Abstract: In order to accurately and efficiently study the transforming relationship between surface water and groundwater, and the renewal capacity of groundwater system in Qaidam Basin, northwest China, a two-dimensional variably saturated numerical model of a typical profile from Nuomuhong River to Amunike Mountain was developed using TOUGH2 simulator. On the basis of previous studies, the evaporation was a function of soil saturation given as an upper boundary to characterize water transport near surface through iterative calculation. The numerical model was calibrated with 41 groundwater observation data based on trial-and-error method. The simulated results fitted well with observation data. The correlation coefficient of simulated groundwater heads was 0.9764 m, and the absolute error was 1.6 m. The simulation results showed that the typical profile of Qaidam Basin could be divided into three orders of groundwater flow system:local groundwater flow system accounted for 58.8% of total amount of groundwater flow with spring as the main discharge; the middle flow system accounted for 27.6% of the total amount of water, with evaporation and river as the main discharge; regional flow system accounted for 13.6% of total amount of water, with evaporation as the most important discharge. The results showed that TOUGH2 with modified boundary condition can accurately describe water transport in variable saturated zone under the conditions of phreatic fluctuation. The study of the renewal capacity of groundwater and the characteristic of groundwater flow system has great significance for the areas lack of intensive studies. In the future research, different supply sources and forms, as well as dispersion and density changes can be taken into consideration to simulate groundwater ages and further study the groundwater systems.

Key words: Qaidam Basin, saturated-unsaturated, groundwater flow system, TOUGH2