| With the development of hydrogeology, the study scale is increasing,even up to the whole basin(watershed). Tóth(1963) created the theory of groundwater flow systems based on gravity driven flow when investigating characteristics of groundwater flow system in a drainage basin. This theory revealed the rule of groundwater flow in drainage basins. In this paper, guided by the theory, Dusitu watershed in Ordos basin was selected as a study area. The shape of water table and flow system features were studied and numerical simulations of groundwater flow were established. To verify this model, geophysical profile and water level features were used to make calibration of model parameters. On this basis, the distribution feature of groundwater age in the discharge area of the study area was also studied.The Dusitu Watershed is located in the northwest of the Ordos basin with an area of 10924 km2. Taken the Watershed as an example, the relationship between the water table and topography was researched. Using the formula in Haitjema et al(2005) as well as the analysis of actual well data in the study area, it can be seen that in arid and semi-arid areas, the shape of water table also conforms to topography control theory. That is to say, when the topography is high, the water table is relatively high, and when the topography is flat, the water table is also low. The water level fluctuation is differently affected by the degree of topographic relief in different regions: near the watershed, influenced by topographic relief, groundwater level is lifted but the rise is modest; inside the basin, where the relief is gentle, the groundwater level uplift amount is very close to the degree topographic of relief. According to the well height and depth in the study area, linear relevant formula between head and surface elevation in different regions were given. The characteristics of water table distribution were calculated using digital elevation data.Applicating water table distribution characteristics to a model with given head upper boundary, a three-dimensional hydrogeological model was established. In order to study groundwater flow characteristics in the aquifer, the grid was refined vertically. The results show that different flow systems including local, intermediate, and regional flow systems can develop even if there is only water table fluctuation. To verify the model accuracy, geophysical profiles were used to verify the discharge area, the result shows that the discharge areas reported by analyzing resistivity data of geophysical profiles can match those arrived at by 3-D simulations. At the same time, we use the penetration depth of the local water system which was drawn by analyzing geophysical profile, and B2 well water level data to calibrate and verify model parameters. The results show that the aquifer anisotropy ratios in the study area were 150 and 200.Based on the model, the groundwater age(convection age) in the discharge area were studied. Overall, the groundwater age distribution shows complexity: the age difference and the groundwater origins differ in adjacent areas. Considering the inhomogeneity of groundwater age in discharge areas, multiple tracking particles were set up at different locations in the selected grid, and the mean value, variance, the maximum and minimum value of groundwater age in each grid were computed. When the variance is small, the water is usually coming from the flow system of the same level; when the variance is large, combined with the groundwater streamline, groundwater may come from flow systems of different levels or of different regions. As for the Dusitu River, the upstream mainly get recharge from groundwater of watersheds at lateral or internal basin. Downstream of the Dusitu River, which is the final discharge area in the whole study area, receives recharge from shallow local groundwater and groundwater from watersheds not far away, thus the groundwater age is relatively young than in the middle of Dusitu River. |
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