| Evaporation is a vital tache of natural hydrology cycle. Soil water and superficial groundwater water movement are both affected by evaporation. In dry inland areas, its influence is much more obvious because of intensive evaporation. Phreatic water can be absorbed and used by crops, due to its supplying to root zone of soil, at the same time, the salt may be move up due to its raising. Thereby, it is very important to the study of soil water and the farmland drainage under evaporation condition. The movement characteristics of soil water and farmland drainage under evaporation condition were studied in the thesis. The thesis, conjoining the study with National Nature Science Fund Project, adopted the mode of theory analysis combing with the experiments both in lab and in fields.The elementary results of the study are as follows:(1) At present, Van Genucthen equation is often used to describe the rational relationship between soil moisture content and radical suction of soil. There is the means of math formation that seek the parameter of Van Genucthen equation by using Damp least square method and simplex algorithm, the methods are programmed in VB in the thesis. It is shown by two examples that the suggested methods for determining parameters of Van Genucthen equation is correct. The calculated result and the experimental result fitted preferably, which approved that the two methods can determine the parameter of Van Genucthen equation for describing soil moisture movement. It is also shown by sensitivity analysis that the result precision is affected by the vriation of a and n. So it is suggested in the thesis that the key matter should focus on choosing their initial value of a and n.(2) It is indicated both by experiments observation in field and theoretical calculation in lab that the evaporation quantity E of surface soil layer increases with potential evaporation Eo, when surface evaporation quantity Eo is small. However, E's value stay almost constant at 4.5mm/d, when Eo's value equal to or larger than lOmm/d. The calculated results fit to the experimental ones rather better.(3) It is found through calculating that phreatic water evaporation intension will increase when the soil conductivity is better and the surface layer vriates from thin to thick; and the grads of change will gradually increase. It shows that the texture and thickness of surface soil layer may affect the phreatic water evaporation. Pheatic water evaporation intension is not only related to the thickness of soil layer but also to the position in longitudinal section. It will minish as the thickness of soil layer with worse conductivity increase. And it will augment as the soil thickness that leads water worse gradually move down. When the soil layer connected with the same quantity of soil, phreatic evaporation intension promptly increase.(4) The parameters of layered soil was calculated through the observed data in field experiments considering of water uptake by roots of winter wheat. The water uptake function was got by regression analysis based on experiments. It was shown by comparison the observed data through experiments with the calculated soil matric potential that the results of presented methods were preferably for calculate phreat water evaporation under plant growth. When the soil parameter, surface soil evaporation quantity and absorption model for plant roots is known, the methods can be used in calculating stable phreat water evaporation under normal condition. It was understood through the analysis of evaperation under stable water table condition that when water table is stable, phreat water evaporation intension will be much larger in period of roots growth; and when roots depth is stable, evaperation will be much smaller with water table becomes deeper.(5) The equation between groundwater drop speed V and table h was derived as: V - p(l - e'9 ), in consideration of evaporation effects. Then, it may get space formula for farm barrel-drain (hidden tubes) by spreading Taylor series, adopting first four items or first three items. It was shown by calsulation result that formula precision is better when using the first four items, and it can be used to calculate space of barrel-drain (hidden tubes). The formula using the first three items can also satisfying the planning demands. It is suggested that n should calculated by formula, because of obvious errors when 1 is constant. It may affected the result a lot when considering evaporation or not, especially in area withviolent evaporation.(6) Under condition of slope ground, though obliquity may affect less on drain process, different water levels in side of barrel-drains may affect more. So that it has to consider the influence of ground obliquity on drain process. For example, there will be different watersheds in different places on farm when water levels at ends of drain are different. Meanwhile, drop speed of water table can be affected differently. When ground obliquity equals to 0.76% 0.57% 0.29° and 0°, the water table will drop 1.13, 1.36, 1.62 and 1.82 meters after draining eight days by calculating. The average drop speed are 0.14, 0.17> 0.20, 0.23 m/d individually.(7) At present, the relation between groundwater evaporation intension ￡ and hiddendepth can be expressed as: e — B q— -A_)?. It was considered they are non linearityrelation when n=2 in the study. The drain space calculation formula for non-symmetry and non-linearity evaporation (n=2) was dirived. In practice, it need to distinguish n=l or n=2, and then addopted to corresponding formula to calculate based on true data of phreat water evaporation and buried depth.(8) In the study, 2 dimension insteable calculating formula for non- symmetry boundary with linear evaporation and symmetry boundary with non-linearity evaporation were presented respectively. Because the boundary condition of the formula in accordance with real ground slope, it can be used to calculate practical farm drain problem in northwest China.(9) When the boundary conditions are complex, the solution may not be got. In the study, the finite element method was drived for anisotropy plane with two dimension instabe flow by using rectangle unit in consideration of evaporation, and its correctness was validated by comparison with the analytical solution. At the same time, it was used to calculate the variation process of farm water level in experimental farm in South of Xingjiang by the model. It was shown by observed data of draining in five days under practical drain system and ground slope that groundwater level can drop to the depth where the evaporation was less, which indicates that the drain results of the farm were rather good.The study was supported partially by the Natural Science Fundation of China (No. 50449009)... |
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