The shortage of water resources is the main limiting factor in agricultural
production and ecological environment construction in the Loess Plateau of China. As an important process for the conversion of surface water to soil water, soil water infiltration not only determines utilization rate of the limited water sources, such as rainwater and irrigation water, but also profoundly affects intensity of surface runoff and soil erosion. Soil water infiltration is affected by many internal and external factors. The stability of aggregates is an important factor affecting soil water movement. Recent researches have demonstrated that soil internal forces, including electrostatic, van der Waals and hydration forces, can be as high as hundreds of atmospheres, which is the main driving force for soil aggregates breakdown. Aggregates disintergration will change the soil pores and thus affect the soil moisture movement. However, there are few studies about the effects of soil internal forces on soil hydraulic properties; and the applicability of the classical water infiltration model considering the interaction forces is still to be further verified. Therefore, in this study, we selected four typical loess parent soils (i.e., Heilu Soil, Loessal Soil, Cinnamon Soil and Lou Soil) as the research object, through the combination of theoretical calculation and experiments, we explored the effect of soil internal force on soil water infiltration. The classical soil water infiltration model and the water characteristic curve model were used to fit the experimental results, and the applicability of the model was verified and compared. The results are as follows:
(1) Quantitatively calculated the distribution characteristics of soil internal forces. Our results show that, with the decrease of electrolyte concentration in bulk solution, the surface potential (absolute value) and surface electric field strength of the four soils increased, and the electrostatic repulsion, DLVO force and net force also increased. The internal forces between the soil particles decreased as the distance between adjacent particles increased. The net force was only attractive at the electrolyte concentration of 1 mol·L-1. The critical potentials of Heilu Soil, Loessal Soil, Cinnamon Soil and Lou Soil were -211.9, -203.6, -223.3 and -232.6 mV, respectively, and the critical concentrations were all 0.01 mol·L-1.
(2) The characteristics of soil water infiltration under different internal forces were systematically studied. Soil internal force had important influence on the process of soil moisture infiltration. With the decrease of electrolyte concentration, the net repulsion between soil particles enhanced, the water infiltration rate and the velocity of wetting front reduced, and the soil moisture accumulation infiltration decreased.
The electrolyte concentration of 0.01 mol·L-1 was the critical concentration for controlling water movement. When the electrolyte concentration is lower than the critical concentration, the parameters such as the water infiltration rate and the migration velocity of wetting front did not change with the electrolyte concentration in bulk solution. Both the Kostiakov model and the Philip model could fit well with the experimental results. Overall, the Kostiakov model was better than Philip model.
(3) The changes of soil water holding capacity under different internal forces were explored. With the decrease of electrolyte concentration, the net repulsion forces increased, and the soil saturated hydraulic conductivity reduced, the soil water holding capacity was enhanced. Both the van Genuchten model and the Gardner model can well fit the water characteristic curves of four soils under different internal forces.Overall, the Gardner model was better than van Genuchten model to fit soil water