Effects of suspended solids in irrigation water on soil water infiltration

Abstract：Reclaimed water and sediment-laden river water for the farmland irrigation play an important role to alleviate the crisis of global freshwater and the shortage of agricultural water resources. The suspended solid particles in these bodies of water can change the structure properties of the irrigated soil, and characteristics of water flow, thereby to increase the management steps for irrigation and fertilization, and the risk for the contamination of groundwater system. In this paper, the irrigation experiments in the laboratory were carried out on the sand columns that filled by hand, under the different concentration of the suspended solid particles in the irrigation water, 0, 1.0, 5.0 and 10.0 g/L, the irrigation frequencies (irrigated every day, once every two days, and once every four days), as well the different irrigation schedules for the suspended solids-laden water and freshwater (irrigated with the suspended solids-laden water only, or with the suspended solids-laden water and freshwater alternatively). The infiltration time of the applied water to fully infiltrate into the sand column was measured to study the effect of the suspended solid particles in the irrigation water on the infiltrability of soil surface. After irrigation, Methylene Blue with the concentration of 10 g/L was used to illustrate the preferential flow patterns under the different infiltration conditions. After the Methylene Blue solution applied to the sand column surface for 48 h, the preferential flow patterns were obtained, and then photographed from the four sides and the excavated horizontal soil layers at the different depths for each experiment, in order to study the effects of suspended solid particles in the irrigation water on the preferential flow characteristics in the irrigated soil. The undisturbed soil cores were also collected from the stained regions of the excavated horizontal profiles at the different depths in each experiment to find the effects of suspended solid particles in irrigation water on the bulk density of soil, porosity and the deposition of suspended solids. The results demonstrated that, 1) the suspended solid particles in the applied water mainly deposited in the upper layer of the irrigated soil; 2) the higher concentration of the suspended particles in the irrigation water can result in the irrigated soil with the greater bulk density of soil, smaller porosity, worse surface infiltrability, and stronger preferential flow behavior with bending coefficient of preferential flow in the 4 sides of sand column and of preferential flow in the whole sand column increased from 2.49 and 4.31 to 3.55 and 11.18, respectively, as the concentration of suspended solids in irrigation water increased from 0 to 10 g/L; 3) the low concentration of suspended solids in the irrigation water also increased the preferential flow heterogeneity, but the surface infiltrability was not changed; 4) the lower irrigation frequency with the higher amount of irrigation can lead to more suspended solids accumulating in the surface of the soil layer, indicating increase in the soil bulk density and decrease in the porosity due to the soil stratification and strong preferential flow behavior with bending coefficient of preferential flow in the 4 sides of sand column and of preferential flow in the whole sand column increased from 3.10 and 7.78 to 3.83 and 12.12, respectively, as the irrigation frequency decreased from once a day to once every four days; 5) the higher irrigation frequency with the lower irrigation quota can lead to more suspended solids that transferred to the deeper soil layer, and thereby to alleviate the soil stratification and preferential flow behavior; 6) the alternative irrigation with the suspended solids-laden water and freshwater reduced the soil bulk density and preferential flow behavior, alleviated the suspended solid deposition, while increased porosity and surface infiltrability. But it was difficult to flush the applied suspended solid particles down to the deeper soil layer to prevent the soil compaction as the suspended solids transmitting depth only increased 5 cm under the alternative irrigation with suspended sediment-laden water and freshwater each day compared to those irrigated with suspended solids-laden water only. This finding can made great contribute to the schedule design for the irrigation of the reclaimed water and solids-laden river water.