| The technique of drip irrigation with plastic film mulch, or called the mulched drip irrigation (MDI) is a combination of drip irrigation (DI) and plasticulture techniques, which is characterized by irrigational water saving, crop being premature and its output increasing, soil temperature increasing, and preservation of soil moisture. This irrigation technique has been used for many kinds of economic value crops over the world, but the water-saving mechanism for the MDI has not been studied thoroughly, especially the research result on the designed parameters for the irrigation technique is so little that is unavailable to the technique development. Experiments had been carried out in laboratory, plot of land, and field, to plant cotton with the irrigation techniques of MDI, DI, and border irrigation (BI) to systematically and comprehensively study the problems about the principle of designing the DI and MDI systems, such as, the design of dripper discharge for DI system, percentage of soil wetted area, soil moisture uniformity for linear source drip irrigation, soil wetting pattern and soil temperature for MDI, cotton growth and its root system distribution in soil with MDI, cotton yield and water use efficiency under MDI, the coupling of soil water and temperature with soil salinity under MDI, cotton growth in the saline land with MDI, and so on. By testing, observing, analyzing, and studying these problems, the results are obtained as follow.(1) With the point source drip irrigation, soil wetting pattern is influenced by the relation between dripper discharge and soil infiltration intensity. When dripper discharge is larger than soil infiltration intensity, a saturated pond occurs on the soil surface and expands around, then soil wetting front moves quickly in horizontal and slowly in vertical. But when dripper discharge is less than the soil infiltration intensity, the horizontal movement velocity of the soil wetting front would be slower than its vertical one due to no saturated pond occurring on soil surface. When dripper discharge is constant but dripping water is increased, the soil infiltration intensity would lessen with time and eventually be less than the dripper discharge and soil wetting front would move quickly in horizontal. Therefore, the width of soil wetting pattern should be the basis to design the dripper discharge for the DI system, while the soil infiltration and the dynamics of soil saturated pond are the foundation to calculate the dripper discharge.(2) Soil moisture uniformity is essential to determine the drippers spacing as well as the dripper discharge and dripping water for linear source drip irrigation system, and it influences the uniformity of crops growing in field. The soil moisture uniformity along the drip line depends on the overlap of soil wetting patterns beneath drippers, and the more the soil wetting pattern overlap, the higher the soil moisture uniformity is. The soil moisture uniformity across the drip line is subject to the width of soil wetting pattern, and the larger the soil wetting width, the higher the soil moisture uniformity. The soil moisture uniformity along drip line is improved by short drippers spacing and large dripping water.(3) With the MDI, the fact that soil saturation pond is limited by plastic film to expand around, results in that soil water content beneath film is far more higher than that outside of the film, and the soil beneath the film is wetted in border strip. So the designed soil wetting area for the MDI is just the soil surface area covered by the film. With plant row spacing of 30+60cm, the percentage of soil wetted area is 67%-83% for MDI, while 33%-67% for DI. The soil infiltration depth of MDI is shallower than that of DI or BI. Under the MDI, soil water content and soil temperature are lager than that under DI. Soil temperature is affected mostly by climatic conditions but by soil moisture conditions only in certain period although soil temperature varies with soil water content in negative-linear function.(4) With the MDI, the cotton root system distributed in the soil beneath drip line and the film is 14-23 times as weight as that distributed in the soil out of the film, and the lateral root of cotton which grows near by the sides of film distributes chiefly in the soil mulched by the film. So the total weight of cotton root under the MDI, which distributes mainly in the soil mulched by plastic film, is larger than that under the DI, but is less than that under the BI. But under the MDI, cotton root weight in the 0-30cm soil layer accounts for 87% of the total root weight, and it decreases exponentially with soil depth increasing although it is the largest in top soil layer. Therefore, the cotton root system irrigated with the MDI distributes shallowly comparing with that irrigated with the DI or BI. Cotton root under the MDI, by which soil temperature is high, grows quickly than that under the DI or BI, but it is obviously early ageing.Comparing with the cotton irrigated with the DI or BI, the cotton irrigated with the MDI grows quickly and its leaves area index is larger, so that it is premature but early ageing.(5) The per unit area yield of unginned cotton under MDI and DI is 40% higher than that under BI. Under the MDI, cotton consumes soil water mainly in 0-60cm soil layer where is covered by plastic film, and the water consumption in the soil layer is 7 times as high as it is in the soil out of the film; and of this soil layer, the top layer has the largest water consumption density. The function of soil water consumption with soil depth under the MDI is relative to the distribution of cotton root system density, and the relation of soil water consumption in soil depth and cotton root system density can be expressed exponentially. The evaporation from soil under MDI is only 28.57% of crop transpiration and is 17.39% of evaporation from the soil under BI.Water use efficiency of cotton under MDI is 1.332kg/m3, which is higher than that under DI by 0.168kg/m3. The production efficiency of irrigation water for the cotton irrigated with MDI is 1.304kg/m3, which is higher than that with DI by 0.201kg/m3. Theoretically, the water consumption of per kilogram unginned cotton under the MDI is 15.4% less than that under DI.(6) When saline soil is irrigated with the MDI, the salinity of soil is distributed in ring-like that soil salinity content is higher in the soil out of film and in deep soil, and is lower in the top soil layer beneath the film. The larger the soil wetting pattern, the more beneficial to crop growing the condition is. Thus the percentage of soil wetted area and soil wetted depth for saline soil is larger than that for un-saline soil. With MDI, the cotton growing on heavy salt soil will be saline stress, so that cotton growing period will be 15 days shorter than that growing normally, the cotton stem is only half of that growing normally, the cotton leaves are early ageing, and cotton yield is low as well as its quality is poor. In addition, the salt soil temperature is usually constant and doesn't vary with climatic conditions comparing with the un-salt soil, because the salt soil is high water content and mineral. In the salt soil under MDI, cotton root system distribution is characterized by evading salt that cotton root grows in the soil of low salt content. So cotton root mainly distributes in top soil layer, and total root weight is little.
|
PDF Full Text Request