Hydrogeological Aquifer Characterisation by Multi-Physical Methods and Numerical Modeling: A case study of the Zhagu subcatchment (Tibetan Plateau)

The Tibetan Plateau (TP) is also called the “Third pole” because its aquifers constitute the origin for several major rivers, which are the water supply for millions of people all over Asia. Groundwater is the most important fresh water resource, however population is continuously growing, resulting in an increasing need of water supply. Due to the remote character of the TP, hydrogeological aquifer information is scarce, which leads to uncertain water resource management. To ensure future sustainable water supply, aquifer characterisation is therefore an important issue on the TP. This study is motivated by the need of increasing hydrogeological knowledge on the TP and provides the physical and numerical hydrogeological characterisation of the Zhagu subcatchment, which is a subcatchment of the third largest lake on the TP: the Nam Co Lake. This project is part of the International Research Training Group “Geoecosystems in transition on the Tibetan Plateau” (TransTiP), funded by the DFG.

Multiple interdisciplinary geophysical (electrical resistivity tomography, ERT), lithological (grain size analysis) and hydrogeological methods (observed hydraulic heads, hydraulic conductivity and recharge estimation) followed by numerical groundwater flow modeling (OpenGeoSys6, OGS6) were applied in order to hydrogeologically characterize the Zhagu subcatchment. The interdisciplinary results reveal the existence of a Quaternary hydrostratigraphic unit (Zhanongtang-Ganmanong aquifer). Furthermore, the results show three hydraulic conductivity zones in the Zhagu subcatchment. Monsoonal recharge in 2018 ranged between 108 and 242 mm and covered 30% to 67% of monsoonal precipitation. The physical results were interpreted into a conceptual model, which was prerequisite for the numerical groundwater flow model. For model calibration, the parameter estimation code PEST was coupled to OGS6. The model was successfully calibrated against hydraulic heads. The simulation results reveal that hydraulic head distribution ranged between 4691 and 5043 m and groundwater fluxes flow from the Zhagu subcatchment into the Nam Co lake by 0.03 m³m^-2s^-1.

This study provides an overall insight into the hydrogeological conditions of the remote Nam Co catchment on the TP.  The new insights of the hydrostratigraphic unit and numerical groundwater flow modeling results are helpful in order to improve current hydrological water balances, which neglect and/or assume groundwater inflow fluxes. In future, the calibrated model can be used for different water extraction and/or climate change scenarios in order to evaluate anthropogenic and climate change influences on the regional aquifer. This study and further future physical and numerical hydrogeological analyses can help to develop a sustainable water management on the TP.