and δ18O), obtained during dry and wet seasons in the period of 2012-2014 are used to determine the relationship between surface water and groundwater. EC and Cl concentrations are found to be unsuitable for analyzing the relationship between surface water and groundwater in this watershed, due to the impacts of chemical weathering. The stable water isotope data indicate a gaining surface water system, i.e. groundwater and gully runoff both recharge streamflow. Further, groundwater appears to dominate the hydrologic cycle in the watershed. Therefore, water resources management on China's Loess Plateau should pay more attentions to groundwater protection.