Calibration of Two-Dimensional Variably Saturated Numerical Model for Groundwater Flow in Arid Inland Basin, China

Alluvial fan is an important recharge area for arid and semi-arid inland basins. In order to study groundwater circulation and quantify recharge amount of the groundwater system in arid inland basins, it is necessary to use numerical models. In this study, a 2D variably saturated numerical model of a typical profile has been developed using EOS 9 module of TOUGH2. The mesh for the fine soil plain area was refined compared with previous studies and the minimum cell size was 0.1 m in thickness. An improved approach in TOUGH2 was applied to calculate the groundwater evapotranspiration more efficiently and characterize water transport more accurately. Multiple calibration approaches were combined to calibrate the model. The results show that, the typical profile can be divided into three groundwater flow systems. The circulation depth for the local groundwater flow system is about 200 m and the shallow discharge accounts for 74.4% of the total amount with groundwater age less than 500 a (year). The circulation depth for the middle flow system can reach 800 m and the amount of discharge accounts for 18.5% of the total amount with groundwater age less than 10 ka (kiloyear). The circulation depth for the regional flow system is from 1000 to 1500 m, and the discharge accounts for 7.1% of the total amount with groundwater age ranging from 10 to 50 ka. The improved TOUGH2 numerical model, combined with multiple calibration approaches, can better reflect regional circulation characteristics and quantify the recharge amount of different groundwater sub-systems in arid and semi-arid inland basins with limited datasets.