Experimental Study On The Movement Of Soil Water And Soil Salt For Soils Under Perforated Plastic Mulch

Owing to the problems involved in water resourees, agriculture and environment, study on the transport of soil water and soil solute has been paid attention to in long time. In this thesis, aiming to the soil salinization issues in northwest regions of China and combining with features of plastic mulch technology, we conducted laboratory soil column experiments as well as theoretical analysis to study disk infiltration characteristics under different soil salt contents, the water and solute transport characteristics during infiltration and plastic film hole evaporation process of saline-alkali soils with sand layers, and the movement of soil water and soil salt at different water repellency under perforated plastic mulch. The main conclu-sions obtained are as follows:(1) Experimental results of the infiltration using disc infiltrometers under different nega-tive water heads (i.e.,-1,-3,-6,-9 and-12cm, respectively) for 4 soils showed that, when the negative head decreased, the sorptivity coefficients decreased linearly, the steady infiltra-tion rate and the unsaturated hydraulic conductivity decreased also. The influences of negative water heads to the characteristics of soil water infiltration were similar for different soil tex-tures. In certain water head ranges, when soil salt contents increased, the steady infiltration rate and unsaturated conductivity increased. In certain ranges of water heads, the sorptivity increased exponentially. Gardner exponential function for describing disc infiltration charac-teris of four soils were found suitable for Lou soil and better for saline soils.(2) Sand-layers restrained soil water infiltration and soil salt leaching to some extent. During evaporation under plastic mulch of different open hole ratios, soil temperature on the surface Ocm were largest at any time, and the minimum soil temperature was at the bottom of the soil column, while 25cm depth was found to be soil temperature influence depth. The ex-istence of sand-layers accelerated the transfer of soil heat from the surface to the bottom of the soil columns but reduced the variations of soil temperature gradients. Plastic mulch could effectively increase soil temperature,. When open hole ratios increased from 0%to 100%, the soil temperature in the profiles decreased during evaporation, the maximum soil temperature was found at open hole ratiao of 0%.(3) At different open hole ratios of perforated plastic mulch and three sand-layer posi-tions, after soil water evaporation finished, there were similar vertical soil water distributions in the profiles. Soil water evaporation affected soil water loss above the sand-layers obviously while affected soil water loss below the sand-layers slightly. At the similar sand-layer posi-tions, when open hole ratios increased, soil water contents decreased at the same depths. The maximum soil water contents in the profiles were found at open hole ratio of 0% and evapo-ration affected its soil water loss slightly.(4) For three sand-layer positions, soil electrical conductivity at vertical profiles showed similar patterns in depths. Soil electrical conductivity at the soil surface was relatively large, at 0 to 5cm depth, soil electric conductivity decreased with depth, while under the 5cm soil-layers, soil electric conductivity increased gradually with depth.(5) The vary of cumulative evaporation with time under different degree of soil water repellency and perforated plastic mulch are consistency for 3 kinds of. soils.The greater the degree of soil water repellency, the smaller of the cumulative evaporation under the same open hole ratios under perforated plastic mulch at the same time. Under the same water repellency and perforated plastic mulch conditions, evaporation coef-ficient shows sand>lou soil> Saline-alkali soil.For different kinds of soil under different degree of soil wa-ter repellency, cumulative evaporation and time shows a linear relationship at open hole ratios under perfo-rated plastic mulch in 0%,0.69% and 2.78%,while open hole ratios under perforated plastic mulch in 7.71%,18.7%,100%,it can be expressed by logarithmic relationship.Cumulative evaporation at unit film hole area (Er) decrease rapidly when the open hole ratios increased, power function between Er and open hole ratios was established.(6) Both water repellency and perforated plastic mulch affected soil water evaporation process. When open hole ratios increased, soil water evaporation increased, and variation rate of soil evaporation on vertical profile decreased. For different types of soil, soil water content profile of evaporation changed with depths. After 15 days of soil water evaporation under perforated plastic mulch conditions, the vary depth of soil water contents in the profile for sand was from 0 to 50cm, while for lou soil it was from 0 to 30cm,and for saline-alkali soil it was from 0 to 20cm.(7) The maximum values of soil salt content in the profiles at different open hole raitios during evaporations were on the surface 0cm. Soil salt content decreased sharply with the depth at 0-5cm. At 5cm depth, it reduced to the initial soil salt content.Soil salt content re-duced to the minimum point at 10cm depth. Soil salt content increased slowly with the depths below 10cm. The larger the open hole ratios, the more variations rate of soil salt in the vertical profile, but the effects of soil water repellency degree on soil water contents were smaller.