Influence of groundwater table depth on the evolution of saline-alkali land in a coastal area

The groundwater table depth, which is typically shallow in coastal areas, is one of the decisive conditions impacting soil salinization. The influence mechanism of shaollow groundwater table depth change on the evolution of saline-alkali land is still unclear.  In this study, Huanghua City, a coastal city of the Bohai Sea in China, was selected as the research area, and four wells with groundwater table depths ranging from shallow to deep were studied. The groundwater table depth was divided into three levels based on the average groundwater table depth over 30 years, namely shallow, medium, and deep. We analyzed the influence of changes in groundwater table depth in each well on the evolution of saline-alkali land within a radius of 2 km. The results showed that changes in groundwater table depth did not cause conversion between saline-alkali land grades in shallow areas, whereas, in medium-depth areas, changes in groundwater table depth caused conversion among saline-alkali land grades. In deep areas, changes in groundwater table depth did not affect the conversion of moderate/heavy saline-alkali land to non-mild saline-alkali land. The lower limit and upper limit of the groundwater table depth response threshold of saline-alkali land were found to be 1.35 and 3.58 m, respectively, in this coastal area. When the groundwater table depth of saline-alkali land remains lower than 1.35 m for a long period, it will become heavily salinized, one appropriate treatment measure in this instance is to plant saline-alkali tolerant plants beacause improving saline-alkali land by reducing the groundwater table depth is a challenge. When the groundwater table depth of saline-alkali land remains greater than 3.58 m for a long period, changes in groundwater table depth will not influence the gradual reduction of salinization, the saline-alkali land should be properly irrigated and drained to avoid secondary salinization caused by the groundwater level rising to the upper limit of the response threshold. When the water depth in saline-alkali land remains between 1.35 and 3.58 m for a long time, the degree of salinization will change with the change in water depth, the underground water depth can be reduced under a subsurface drainage system in order to treat the saline-alkali land. The research results can provide a scientific basis for improvement of saline-alkali land.