Water infiltration under different CaCl2 concentrations for soil with mainly permanent charges

Abstract

It is well known that, electrolyte concentration can influence soil water movement through its influence on the osmotic potential of water; on the other hand, electrolytes also can influence soil water movement through its influence on the matric potential of soil water, because soil electric field is closely related to the electrolyte concentration. Therefore, the two effects arising from electrolytes on soil water movement are coupled, and the object of this study is to explore this coupling effect by adopting a 2:1 type electrolyte with different concentrations. In this study, water movement in a purple soil was evaluated through a one-dimensional vertical infiltration experiment through determining water infiltration rate and advance speed of the wetting front. The results revealed that (1) the critical electrolyte concentration of CaCl2 in soil water infiltration and migration is 10−2 mol L-1; (2) 10−2 mol L-1 CaCl2 is also the critical concentration for the relative total water potential, which is the sum of the matric potential and the osmotic potential; (3) detailed analysis of the effects of the matric potential and osmotic potential on soil water movement indicated that soil water infiltration is dominated by the matric potential when the CaCl2 concentration is lower than the critical concentration, while it is dominated by the osmotic potential when the CaCl2 concentration is higher than the critical concentration. It can be speculated that the matric potential based on an electric field dominates soil water infiltration when the soil moisture content is high, while osmotic potential dominates soil water infiltration when soil moisture content is low and that the concentration of 0.01 mol L-1 CaCl2 is the critical point at which water infiltration changes from matric potential control to osmotic potential control.