Abstract:

Accurate estimation of the groundwater table dynamic variation in a riparian zone under ecological water conveyance conditions is of great importance for the sustainable development of water resources management in arid areas. Based on the data of river discharges from ecological water conveyance and the groundwater tables of riparian monitoring wells from 2001 to 2020, this study simulated the dynamic variation of groundwater table at three sections of Yingsu, Alagan, and Yiganbujima in the lower reaches of the Tarim River, Xinjiang, China using a developed quasi-two-dimensional groundwater model, including the groundwater and soil water motion equations. In the calibration period, the observed groundwater table data was used to adjust the critical parameter (i.e., horizontal hydraulic conductivity) of the quasi-two-dimensional model, and the average simulation error of the monitoring wells at each of the three sections was approximately 0.2 m. In the validation period, the groundwater table data from 2011 to 2020 at the monitoring wells of the three sections were compared with the simulated results, and the average error at each of the three sections was approximately 0.5 m. Therefore, the simulated results of the groundwater table were acceptable under the lack of accurate river level data and the model was suitable for simulating the variation of the riparian groundwater table under ecological water conveyance in the lower reaches of the Tarim River. Thus, based on the simulation results from 2001 to 2020, the groundwater table and soil moisture variations were analyzed to demonstrate the response of riparian groundwater and soil moisture to the ecological water conveyance project. The results show that the groundwater table and soil moisture at the three sections evidently increased in 20 years of water conveyance periods. Specifically, the groundwater table increased from approximately 8 m before ecological water conveyance to nearly 4 m, and the corresponding soil moisture rose from 0.20 to 0.35 above. The increasing trend is more significant with the substantial increase of the annual water conveyance amount since 2009. Moreover, the variation of groundwater table presented a certain lag following the change in the water conveyance amount, and a positive correlation between soil moisture and groundwater table makes the soil moisture lags behind the water conveyance amount. Compared with the river discharge, the groundwater hydraulic conductivity plays a more important role in the variation of groundwater table at cross-section; thus, increasing the upswing amplitude of the groundwater table at Yiganbujima sections of the lower segment compared with that of the Alagan section of the middle segment in lower reaches of the Tarim River. Additionally, the inter-annual variations of groundwater and water conveyance amount also indicate that the intermittent ecological water conveyance is essential to achieve a sustainable ecological benefit in the lower reaches of the Tarim River.

Key words: quasi-two-dimensional groundwater model, ecological water conveyance, long-term water table simulation, the lower reaches of Tarim River