# Assessing the Long-Term Evolution of Abandoned Salinized Farmland via Temporal Remote Sensing Data
## 4 Results
### Desalization Progress in Hetao  
A significant exponential relationship between the SI and measured soil EC values is shown in Figure 9. The EC map was estimated from the SI via the SI-EC relationship in Figure 9, and the salinity classes were determined by EC values using the widely used criteria as follows: (1) 0-4 dS/m: salt-free or slightly saline; (2) 4-8 dS/m: moderately saline; (3) 8-16 dS/m: highly saline; and (4) >16 dS/m: extremely saline [39]. The area was calculated by counting the number of pixels for each saline class (Figure 10). Some research has indicated that the \(\mathrm{SI}=\sqrt{\rho_{blue}\times\rho_{red}}\) shows the best results [38], while the results of that study were for the entire study area; different degrees of salinity were not studied separately. In the future, more samples should be collected to analyze different degrees of salinization separately. Only imagery collected in March and April was used to calculate the SI when the study area was crop-free and unfrozen. The missing data for specific years indicated that there was no cloud-free imagery available from March to April. The results show that the area of non- (or slightly) saline conditions continued to expand from 1988 to 2019. In contrast, the rate of decline for the moderately saline area has accelerated since 2000, while the highly saline area shrank sharply from 1993 to 2001. The area with extreme salinity decreased to a low level after 2000, indicating the construction of drainage facilities and promoting water-saving irrigation technology, which shows a positive effect on desalinization.  
Figure 8: The evolutionary progress of cropping structure in Hetao from 1988 to 2019. The dashed lines represent the statistical area of Bayan Nur city. The solid lines are the detected cropland area in the Hetao Irrigation District.  
Figure 10: The dynamics of desalinization in Hetao: (**a**) salt-free/slightly saline and moderately saline; (**b**) highly saline and extremely saline. The area for each year was statistically analyzed using mono-temporal SI data from March or April when the study area was uncropped. The trends are depicted by moving average data with a window of 5.  
Figure 9: Exponential relationship between the SI (Sentinel 2) and measured EC. The SI was calculated from Sentinel 2 imagery from 22 April 2019 when the area was uncropped and unfrozen.  
The spatial distribution of saline classes in Hetao from 1988 to 2019 is visualized in Figure 11. In 1988, most lands in Hetao suffered varying degrees of salinity, and the areas with extreme salinity were scattered throughout the study area. Due to the construction of drainage facilities, this situation was relieved in the next two decades. A pumping station was set up at the outlet of the irrigation district (located at the easternmost boundary of Hetao) in 1975, and a field drainage system was built from 1988 to 1996, which drains water into Ulansuhai Nur Lake (east of Hetao). The facilities turned eastern Hetao salt-free (or slightly saline) (Figure 11b,c). However, the drainage system ran at low efficiency in western Hetao due to the flat terrain, which caused western Hetao to still be affected by severe salinity even in 2010 (Figure 11b,c). In the last decade, the changes in the cropping structure (planting more sunflower and corn instead of wheat) required less irrigation. This was because the deeper roots of sunflower and corn enhanced their ability to uptake water from the shallow groundwater table. In addition, the promotion of water-saving techniques reduced the inlet salt from the Yellow River. As a result, the saline situation was significantly mitigated in 2019, when most of western Hetao turned salt-free (or slightly saline) (Figure 11d).