The Effects of Concentration of Ponding-Infiltration Water on Water and Salt Movement in Soils

【Background】 Seasonal water shortage in southern China has made water supply unable to meet the demand and using unconventional water as a supplementary resource for irrigation can partly relieve this pressure. The widely distributed red soils in southern China are weakly acidic characterized by low porosity, and to what extent irrigating such soils with salt-rich unconventional water affects their transport properties is poorly understood. 【Objective】 This paper is to fill this gap by systematically studying the effect of irrigation water salinity on water flow and solute movement in the red soil aimed to improve unconventional water irrigation management in these regions. 【Method】 The experiment was conducted in repacked columns with distilled-water irrigation taken as control (CK); the concentration of the irrigation water ranged from 1.0 to 10 g/L. In each treatment, we measured spatial distribution of water and salt in the soil profile, as well as pH at the end of infiltration. 【Result】 Compared with CK, saline water irrigation impeded water infiltration, with the impedance increasing with salinity concentration. Water-holding capacity of the soil did not show noticeable change when the salinity was less than 3 g/L, but increased significantly when the salinity was 5 g/L (p<0.05). Water infiltrating rate decreased as time elapsed, and 160 min after inception of the infiltration, the advancing speed of saline water front became slower than that of freshwater. Both Kostiakov and Philip formulate can accurately describe the change of cumulative infiltration with time, with the former working better when salinity was less than 3 g/L and the latter more accurate in other treatments. The cumulative infiltration linearly increased with the advancing distance of the wetting front, while the water-holding capacity and infiltration parameters varied with salinity in a way that could be described by a cubic polynomial function with R2>0.95 and RMSE<0.06. Irrigating with saline water in the range of 1~5 g/L increased water content in the 5~25 cm of soil by 0.09%~4.61%. It was found that EC, Na+ and Cl- content decreased with soil depth, and that the effect of irrigation water salinity on soil salinity was more significant in the top 0~25 cm soil than in the subsoil. Compared with CK, irrigating with water at salinity 1~5 g/L acidified soil, while when the water salinity increased to 10 g/L it alkalized the soil. 【Conclusion】 Irrigation water salinity not only modulates water flow and solute movement in the red soil but also affects its physical properties. Our results showed that irrigating the red soil with unconventional water in southern China should consider the consequences for water and salt redistribution, as well as soil acidification or alkalinization.