Effect of long-term saline mulched drip irrigation on soil-groundwater environment in arid Northwest China

Water shortage and soil salinization are the two main factors that are limiting the sustainability of agriculture in arid and semi-arid areas. The mulched drip irrigation (MDI) with brackish groundwater is widely used in the arid areas of Northwest China. In this study, field experiments were carried out to study the effect of long-term MDI with brackish groundwater on the soil and groundwater environment. It was found that the groundwater level decreased in the Peacock river watershed steadily from 2008 to 2019, resulted from escalating groundwater exploitation due to the expanding agricultural irrigation area and increasing irrigation water demand. The decline of groundwater level reduced the evaporation of phreatic surface (ET_g) and groundwater recharge from MDI (R_g). The ET_g and R_g would be very small, where ET_g tended to be zero and R_g would decrease to a constant value, while the water table depth was larger than 3 m. In addition, MDI had little effect on the soil moisture content (SMC) during the MDI period while the groundwater level was shallow (less than 1.9 m), and it increased SMC gradually as the cycle of irrigations increased while the groundwater level was deep (greater than 4.2 m). MDI reduced the concentration of soluble salt ions (Na⁺, K⁺ and Cl^-) and increased the concentration of Ca²⁺ and SO₄^2- in the soil. The accumulation of Ca²⁺ and SO₄^2- in bare soil was more serious than that in the mulched land. The SMC, soil ions concentrations, soil salinity and the total dissolved solids of groundwater decreased significantly with the decrease of the groundwater level, and the salinization degree of the soil and groundwater tended to be weak in the field experimental site. However, groundwater level dropped too much caused by increasing agricultural irrigation would be harmful to the sustainable ecological environment.