Long-term gully dynamics over cropland in the black soil area of China based on systematic sampling

Understanding the large-scale spatial distribution characteristics of gully development dynamics, particularly over long periods, can help in accurately identifying areas with severe gully erosion and is thus crucial for targeted gully prevention and rehabilitation efforts. This study aimed to investigate the long-term dynamics of permanent gullies on cropland in the Songnen typical black soil region (SBR), which is the most important commercial grain production area in China, covering an area of 212,000 km². For this purpose, 998 sampling units were selected using the systematic sampling method. Based on Corona KH-4B images from 1970 and Google images from 2018, all gullies within each sampling unit were visually interpreted. In the past 50 years, the number of permanent gullies on cropland in SBR increased by 24.55 %, but the average linear density of gullies in the cropland sampling unit decreased from 0.47 to 0.45 km·km⁻² because the average lengths of gullies decreased from 285.90 m to 233.15 m. While 50.50 % of gullies found in 1970 disappeared from the images of 2018, more gullies formed and were widespread in the east part of the study area characterized by a topography of rolling hills. In particular, 66.70 % of gullies were active, including all newly formed gullies and 16.28 % of long-standing gullies (LSGs), and the average gully retreat rate of LSGs was 0.53 m·yr⁻¹, with active LSGs grew at a rate of 3.26 m·yr⁻¹ on average, indicating the severity of gully erosion and limited effectiveness of efforts made to control gully erosion in the black soil region of China. The threat of gully erosion is more serious in the eastern part of SBR, with 44.69 % of cropland suffering gully erosion and 66.14 % of the gullies being active. Moreover, the trend of increased gully erosion in the centre and the west requires further attention. The findings highlight the need for studies on more effective and targeted measures for gully control and their wide application in order to ensure the sustainable utilization of the valuable black soil resources.