Effects of succession processes of marsh wetland and farmland on groundwater in the Sanjiang Plain

The Sanjiang Plain is an important base of commodity grain production and contains lots of marsh wetlands in China. In the past 60 years, with the continuous increase of farmland area and the adjustment of planting structure, the decline of wetland and groundwater level has attracted great attention, whether the over-exploited groundwater is controversial. The remote sensing image data from 1956 to 2019 were selected to the analyze the evolution process characteristics of wetland and farmland using single land use dynamic degree. Based on the simultaneous measurement data in 1980 and 2019−2021 and the national groundwater monitoring project data, the influence of wetland reclamation on the groundwater level was explored. The results show that: (1) From 1956 to 2019, the marsh wetland presented a decreasing trend, while the dry land showed a trend of increasing first and then decreasing, and the paddy field had a trend of increasing first and then stabilizing. Sanjiang Plain was characterized by “wetland turning into dry land” with the area of 2.36×10⁴ km² from 1956 to 1996, while it presented “dry land turning into paddy field” with the area of 1.15×10⁴ km² from 1996 to 2019. (2) The decrease in groundwater level in the area of 36 546 km² in the Sanjiang Plain from 1980 to 2021 was less than 5 m, and more than 10 m in the eastern area with 3 669 km². The groundwater in the Jiansanjiang area is being over-exploited. (3) Compared with the groundwater level in the dry season in 1980, the groundwater depression cone area was 3 669 km² in 2021 with the depth of 10 m as the standard, which was 269 km² larger than that in 2019, and slightly expanded to the northeast direction. (4) In 2019, it was difficult to reach the natural regulation of “abundance to make up for deficiency” under the condition of heavy rainfall in the Jiansanjiang reclamation area, and the groundwater storage decreased by 5.81×10⁸ m³. The results of this study provide a foundation for the study on regional water balance and are of great significance to the scientific understanding of the rational development and utilization of water and soil resources.