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

In order to investigate the dynamics and distribution of soil water, heat and salt in cotton field under mulched drip irrigation in southern Xinjiang, we conducted field experiment under different water and salt irrigation treatments at Xinjiang Alar Modern Agriculture Academician and Expert Workstation in 2024. The experimental settings include three irrigation levels (75%, 100%, and 125% of crop irrigation water demand for cotton, respectively) and three irrigation salinity levels (1.5, 3.5, and 5.5 g/L, respectively). The soil moisture content, soil salt content and temperature of 0-80 cm layer were measured during the whole cotton growth period. Then we used HYDRUS-2D model to explore the effects of different irrigation amounts and irrigation salinity on the 2-D migration and distribution characteristics of soil water, heat and salt in the cotton field. Through scenario simulation, the mechanism of soil salt leaching/accumulation under different irrigation schedules was revealed, and the appropriate irrigation schedule under brackish water irrigation was proposed. The main results are as follows. In the early stage of growth, the horizontal moisture of soil profile was unevenly distributed, showing two-dimensional distribution characteristics. In the horizontal direction, the moisture content of soil gradually decreased from the middle of mulched area to the bare soil between the mulch. While in the later stage, the lateral soil moisture distribution was gradually more uniform. The wet range of soil and irrigation uniformity increased with the increase of irrigation quota. The higher irrigation quota, 125% of irrigation water demand, could not effectively increase the soil water content in root zone compared to 100% of irrigation water demand . With the increase of irrigation amount, soil salt leaching efficiency increased. Salt accumulation position moved to lower soil layer (40-80 cm) and bare soil under low irrigation salinity, while moderate and high irrigation salinity levels increased 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. With the increase of irrigation salinity, soil salt accumulation dominated and the depth of salt accumulation decreased. Salt accumulation was obvious at 60-80 cm depth under the mineralization degree of 5.5 g/L. During the whole growth period, the differences in the distribution of soil temperature showed that soil temperature under mulch was higher than the un-mulched area, and soil temperature decreased with the increase of soil depth. Mulching significantly enhanced soil warming effects in the early growth stages at 0-40 cm depth. HYDRUS-2D could reliably simulate the transport of water, heat, and salt in cotton field under mulched drip irrigation, where the simulated values of soil moisture content, salt content and temperature within 0-60 cm show good consistency with the measured values, and all the R² values were higher than 0.56. Scenario simulation results indicate that under full irrigation conditions, the highest irrigation water salinity to prevent salt accumulation in the soil beneath the film (0-40 cm) was 3.2 g/L. When the irrigation amount was 90% of irrigation water demand, the salinity of 3.5 g/Lwas the maximum threshold to ensure salt balance in the root zone soil beneath the film.