Simulation and prediction of groundwater considering seasonal freezing-thawing in irrigation area with conjunctive use of groundwater and surface water

Abstract: The conjunctive use of groundwater and surface water is a promising measurement to alleviate the water resources shortage in Hetao irrigation district. It can not only decrease the amount of water diversion from the Yellow River but also prevent the deterioration of soil salinization by decreasing the groundwater table and evaporation. However, it might have adverse effects on ecological environment if over-exploitation happens. Therefore, it is necessary to evaluate the impacts to groundwater after conjunctive use of groundwater and surface water. In this paper, we selected Hetao irrigation district as the study area to estimate the temporal and spatial groundwater dynamics under conjunctive use of groundwater and surface water. The freezing and thawing period in Hetao irrigation district lasts for nearly half a year, during which the mechanism of groundwater dynamics is quite different from that of unfreezing period, with multiple complex impact factors. Studies showed that temperature was the most important factor that drives the groundwater table change during the freezing and thawing period. There were good correlations between water table depth and air temperature. The water table depth of Hetao irrigation district in freezing-thawing period was related to the air temperature before 48 days. Trigonometric function was used to fit the change of temperature and depth. A sub-model was developed to calculate the source term of groundwater system in freezing and thawing period by establishing the relationship between groundwater recharge/discharge flux and air temperature. The sub-model in the freezing-thawing period was integrated with the three-dimensional groundwater model Modflow for the whole year simulation. Model parameters were calibrated with datasets from 2006 to 2010 and were validated with datasets from 2011 to 2013. Comparisons between simulated and observed water table depth showed that the coupling numerical model was able to predict change of groundwater table reasonably well. Then, the numerical model was applied to estimate the impacts of conjunctive use of groundwater and surface water under 18 water saving scenarios with 3 different upper limit of groundwater salinity (2.0, 2.5, 3.0 g/L) and 6 different conjunctive ratios. The simulating results indicated that both the upper limit of groundwater salinity and conjunctive ratio had great impact on groundwater table, since they determined the amount of groundwater withdrawal. Higher groundwater salinity upper limit and smaller conjunctive ratio resulted in lower groundwater table. From the results of 18 water saving scenarios simulation, we concluded that after implying the conjunctive use measurements, the annual average groundwater table increased relatively by 0.103 m to 0.445 m in summer irrigation period, 0.076 m to 0.243 m in autumn irrigation period, and 0.096 m to 0.216 m in freezing and thawing period. The value of groundwater table decline ranged from 0.346 m to 0.635 m in the conjunctive use of groundwater and surface water irrigation district, less than 0.07 m in surface water irrigation district, and 0.096 m to 0.316 m in the whole district on average. The total recharge and discharge volume of groundwater were basically balanced after conjunctive use of groundwater and surface water. The phreatic water evaporation and water diverted from Yellow River could be respectively reduced by 2.243×108m3/a- 5.120×108 m3/a and 3.765×108 m3/a-8.401×108m3/a. The less phreatic water evaporation was the key point of saving water by the conjunctive use of groundwater and surface water. It decreased useless evaporation and increased water use efficiency so that water resources for agricultural irrigation can be decreased. The river leakage increased by 3.1%-4.4% than before. Thus, the conjunctive use of groundwater and surface water does not result in an unacceptable increase in the amount of river leakage. The research provides important information for conjunctive use of groundwater and surface water in similar seasonal freezing-thawing irrigation district.