| Yanqi basin is located in an arid area of northwestern China which contains the biggest inland fresh lake-Bosten Lake. Since the 1950s the basin has been reclaimed, the flooding irrigation, incomplete drainage system and enormous surface-water conducting caused the sharply rising of groundwater table and decreasing of flow into Bosten Lake. These human activities plus the special conditions of local climate and soil, have lead to a series of serious environment problems such as secondary salinization of soil and deterioration of groundwater and Bosten Lake. To solve the above problems, it is necessary to study the temporal changes and spatial distribution of water and salts in soil and groundwater, construct the applicable water flow and solute transport model for unsaturated-saturated groundwater, and propose the applicable macroscopical controlling patterns for water and salts of soil and groundwater.Firstly the author analyzed the water-bearing medium of Quaternary sediment, and the temporal and spatial distribution of water and salts in the unsaturated zone and saturated zone. Secondly HYDRUS1D software was applied to construct the 1-D variant saturated flow and solute transport simulation model in a representative area, and design the reasonable field irrigation schedule. Thirdly the regionalization maps for soil-texture types and depth of groundwater were drawn, which helped to obtain the integrated regionalization for areal unsaturated domain. In different unsaturated subregions representative soil profiles were selected to study the water and salts exchange across the interface between soil and groundwater, and the unsaturated-saturated coupled model for water and salt transport was constructed. Finally the calibrated coupled model was utilized to predict the evolvement of flow field and chemical field under the actual and suppositional condition of water and land use, based on which the applicable macroscopical pattern for controlling water and salts in soil and groundwater was put forward and concrete methods for solving secondary salinization was suggested. Some essential results and conclusions are as follows.1. Quaternary groundwater containing system includes both unconfined aquifer subsystem of piedmont plain and confined-unconfined aquifer subsystem of inner plain. Quaternary groundwater flow systems can be on a local, intermediate and regional basis, and the regional system contains four intermediate ones. Groundwater flowes from piedmont plain into Bosten Lake, and water-table varies from 1140m to 1050m with the hydraulic gradient from 0.002 to 0.0005. The regime type of water table is irrigation-evaporation. TDS of unconfined groundwater varies mainly from 0 to 50g/L increasing from the piedmont plain to Bosten Lake, and it is lower along riverside and higher around drainage canals. The Shukarlev classification for chemical types of groundwater changes from dicarbonate type with lower mineralization in the western and northern piedmont plain to sulfate-chloride type with higher mineralization around the lake, and is sulfate-chloride type or chloride type with higher mineralization in the east and south of Yanqi Basin. 2. Spatial distribution of soil texture was determined, and the regionalization maps of soil texture types for single layer and texture combination types for soil profile were obtained. The soil texture become more and more fine varying with gravel, intermediate-fine sand, very fine sand, silt, and silty clay from the piedmont plain to the vicinity adjacent to Bosten Lake. Soil profiles show textures are fine at the upper and coarse at the low, and the thickness of fine-grain soil become thinner near the edge of the basin.3. Temporal and spatial distribution of water content and salinity of unsaturated soil for different types of land use were discussed. Water content of shallow soil is obviously affected by irrigated water in farmlands for irrigation period but not in wastelands. And several peak values occur at the regime curve of shallow soil moisture for farmlands in irrigation period but not for wastelands. Shallow soil salinity increases from piedmont plain to Bosten Lake, and which is lower at riveside of Kaidu River. According to the amount, variability and surface accumulation of soil salt content, salinity profiles can be classified into three types: equably distribution type, surface accumulation type and oscillation type. And the regime curve of soil salinity can be demarcated into three parts: the first part showes the alternate of desalting periods and salt return periods of spring-irrigation stage from April to June, the second part is relatively stable period from July to September and the last part is salt return period at the winter-irrigation stage from October to next March. Chemical components of soil salts are mainly sulfate and chloride ions with the relative ratio about 40% and 20% separately. Classification of salinized soil is mainly chloride-sulfate type, and the grading of soil salinity in wastelands with saline soil is higher than that in farmlands with strong