Water Movement In Unsaturated Layered Soil Under Constant Waterhead

Layered soil directly influences water movement in it, thus water movementrules in layered soil have been research focus of agricultural water-saving irrigationand water resources protection.This paper does water movement experiment in twolayerd soils, TypeⅡA,consisting of70%fine sand and30%clay,and Type ⅡB,withopposite structure to TypeⅡA，as well as homogenous soils that have similar soiltexture to layered soil.By analyzing wetting front、water content、matric potential andunsaturated hydraulic conductivity,this paper studys and compares characters ofwater movement in layered soils and homogenous soils.Main research conclusionare:1、 Wetting front velocity is constant in homogenous soils;For layeredsoils,wetting front can be divided into two phases at the upper layer,the first phasehas a simiar wetting front to homogenous soil while the second phase has a lowerwetting front velocity. Wetting front velocity in subsoil is constant.2、Both in homogenous and layered soils, water content appears liner growth atinitial stage,after the liner growth peak,water content rising rate greatly reduces andfinally tends to a constant value. For layred soilⅡA,water content rising rate in uppersoil near the interface is obviously higher than homogenous soil with similar soiltexture,while the rising peak value is lower than homogenous one；water contentrising rate and peak value are both lower than homogenous soil with similar soiltexture.For layered soilⅡB,water content rising rate and its peak value are both lowerthan homogenous soil with similar and the subsoil has a opposite condition.3、Relationship between matric potential and time can be described as h=A*exp(-t/α)+B. For layered soilⅡA,matric potential decreasing rate both in upper soil near interface and in subsoil is lower than homogenous soil with similar soiltexture.For layered soilⅡB,matric potential changing rate in upper soil is clear tohomogenous soil with similar soil texture while matric potential in subsoil decreasesmore slowly than homogenous soil with similar soil texture.4、Both in homogenous and layered soil, change-over-time of unsaturatedhydraulic conductivity can be divided into linear phase and nonlinear phase. Forlayered soil ⅡA, unsaturated hydraulic conductivity increasing rate both in upper soilnear interface and in subsoil is lower than homogeonous soil with similar soiltexture.For layered soil ⅡB,unsaturated hydraulic conductivity increasing rate inupper soil near interface is greater than homogenous soils with similar soil texturewhile in subsoil it is smaller than the homogenous one. For layered ⅡA, both inupper soil and subsoil,hydryaulic conductivity of the turning point where unsaturatedhydraulic conductivity changes from linear growth to nonlinear growth is smaller thanhomogenous soil.For layered soil ⅡB，hydraulic conductivity of the turning point inupper soil near interface is higher than homogenous soil while in subsoil suchconductivity is lower than it.