Two-dimensional transport of soil water, heat and salt with mulched drip irrigation under brackish water in cotton fields and appropriate irrigation schedule in southern Xinjiang of China

This study aims to investigate the dynamics and distribution of soil water, heat, and salt in cotton fields under mulched drip irrigation in southern Xinjiang. The field experiment was conducted under different treatments of water and salt irrigation at Xinjiang Alar Modern Agriculture Academician and Expert Workstation in 2023. Three irrigation levels (75%, 100%, and 125% of crop irrigation water demand for cotton) and three levels of irrigation salinity (1.5, 3.5, and 5.5 g/L). The soil moisture content, soil salt content, and temperature of 0-80 cm layer were measured in the whole period of cotton growth. Then HYDRUS-2D model was used to explore the effects of irrigation amounts and salinity on the two-dimensional (2-D) migration and distribution of soil water, heat, and salt in the cotton field. Scenario simulation was implemented to reveal the mechanism of soil salt leaching/accumulation under different irrigation schedules. The appropriate irrigation schedule was proposed under brackish water irrigation. The main results were as follows. The horizontal moisture of the soil profile was unevenly distributed in the early stage of growth, indicating two-dimensional (2-D) distribution. The moisture content of the soil decreased gradually from the middle of the mulched area to the bare soil between the mulch in the horizontal direction. While there was a more uniform lateral distribution of soil moisture in the later stage. The wet range of soil and irrigation uniformity increased with the increase of irrigation quota. The higher irrigation quota, 125% of the irrigation water demand failed to increase the soil water content in the root zone, compared with 100% of irrigation water demand. The efficiency of soil salt leaching increased with the increase of irrigation amount. Salt accumulation position was shifted to the lower soil layer (40-80 cm) and bare soil under low irrigation salinity. While the moderate and high irrigation salinity levels increased the soil salinity beneath the drip tape and along the centerline of the plastic film. Higher water volumes under low irrigation salinity raised the salt content in the soil of the root zone under the mulch. Soil salt accumulation was dominated to decrease the depth of salt accumulation with the increase of irrigation salinity. The outstanding salt accumulation was observed at 60-80 cm depth under the mineralization degree of 5.5 g/L. Furthermore, the soil temperature under mulch was higher than that in the un-mulched area in the whole growth period, where soil temperature decreased with the increase of soil depth. Mulching has significantly enhanced the soil warming at 0-40 cm depth in the early growth stages. HYDRUS-2D platform reliably simulated the transport of water, heat, and salt in cotton fields under mulched drip irrigation. Specifically, excellent consistency was found in the simulated and measured values of soil moisture content, salt content, and temperature within 0-60 cm, where all the R² values were higher than 0.56. Scenario simulation indicated that the highest salinity of irrigation water was 3.2 g/L to prevent salt accumulation in the soil beneath the film (0-40 cm) under full irrigation conditions. Once the irrigation amount was 90% of the irrigation water demand, the salinity of 3.5 g/L was the maximum threshold for the salt balance in the root zone soil beneath the film.