Effects of irrigation with reclaimed water on hydraulic characteristics of red soil

Abstract: With the rapid development of China's economy, the requirement for water resources from all walks of life is increasing. Using reclaimed water and other unconventional sources can greatly alleviate the contradiction between supply and demand for fresh water. However, long-term reclaimed water irrigation is one of the key factors that leads to the evolution of hydraulic characteristics of soil, and likely to increase the risk of soil degradation due to the high salt content of reclaimed water. As the irrigating water source, the reclaimed water discharged from the sewage treatment plant was selected in this test .During the wetting-drying cycle irrigation test, tested red soil was irrigated persistently with four kinds of reclaimed water with different concentrations, which were original reclaimed water (RW) and diluted concentration of 2 (RW-2), 4(RW-4), 6(RW-6) times. The selecting distilled water irrigation was control treatment (CK). Meanwhile, two irrigation modes applied to this study, such as continuous irrigation and alternate irrigation (irrigation with reclaimed water and distilled water alternately/ the corresponding treatments were expressed as: ARW, ARW-2, ARW-4, ARW-6). Finally, sampling was not started until the infiltration rate tended to stabilize after three times under the same treatment. In order to analyze the hydraulic characteristics of red soil, the soil water characteristic curve was measured by pressure membrane method, relying on van Genuchten (VG) model to extract the parameters of the water characteristic curve after fitting it. The results indicated that reclaimed water irrigation significantly affected the water retention of red soil and accelerated the evolution of pore's size, quantity and distribution in red soil. The variation law was closely related to the concentration of reclaimed water and irrigation mode. Compared with CK treatment, continuous irrigation RW, RW-2, RW-4, RW-6 treatments, available water content in turn increased by 5.4%, 3.6%, 14.6%, -8.1%;Alternate irrigation ARW, ARW-2, ARW-4, ARW-6 processing, available water content reduced by 9.9%, 23.3%, 26.5%, 16.9%, respectively. Compared with the control group, continuous irrigation had no significant effect on the unsaturated hydraulic conductivity of red soil, while alternate irrigation had significant effect. In the same moisture content, red soil unsaturated hydraulic conductivity and water diffusivity from big to small both could be described as CK?ARW-4>ARW-6>ARW>ARW-2 under alternative irrigation generally. Reclaimed water irrigation had significant influence on VG model parameters (θs', θr', α, n ), the pH, conductivity and sodium-adsorption ratio of reclaimed water were significantly correlated with the available water content of red soil. These results provided theoretical basis and technical support for the long-term utilization and risk assessment of reclaimed water in red soil areas. At present, reclaimed water and other low-quality water have not been put into agricultural production in south China. The change rules of water movement and retention in red soil under reclaimed water irrigation analyzed in this paper will have certain value and significance for the sustainable and efficient utilization and development of water resources, and also provide reference for the scientific irrigation of reclaimed water in south China because the red soil is the typical farming soil in this region.