The water body inside the constructed wetland is affected by various factors, and the flow state is relatively complicated. There will always be a certain degree of low velocity area and rapid outflow phenomenon, which makes part of the space in the wetland unable to be effectively used. Based on Computational Fluid Dynamics (CFD) technology, this paper uses Fluent's porous media model and discrete phase model to establish a hydrodynamic model of up and down baffled subsurface flow constructed wetland system. The internal flow field of the wetland is simulated, and the hydraulic performance of different baffle settings and substrate laying methods in the wetland is systematically evaluated. The results show that when the number of baffles is the same, the hydraulic efficiency is higher when the first baffle is located on the lower part of the substrate. Compared with the position of the baffle, the increase in the number of baffles does not significantly improve the hydraulic efficiency of the constructed wetland. The substrate layer thickness ratio has a significant effect on the two parameters of the variance of the hydraulic residence time distribution (σ2) and the flow divergence (σ02). By increasing the thickness of the middle substrate, the low flow rate phenomenon caused by the small porosity substrate area of the upper layer and the rapid outflow phenomenon of the lower substrate can be improved to a certain extent, the utilization efficiency of the middle substrate layer is improved, and the hydraulic performance is increased. The research results are of great significance for improving the utilization of wetland space and ensuring its efficient decontamination and purification function.

Bromide was used as tracer in the Weishan Irrigated District to determine the groundwater recharge as well as to evaluate the impacts of different irrigation basin locations, irrigation regimes, and crop types on the recharge. The comprehensive recharge coefficient and the Kriging Spatial Interpolation methods were used to distinguish the effects of precipitation and surface water irrigation on the groundwater recharge rate. The results show that the recharge rates ranged from 85.8 to 243 mm/a, with an average of 168 mm/a. The average recharge rate in the upstream district is greater in the downstream and the average recharge rate of irrigated land (193 mm/a) is greater than non-irrigated land (110 mm/a). The recharge rates in fields of winter wheat-summer maize and cotton with irrigation are 210 mm/a and 140 mm/a, respectively, while they are 115 mm/a and 94.1 mm/a under no irrigation conditions. The comprehensive recharge coefficient of groundwater in the upstream irrigation area is larger than that in the downstream. By comparing the spatial distribution of the groundwater level and the comprehensive recharge coefficient, it is found that there is a positive relationship between the groundwater level and the comprehensive recharge coefficient. The results of this study can provide reference and guidance to a water resources analysis of the Weishan Irrigated District.

Biochar has been extensively used for the improvement of soil environment in fields. However, the effects of biochar on soil water movement and crop productivity under different irrigation rates were not well clear or consistent. The objectives of this study were to investigate whether biochar could prevent the production of preferential soil flow, and to study the effects of biochar and irrigation on soil properties, nutrient leaching, crop growth and yield, which was of great significance to improve crop water and fertilizer use efficiency and protect the ecological environment. The experiments included two parts, i) short-term blue dye tracer test, to investigate the effect of biochar application (20 and 40 t/hm2) on the degree of preferential flow, and ii) a field trial with winter wheat (Triticum aestivum L.) as test material was conducted in the lysimeters in 2018-2019, to investigate the effect of biochar application (20 t/hm2) on the soil nitrate nitrogen distribution and wheat yield under two irrigation rates (0.6 reference crop water requirement (ETc), 0.8 ETc). The results showed that i) within the designed biochar application rates, the degree of preferential flow was decreased with the increase of biochar application, but the decreasing speed slowed down significantly, with the preferential flow fraction (PF-fr) reduced by 6%~17.5%, especially under high irrigation, ii) compared with the control treatment, biochar improved the capacity of the soils to retain nutrients, alleviated the negative effect of water shortage, and increased winter wheat productivity. Our results suggested that the 0.8 ETc irrigation and 20 t/hm2 biochar was recommended in the field condition.