| China desertification areas generally exist the internal closed drainage areas composed of meadow dunes, and form a system of regional scale in the process of water cycle. But at present the specific hydrological process of moisture migration and transformation mechanism is still not clear. This article in Horqin sandy dunes-meadow landform area is selected as the typical study area, using GSPAC (Ground Water-Soil-Plant-Atmosphere Continuum) system theory as the foundation, the Ground water-Soil-Plant-Atmosphere as a unified continuum, with years of field test data of sand dunes and meadows, study the main plants growing period rainfall infiltration of soil water movement and the formation of condensation water mechanism in a GSPAC system, and simulate the sand dunes, meadows water transport followed the SWAP model. The content and the results are summarized as follows:1.Choose the2009flood period from May to September five times typical rainfall, through the analysis about the different types of Geomorphology (dune, meadow) soil vertical profile in water content and matrix potential changes, further research on rainfall to soil water and groundwater transformation mechanism from the energy angle.The results show that, the sand dune, the total soil is in a dry state, even if there is the occurrence of certain rainfall, the maximum of matrix potential is not exceeding-500cm. Relative to the other layer, the depth of dune is about20-40cm, water content is the most, after rainfall the change amplitude is the most, the matrix potential is the largest, too. When the rainfall is less than10mm, most of the time before and after the rainfall below the ground about20-40cm forms a divergent zero flux plane, above the zero flux plane, the soil water moves upwards and consumes in evaporation and transpiration; downward the zero flux plane, water movement is forming infiltration to the lower excretion. The rainfall is stronger, the zero flux plane moves deeper. Experienced a greater rainfall (e.g.>17.0mm),1Day later the infiltration of water is a single type of rainfall infiltration, the3Days later about20cm below the ground forms a divergent zero flux plane, and with the time of zero flux plane moves downward, the8Days moves down to about underground30cm. Because the A3’s groundwater is buried below120cm, with the increase of the depth, the matrix potential is increasing; soil water moves upwards, in the120cm forms the convergence of zero flux plane.The transformation mechanism of water movement in meadow rainfall is more complex than the soil sand dune. Before the rainfall, the soil moisture of about0-20cm is smaller than the surface, matrix potential decreases with the depth. The ground water is worked by the soil water potential gradient, slowly moves upward to replenish the soil water, when the rainfall stars, the surface soil matrix potential and water content of soil is increased rapidly, the soil plays as the single infiltration type water distribution, the water moves downward, immediately recharge of groundwater, but with soil and vegetation transpiration, soil water content and matrix suction and water content decreases gradually, and the water moves upward again, at20cm forms the divergent zero flux plane. Eventually the rainfall effect disappeared, because the transpiration makes the surface soil moisture decrease ceaselessly, until the next rain supplies. Variation of soil moisture is most notably in0-20cm soil. When the heavier rainfall (>17.0mm) occurs, the soil moisture content changes and influences to the ground below about100cm, the infiltration of wetting moves to exist, it is infiltration surface recharge.2.Analysis the2009flood period from May to September the fluctuations of the overall response trend of the rainfall and groundwater level, and the level of groundwater of the different landforms responding to the rainfall, sums up the form of different landforms groundwater recharge rules and response mechanism. Studies show that, no matter the meadow ground which has smaller depth of groundwater or the sand dune that has the depth about11m, are effected by the different levels of rainfall on groundwater level fluctuations. This shows that after the rainfall, the atmospheric pressure, the soil water content and the water potential change, result the profile of the complex water transport can supply the underground water. The meadow responds precipitately to the rainfall, the groundwater level fluctuations is greater than the sand dune, the meadow embodies the groundwater level fluctuation in one day, the fluctuation of sand dunes is in2to4days, so the sand dunes of the hysteresis effect is obvious. The meadow C3is in each representative the groundwater lifting amplitude and rainfall basically linear increase trend after the rainfall. But as a result of the buried depth of groundwater in sand dunes, influences by the rainfall and the groundwater level fluctuation of rainfall was not well correlated.3.Selects MLS value (2009May1st to October1st2008, between sunset and sunrise the next day) as the research object that the condensation of water is closely related to the change, combines the analysis of geothermal, analysis of condensate formation and transformation dynamics. Research shows that, the formation of coagulating water of soil are closely related with the soil temperature, air temperature, relative humidity, wind speed near ground etc. meteorological elements and the depth of groundwater, underlying surface conditions and other factors. Sand dunes and meadows these two topography, from the sunset to the sunrise, the MLS values which is connected change has higher frequency than the disconnected, this indicates that a considerable part of soil condensation water is from the lower part of soil moisture migration and condensation water. The A3sample has the higher MLS change values appear than the C3sample in frequency, especially the not connected case it is2.53times higher, it indicates that the soil properties also affects the occurrence of condensation process.Dune isogeotherms appears the left convex, this should be in the March to August the air temperature and surface temperature is rising, the sand surface in the presence of a certain thickness of dry sand bed which accumulates a certain quantity of heat, and the surface temperature changes with the circadian temperature, this results phenomenon that is the highest ground temperature in a certain depth below is higher than the surface temperature; here forms the heat zero flux plane, the zero flux plane above to the surface, the water vapor moves upward, and forms the condensation water. For the C3sample, in June to July, near surface air is relatively humidity, forms more condensation water, C3MLS changes greater. This also reflects that the air humidity determines the amount of steam condensation water.4.According to the data about2008,2009’s weather, plants, soil, the depth of groundwater, SWAP model is used to simulate the water movement in Horqin sandy dunes and meadows, which were calibrated and verified, the results are reliable and can reveal the water transport processes in the study area GSPAC system. The results showed, in the simulated time no matter how much the rainfall was, the sand dune soil evaporation and evaporation process did not have too much change (because of the dry sand bed on surface.), and the response of the deep seepage was more obvious, if the rainfall was heavier the amount of deep seepage produce would be larger, so the sand dune is helpful for water conservation. In the condition of absence vegetation, the soil water characteristic curve parameter and the saturated hydraulic conductivity influence the deep seepage obviously; the meadow soil and the sand dunes are quite different, water consumption is mainly by the vegetation transpiration, although in the medium-term of the plant growth, the rainfall infiltration recharge groundwater and groundwater recharge to soil water alternately, in the whole growth period of plants mainly recharge the groundwater to the soil water, it is consumed by the plant transpiration,1the most sensitive parameters of the mode are the vegetation minimum canopy resistance, the extinction coefficient and the leaf area index, the maximum rooting depth, the root distribution etc the important parameters about vegetation. |
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