On the basis of dynamics principle for soil water and solutes transport, the numerical modeling is used for analyzing the regularity of the transport of water and Cl - in the soil with clay layer under evaporation in this paper. The layered soils used for simulation are composed of loam and two kinds of clay layer respectively. The depth of groundwater is 1.5 m. The differences and causes in the effects of the position and thickness of the clay layer on the transport of soil water and solute between two kinds of clay layer are focused. The results show that the degree of the effects depends on the soil hydraulic properties of the clay and loam in the layered soil. In case 1 where the layered soil is composed of loam and heavy clay Y, the basic condition is, the higher the position of the clay layer and the thicker the clay layer are, the lower the evaporation, up inflow rate and the accumulation of Cl - in soil are. In case 2, where the layered soil is composed of the same loam as above and light clay R, the highest evaporation and up inflow rate and accumulation of Cl - appear in the layered soil with clay layer at the top position.
The next higher level appears in the case where the clay layer locates in the bottom of the soil and the lowest appears in the soil with clay layer in the middle position. The causes of above differences are resulted from the different hydraulic properties of the two clays. The hydraulic conductivity curves ( K versus h ) of clay R and loam cross at about -1 000 cm of h . Therefore, when the surface soil is drying and h drops to less than -1 000 cm, the K of clay R will be higher than that of loam. But the h value of the intersecting point for clay Y and loam is rather lower than above value. The accumulation position of Cl - in soil profile under evaporation is mainly at the soil surface, clay layer in soil profile only plays the role of retardation in the transport of solutes to the surface and of temporal accumulation place.