salinization and middle salinization.4. The 1-D unsaturated water flow and salt transport simulation model for a representative area of shallow groundwater table was built and the suitable field irrigation schedule was designed. The water recharge and discharge of unsaturated zone was 1187.1 mm/a and 1039.0mm/a separately. The irrigated water infiltration was the major component of recharge (relative ratio was 91.8%) and water leakage into groundwater was the major component of discharge (74.6%). And the change in soil water storage was less than 5% which indicated the capacity of soil-water reservoir regulation was limited. The salts recharge and discharge was 1210 g/m~2/a and 5322 g/m~2/a separately. Salts taken by irrigated water was the major component of recharge (50.2%) and those by soil water leakage was the major component of discharge (99.4%). And the change in salts storage accounted for 30% of original storage indicating the desalination of soil. The constructed and calibrated simulation model was utilized to predict the tendency of changes for soil moisture and salinity, to determine the reasonable irrigation of 800mm/a for crop growing and washing soil salts, and to design the suitable field irrigation schedule mainly based on the discipline of crop growing.5. Integrated regionalization of unsaturated domain was determined according to the soil texture, depth of groundwater table and distribution of irrigation areas. The study area could be divided into groundwater deeply-buried subareas (water table deeper than 3m) and groundwater shallowly-buried subareas (water table shallower than 3m). In irrigation areas when depth of water table was shallower than 3 m water and salts exchange occurred cross the interface between saturated and unsaturated zones, and when depth of water table was deeper than 3m only water and salts of soil infiltrated into phreatic water. In non-irrigation areas when depth of water table was shallower than 3m only groundwater recharged soil water by evapotranspiration, and when depth of water table was deeper than 3m there was no exchange cross the interface. Combining the distribution maps for soil texture, depth of water table and irrigation areas, the unsaturated domain of the study area could be divided into eighteen subregions, where fourteen occurred water and salts exchange between soil and groundwater where representative soil profiles (S1～S14) were selected. Water flow and salt transport models for fourteen representative soil profiles (S1～S14) were constructed by applying HYDRUS-1D and were simulated to determine the water and salt exchanges cross the interface between saturated and unsaturated zones. In irrigation areas where the depth of water table was shallower than 3m infiltration recharge rate for groundwater was 591～966 mm/a (about 49%～80% of irrigated water), and actual evapotranspiration rate was 391～1065 mm/a. In irrigation areas where the depth of water table was deeper than 3m, only infiltration for groundwater occurred and the recharge rate was 172～389 mm/a (14%～32% of irrigated water). In non-irrigation areas where the depth of water table was shallower than 3m, only evapotranspiration of groundwater occurred and its rate was 123～145 mm/a. And the rate of infiltration recharge and evapotranspiration of groundwater increased when soil texture became fine. Salts exchange between soil and groundwater was affected by water flow, soil salinity and TDS of groundwater.6. Water and salts budget of groundwater were analyzed and the coupled simulation model of water flow and salt transport for unsaturated-saturated groundwater was constructed. Water recharge rate for groundwater was 11.051×10~8m~3/a where the major compositions were lateral recharge and canal leakage accouting for 40.42% and 38.47%. And water discharge rate was 11.006×10~8m~3/a where the major composition was evapotranspiration accouting for 52.40%, the secondary ones were drainage discharge and river discharge accouting for 20.38% and 15.28%, and the artificial extraction was very low with the ration of 8.4%. Salts recharge rate of groundwater was 253.897×10~4t/a where the major compositions were from canal leakage accouting for 50.23% of tall and secondary ones are lateral recharge and irrigation infiltration accouting for 27.92% and 20.26% respectively. And the salts discharge rate was 255.615×10~4t/a where the major compositions were drainage discharge and evapotranspiration accouting for 33.61% and 32.71%, and the artificial extraction was very low accouting for 6.4%. The coupled model for unsaturated-saturated zone thought of the water table as coupling interface (low boundary of the unsaturated and upper boundary of the saturated) and calculated real-time water and salts exchange cross the interface. As a result determination for the sink and source through the upper boundary of groundwater was more reliable and the emulation accuracy of simulation model was improved.7. Under the actual condition for water-use and land use the tendency of spatial and temporal changes for water table and TDS of groundwater was predicted