ANALYSIS OF THE RELATIONSHIP BETWEEN GROUND-WATER AND SALTS ACCUMULATION IN SOIL

To control the ground-water table on irrigated lands is vitally important in arid and semi-arid regions. In order to determine the standard of water table control, considerable attentions were paid to the (or safe depth) of ground-water table. The salts accumulate in the top soil if the water table is not deeper than this depth. The conditions that the salts accumulate in any layer of the soil owing to the movement of soil moisture are: (1) the supplement of soil moisture from neighboring layer in the liquid phase (soil water); and (2) the soil moisture loss in vapor phase (soil vapor). This paper describes the soil moisture-content distribution profile, the classification of capillary water forms in the soil and the effect of moisture-content on the movement of soil moisture in liquid phase and vapor phase. There is a critical content of soil moisture W_R, which corresponds to the moisture-content of rupture of capillary bonds. When the moisture content is W_R, the rate of movement of moisture in soil, and the evaporation of the moisture from soil surface change (increase or decrease) with moisture content considerably. The value of W_R of light clay-loam equals to 16—18% (by weight) approximately. The critical depth may be expressed as H_c=H_R+Δ,where H_R is the distance from water table to the point of W_R on the soil moisturecontent distribution profile, and Δ is the depth of soil tillage (about 0.20m). Comparison of the data obtained from the field tests and the soil investigation sites of several irrigated areas in the northern part of the country with results determined by the above equation shows that the majority of them is consistent.