Threshold value of ecological water table and dual control technology of the water table and its quantity in the salinized farmland around wetland in arid areas

The farmland around wetland in the arid areas in northwest China is easily salinized. Reasonably controlling and reducing groundwater levels in real time is an effective way of protecting wetland and preventing and controlling farmland salinization. The typical demonstration zone is selected in the transitional zone of wetland and farmland near the Dengmaying Lake of the Shiyang River Basin in the arid area. The dynamic characteristics of groundwater depth and the synergic relationship between groundwater depth and salt content of topsoil are analyzed. The threshold value of the ecological water table is confirmed. The Intelligentized dual control technology of water table and its quantity is developed, which is based on groups of radial wells connected by siphons. The key points of the technology are as follows: one well collecting many radial wells by siphons is used for increasing the water yield of the well in the aquitard and contributing to control the water level in the form of a plane. The subsystem consisting of the electricity system, signal system and controllers is used to control the groundwater level and water yield in real time. Demonstration test of the key technology application is carried out and the application effect is remarkable. The results show that when the groundwater depth increases, the risk of salinization and bulrush coverage fraction in wetland decreases. The threshold value of groundwater depth for protecting wetland and preventing and controlling farmland salinization ranges from 1.9 to 3.0 m. The intelligentized regulatory system could make the groundwater depth stay in the suitable ecological range in the stage of evaporation from phreatic water in July and August of each year, when is the critical period of topsoil salification. The regulation not only can decrease the increasing amplitude of soil conductivity caused by irrigation, but also reduce the speed of topsoil salification. The effect of dissolving and controlling salt by the dual control technology of water table and its quantity is more obvious under the condition of fresh water irrigation than under the condition of brackish water. The technology can help realize the accurate regulation and control of groundwater level and it is of realistic significance for wetland protection and preventing and controlling farmland salinization in arid areas.