# Assessing the Long-Term Evolution of Abandoned Salinized Farmland via Temporal Remote Sensing Data
## 2 Study Area  
Covering a total area of 1.12 \(\times\) 10\({}^{6}\) ha, the Hetao Irrigation District (40.1\({}^{\circ}\)-41.4\({}^{\circ}\) N, 106.1\({}^{\circ}\)-109.4\({}^{\circ}\) E) is one of the largest irrigation systems in China (Figure 1). The study area is approximately 250 km in width from west (Wulanbuhe Desert) to east (Wuliangsuhai Lake) and 50 km in length from south (Yellow River) to north (Langshan Mountain). There are mainly five types of surface coverage in the Hetao Irrigation District recorded in the 2020 land cover products provided by GlobeLand30 ([[http://www.globallandcover.com/](http://www.globallandcover.com/)]([http://www.globallandcover.com/](http://www.globallandcover.com/)), accessed on 31 August 2021). The sand dune area is mainly distributed in the western part, accounting for approximately 7% of the total area. The area with water bodies is approximately 6%. Wuliangsuhai Lake, the largest lake in Hetao, is located in the eastern part of the study area and receives drainage water from the irrigation system. The vegetative area and residential area account for 77% and 8% of the total area, respectively. Spring wheat, maize, and sunflower are the major crops in Hetao, with areas of 4.9 \(\times\) 10\({}^{4}\) ha, 3.0 \(\times\) 10\({}^{5}\) ha, and 2.8 \(\times\) 10\({}^{5}\) ha in 2020, respectively ([[http://ij.bynr.gov.cn/](http://ij.bynr.gov.cn/)]([http://ij.bynr.gov.cn/](http://ij.bynr.gov.cn/)), accessed on 31 August 2021). Due to the typical arid and semiarid climate, the Hetao Irrigation District has deficient annual average precipitation (ranging from 60 mm to 280 mm), which is mainly concentrated in July and August, while the potential evaporation reaches 2000-2400 mm per year. The large divide between crop evapotranspiration and actual rainfall provokes high dependence on irrigation water for agriculture.  
In Hetao, the main irrigation method is flood irrigation. In response to the national water-saving irrigation policy, drip irrigation has been deployed in Hetao since 2015. However, most areas with drip irrigation are small demonstration areas. Approximately 4.79 billion m\({}^{3}\) of water per year (2020, Statistics Bureau of Bayannur City) is diverted into the Hetao area from the Yellow River. The enormous scale of irrigation has lifted the groundwater table of Hetao, which varies between 0.5 and 3.0 m annually. In the meantime, irrigation water transports salt into the groundwater, which makes the salinity of the groundwater increase. Capillary water with high saline content brings salt into the field, and thus, soil salinization occurs.  
An artificial drainage system was established to combat the soil salinization process in the Hetao Irrigation District. However, this system operates with low efficiency due to the gentle slope of the terrain in the Hetao area, by which merely thirty percent of the inlet salinity was drained out of the irrigation district. To balance the soil salinity in the field, local farmers deploy a typical irrigation pattern, termed autumn irrigation; that is, irrigating the uncropped field after harvest (usually conducted from mid-October to mid-November). An amount of water is applied in the autumn irrigation to leach the soil salinity downward, which balances the annual soil salinity. Unfortunately, autumn irrigation only leaches the salt-affected surface soil, and there may be an adverse effect if an overdose of irrigation is applied. The soil saline content in the over-irrigated land bounces back in spring after the thawing of the soil water.  
Figure 1: Overview of the Hetao Irrigation District: (**a**) geographic location, (**b**) land surface of the study area depicted by Landsat 8 OLI imagery captured on 30 October 2019, and (**c**) soil sampling locations for EC measurement.