Spatio-temporal changes in subsurface soil salinity based on electromagnetic induction and environmental covariates at the Tarim River Basin, southern Xinjiang, China

Abstract

Changes in water and salt content in the root zone directly affect the survival strategies and health status of natural vegetation in arid regions. Since the 1980s, climate change and intensified human activity have resulted in spatial changes in soil salinity in the root zone (SSRZ, 30–100 cm) of the oases in the Tarim Basin. These changes have not been explored adequately. Therefore, this study quantifies the SSRZ based on Landsat OLI, electromagnetic induction (EMI), and auxiliary variables to evaluate the difference between current conditions and the historical record. Environmental covariates derived from the SCORPAN framework were employed for SSRZ modelling using the random forest (RF) algorithm. The results were as follows: (1) The EC_a spatial model established based on ECa readings and environmental variables showed that the R² values of the validation models were between 0.77 and 0.84 at four EMI modes. (2) The EC_a maps were then used as environmental variables for RF-SSRZ model construction with 84 % modelling accuracy using a calibration dataset (70 %) and validation dataset (30 %), which indicated that 83 % of the SSRZ spatial variation could be explained by this method. (3) Variable importance calculated by recursive feature elimination (RFE) showed that the contribution of four EMI dipole modes occupied the top four positions in the variable importance ranking. (4) The spatial variability pattern of the ECa maps and the RF-SSRZ map was approximately similar to the EC_eHWSD (30–100 cm) affiliated with the Harmonized World Soil Database (HWSD) generated in the 1980 s. (5) The SSRZ change over the past 40 years was calculated based on the EC_eHWSD and SSRZ maps with HWSD soil type units, and 43 % of the area showed alleviation. The three desalination levels were 2.88 % for –10 to 0 dS·m⁻¹; 12.49 % for –10 to –28.33 dS·m⁻¹; and 27.58 % for <–28.33 dS·m⁻¹. Within the oasis and on both sides of the Tarim River, a 57.05 % increasing trend was computed between 0 and 2.08 dS·m⁻¹.