Modeling and assessing feasibility of long-term brackish water irrigation in vertically homogeneous and heterogeneous cultivated lowland in the North China Plain

In the North China Plain (NCP), increased water shortages and food yield increase present serious threats to the sustainability of cultivated lands. Irrigation by brackish or saline water is a possible solution to alleviate freshwater shortages. It is critical to understand the soil salt variations and the characteristics of salt accumulation and leaching under long-term brackish water irrigation in cultivated croplands. In this study, the HYDRUS-1D model was calibrated and validated, and then applied to evaluate that how 20 years of irrigating with brackish water affects soil salinity and soil salt transport in wheat-maize cultivated lands with different texture layers in the lowland of NCP. The results showed that the simulations of soil matric potential and soil salt concentrations fitted well with the measured values. Soil salt dynamics were dominated by seasonal precipitation and hydrological years. Seasonal rainfall distribution determined seasonal characteristics of salt changes in shallow layers (0-100 cm). Soil salt accumulated during the growth period of winter wheat corresponding to the dry season, and was greatest at the harvest. Soil salt leached down to deeper soil layers under abundant rainfall during growth period of maize. The annual soil salt in 300 cm profiles were driven by hydrological years, and soil salt could be accumulated in dry and normal years and leached in wet years, especially in the extremely wet year. Soil salt accumulation was higher under two brackish water irrigations compared to one irrigation for winter wheat, and soil salinity in homogeneous soils was less than in the heterogeneous soil with clay loam interlining. The results showed the effect of irrigation times of brackish water was greater than that of soil texture on soil salt accumulation. Our results demonstrated that wheat and maize could be grown well under one irrigation with brackish water a year. Salt accumulation increased with the number of irrigation events that used brackish water for winter wheat. The brackish water irrigation was evaluated and the results demonstrated that partial substitution of fresh water by brackish water for irrigation is feasible in the lowland area of the NCP. It is more suitable for long-term brackish water irrigation in relatively homogeneous soil than in heterogeneous soil. These simulated results are helpful to provide appropriate management measures for long-term brackish water irrigation and fresh water saving, and provide a basis for assessing the environmental effect under long-term brackish water irrigation.