Evaluating soil water and salt transport in response to varied rainfall events and hydrological years under brackish water irrigation in the North China Plain

Soil salination challenges sustainable agricultural production and management in the lowland area of the NorthChina Plain (NCP). This study investigated the effect of varied rainfall events and hydrological years on deepdrainage and salt leaching in Nanpi County in the NCP. Field experiments were conducted to measure soil matricpotential (SMP) and soil water electrical conductivity (ECw) in 300 cm profiles of an irrigated cropland (CL) andunirrigated grassland (GL). The HYDRUS-1D model was calibrated and validated to optimize soil water and salttransport parameters using the SMP and ECw data obtained in 2015 and 2016, respectively. The results showedthat the HYDRUS-1D model was reliable to simulate soil water and salt migration in 300 cm profiles at both sites.The salt leaching depths and degrees were positively correlated with precipitation amount. The heavy rain (45mm) and rainstorm (68 mm) favored salt leaching within 100 cm, and severe and extreme rainstorms resulted insalt leaching within 150–200 cm both in the CL and GL. The SDP in the CL was greater than that in the GL in allhydrological years owing to agricultural cultivation and irrigation. Salt leaching in the 300 cm profiles mainlyoccurred in the rainy seasons of wet year, with the salt decrease percentage (SDP) being 61% in the CL and 42%in the GL. Salt leaching due to precipitation was sufficient in the irrigated CL in all hydrological years, andprecipitation in normal rainfall years could remove 70–80% of soil salts within 50–63 cm soil depth. Grassgrowth without irrigation led to water deficits and salt accumulation when the annual precipitation amount wasless than that in wet years. The results demonstrated that precipitation, combined with irrigation using ofbrackish and freshwater, could be usefully for soil salt leaching. A recommended irrigation scheme for watersaving and salt control in CL and GL is one brackish water irrigation application and three freshwater irrigationapplications each year in the NCP.