Coupling Simulation Of Soil Water-salt Movement In Plastic Film Mulched Cotton Field Under Saline Water Irrigation

Researches on saline water security irrigation for cotton, a salt-tolerant crop in the Hebei low plain area are of great importance to alleviate regional freshwater resource shortage, to promote saline-dry land improvement and saline water exploitation and utilization, and to ensure the sustainable development of cotton industry. In this study, the conventional border irrigation and cotton planting with film mulching were investigated in Shenzhou City, Hebei Province, from 2012 to 2014. Four salinity water treatments were designed, which were 1, 3, 5, 7 g/L, respectively. Soil water and salt movement in the cotton field with film mulching were measured and simulated under different salinity water irrigation. The responses of cotton growth to saline water irrigation were revealed. Change trend of soil water and salt content in the future was predicted under different scenarios of irrigation water salinity. The main results were as follows:(1) Elaborating the water and salt variations within and among years under saline water irrigation and film mulching. During cotton growing period, soil water and salt content fluctuated with variation of rainfall, irrigation and cotton evapotranspiration. Soil moisture and salt content increased with the increase of irrigation water salinity. In wet years, soil moisture of root layer was sufficient and salt was completely washed out; while in drought years, soil moisture in cotton root zone was low and salt accumulated in the root zone. After continuous saline water irrigation over the 3 different hydrological years, soil salt content in cotton root zone did not accumulate. On the contrary, compared with the beginning of the cotton experiment in 2012, the treatments of 1, 3, 5, 7 g/L all appeared desalination at cotton harvest in 2014. The distribution of water and salt was uneven due to film mulching. At seedling stage, the soil moisture under mulching was higher than non-mulching, while the salt showed opposite trend. After budding stage, the water and salt under mulching were lower than non-mulching.(2) Clarifying the effects of different salinity water irrigation on the cotton emergence rate, the above- and below-ground growth, yield composition and fiber quality. Three consecutive year experiment illustrated that there were no significant differences among the effects of 1g/L, 3g/L and5g/L irrigation on the cotton emergence rate, number of bolls, single boll weight and raw cotton yield, while the 7g/L significantly decreased all of these parameters. The electronic conductivity in the root zone was 7.1~8.8d S/m when the raw cotton yield began to decrease.(3) Presenting the estimation method of cotton evapotranspiration under saline water irrigation. Soil evaporation had an increasing trend with the increase of irrigation water salinity, but the differences in crop water consumption were not significant among the four treatments. There were no significant differences in water use efficiency among the treatments with salinity of 1g/L,3 g/L and 5 g/L. However, the value of water use efficiency of cotton irrigated with salinity of 7 g/L was significantly reduced. Calibrating the basic crop coefficient with the salt stress coefficient, evapotranspiration in cotton field with film mulching under saline water irrigation was simulated using the dual crop coefficient model-SIMDual Kc and the estimation accuracy reached up to 93.6%.(4) By localizing the parameters of HYDRUS-2D and identifying the proper simulation domain and boundary conditions, the two-dimensional movement of water and salt movement under saline water irrigation and film mulching were better simulated. The simulation error of water and salt concentration was controlled in 10% and 20% respectively.(5) Predicting the dynamics of water and salt in cotton field under multi consecutive year saline water irrigation. In a 20 year consecutive irrigation scenario, 3g/L saline water did not lead to significant salt accumulation in the root zone. The 5g/L and 7g/L lead to secondary salinization, while the 5g/L just lead to lower salinization. Considering the soil salt variation and cotton salt tolerance, the saline water under 5g/L can be directly used for cotton sowing and supplemental irrigation. According to the simulation result, the appropriate irrigation program on cotton was presented for the Hebei low plain.