Remotely sensed estimation of root-zone salinity in salinized farmland based on soil-crop water relations

Accurate monitoring and evaluation of root-zone soil salt content (SSC) are critical for sustainable development of irrigated agriculture in arid and semi-arid areas. Based on soil-crop water relations and farmland evapotranspiration (ET) fused through remote sensing data, this study developed an inversion method to estimate root-zone SSC using a case study from cotton fields under film mulched drip irrigation (CFFMDI) in the Manas River Basin (MRB) over 21 years (2000–2020). Two hypotheses were set as: (1) relative transpiration can be approximated by relative ET; and (2) the soil water stress response function is linearly proportional to the ratio of relative water supply. Measured data from a field experiment and collected data from regional survey and literature retrieval were used to optimize parameters and verify the hypotheses and method. The method was then applied to analyze the spatial and temporal distribution characteristics and cumulative effects of root-zone SSC. Results showed that the hypotheses and the method were reasonable and reliable in estimating root-zone SSC (with coefficient of determination R² > 0.50). Along with the popularization of film-mulched drip irrigation and the expansion of CFFMDI over the past 21 years, regional-scale root-zone SSC declined significantly with an annual attenuation rate of about 0.09 g kg⁻¹. Due to the gradual reduction of irrigation amount per unit area, the decline was more rapid before 2011 (0.18 g kg⁻¹), but slightly slowed down or even reversed at the end of the second decade (2015–2020). By 2020, the mean regional root-zone SSC reached 3.93 g kg⁻¹. At the beginning of this century, MRB was mainly composed of mildly- (59.8%) and moderately-salinized CFFMDI (39.9%). However, by 2020, non- (69.7%) and mildly-salinized cotton field (28.2%) dominated the basin. The inversion method of root-zone SSC fully considers the water consumption mechanism of soil-crop system, thus shows great potential in effective planning and management of soil and water resources in arid salinized areas such as MRB.