| In the arid zone such as Xinjiang and so on, the rainfall is scarce and the evaporation is intense, and consequently, the saline-alkali soil is serious. Prevention and treatment of saline-alkali soil are an important factor relating to the development of agricultural production, but the reduction of phreatic water evaporation, the controlling of groundwater table is important to prevent saline-alkali soil. Therefore, it appears especially important to research the phreatic evaporation and agricultural drainage. Taking the experimential data as a foundation, this paper researched the phreatic evaporation characteristics and the determination method of drainage system's parameters in the arid region using the theoretical analysis method, and obtained the following findings:1. According to the measured data of phreatic evaporation in Xinjiang, the paper analyzed the laws of phreatic evaporation, the power function was established between the ratio of phreatic evaporation intensity at any depths to that of the depth of 0.25m and the amplify of groundwater depths, the error existed in other formulas which was caused by E 0 will be avoided by using this formula, it provided reference for the accurate calculation of phreatic evaporation quantity. At the meantime, the influence of E 0 to the computation precision of phreatic evaporation was analyzed, the result indicated that, except for fine sand and gravel, if the phreatic evaporation intensity at zero watertable is replaced by the atmosphere evaporation capacity, there will be some big inaccuracy in calculating the evaporation quantity at different watertables. Therefore, we should choose the land surface evaporation intensity at zero watertable as E 0 in the calculation of phreatic evaporation.2. The paper proposed the determination method of the limiting evaporation depth Hmax in Averiyanov empirical formula and the limiting evaporation intensity Emax andηin the Tsinghua formula, which provided certain reference for the parameters calculation and the extending application of these two formulas. Four kinds of commonly used empirical formulas were selected to fit the actual data of phreatic evaporation measured, and the results indicated, the Tsinghua formula has a widespread versatility in Xinjiang; when the groundwater depth is shallow, Averiyanov equation, Ye equation and power function have a relative high precision, but the precision is low when the depth is deep. The difference between the value calculated using power function formula and the actual value is the biggest. The Averiyanov equation, Ye equation and power function fit well in loam than coarse soil. This research provides guidance for selecting the appropriate empirical formula to calculate the phreatic evaporation in Xinjiang.3. According to the Tarim Basin measured soil salt content, the dominate component content of soil salt, degree of mineralization and groundwater table, the features of main component of soil salt were analyzed and the function relationships among soil salt content of the different depths in 0-30cm, 0-60cm and 0-100cm, groundwater table and mineralization degree were established. The results indicated that the salt accumulation in soil had obvious dependency relation with groundwater degree of mineralization and groundwater table in arid region; it can provide guidance for determining groundwater critical depth and groundwater management according to crop's salt endurance.4. This paper has analyzed the differential equation of spacing calculation under pure draining condition and the condition influenced by evaporation separately using the instantaneous stabilization method, and has solved the equation. The results indicated that, the spacing calculated considering the evaporation influence is bigger than that under pure draining conditon, which announced the non-neglectable affect of evaporation to draining. A new definition and calculation method of groundwater allowable depth was proposed, and the calculation method of drainage system of desalination has been analyzed preliminarily.
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