by applying the calibrated coupled model of groundwater. Water-table prediction for observed boreholes showed: water table would rise gradually in Quhui Village and Wushitala Village within water deeply-buried areas, and would keep stable except for fluctuate of water-table expanding caused by irrigation in Wulasitai Village, Baoerhai Village and Chahannuoer Village within groundwater shallowly-buried areas. And the predicted flow field showed: water table would increase obviously at the rate of 0～0.30m/a in Heshuo County and would decrease at the rate of 0.03～0.11m/a in the areas west to Bosten Lake, along downstream of Kaidu River(south to Yanqi County), Huangshui River, Qingshui River and the local areas south to Bosten Lake. The TDS predication for observed boreholes showed: TDS of groundwater of most boreholes would increase particularly in those located in Twentieth Regiment, Qigexing Town and Telier Town. The predicted concentration field showed: TDS of groundwater would increase in the broad plain west and northwest to Bosten Lake, the areas around Quhui County and Wulasitai County, and some areas in the south of Yanqi Basin.8. Under different suppositional patterns for water-use and land use, the tendency of spatial and temporal distribution of water table and TDS of groundwater was predicted and the applicable macroscopical pattern for controlling water and salts in soil and groundwater was selected. Planning theme-I is designed to increase the withdrawl of groundwater from 0.924×10~8m~3/a at present to 4.056×10~8m~3/a in future while other conditions are unchanged. Based on the theme-I, planning theme-II is designed to decrease field irrigation amount to 800mm/a as the reasonable irrigation. The target of I was to change the relative ratio between the surface water and groundwater as resources of irrigated water in order to reduce water loss from canal transmission and improve groundwater environment. On the base of inheriting advantages of I, target of II was to further control the rising of water table and TDS of groundwater and realize the water-saving irrigation.Comparing I with II, the prediction shows that the water table and TDS of groundwater would descend and the effect of II was better than that of I. From the long-term tendency, the water table would fall gradually and come to constant eventually, and would control the increase tendency of TDS in general under II. So II could be considered and selected as the applicable controlling pattern for Yanqi Basin. At the same time, to keep the steady of groundwater field, the pumping rate of groundwater should be reduced in the areas around Baoerhai irrigation sources site, and that could be properly increased at the Group 89800 in the northeastern basin.9. Concrete methods of preventing and treating soil salinization were put forward. From the point of irrigated water sources, groundwater withdrawal could be increased from 0.924×10~8 m~3/a to 4.056×10~8 m~3/a and surface water should be decreased. From the point of irrigation schedule, the field irrigation ration of 800mm/a would be suitable for saving water and should be time-distributed properly according to crop growing. From the the point of irrigation style, blooding furrow irrigation or spout irrigation could be extended for the areas of soil salinization, drip irrigation for the areas of irrigating saltish water and seep irrigation for the areas of groundwater deeply-buried and high permeability of the soil. From the point of drainage style, in the areas of water deeply-buried well-irrigation and well-drainage could be applied to low the soil salinity, and in the areas of water table shallowly-buried capacity of horizontal discharge for salts should be strengthened, the depth of drainage canal should be deeper, and the combination with multilevel canal system and underground pipes could be utilized. Further more, the biological, physical and chemical amendments could be employed in association.The characteristics and innovations of the present paper are that: 1) it discussed the spatial distribution of soil texture types for different single layers and texture type units for soil profile; 2) regionalized the areal unsaturated zone mainly based on the soil texture , depth of water table and irrigation-area distribution, and simulated the water and salt exchanges cross the interface between soil and groundwater at different subregions applying the 1-D water flow and salt transport modeling; 3) constructed and calibrated the coupled simulation model of water flow and salt transport for unsaturated-saturated groundwater regarding the phreatic surface as the coupled interface for matter exchanges; and 4) predicted the tendency of temporal and spatial distribution for water table and TDS of groundwater under actual and suppositional patterns for water use and land use, put forward the applicable macroscopical pattern for controlling water and salts of soil and groundwater, and suggested the concrete methods for preventing and amending the soil salinization.
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