(1) simulation range

The lower reaches of Heihe River Basin mainly refer to Jinta-Huahaizi Basin and Ejina Basin, and its southern boundary is Jiashan and Nanshan. The western, northern and northeastern borders are Huahai Farm and Beishan respectively; The southeast border is Badain Jaran Desert, covering an area of about 32,900km2.

(2) Structural generalization and boundary conditions of groundwater system.

The lower reaches of Heihe River Basin is a hydrogeological basin with only lateral inflow and no lateral outflow. The structure of Quaternary groundwater system has the following characteristics: in the plane, the Shuanggu Town-Huoshao area in the northwest of Jinta-Huahaizi Basin is a multi-layer phreatic-confined water system in the west and a single-layer phreatic system in the southeast. From the northwest mountain to the northeast, the single-layer structure of Ejina basin is transformed into a double-layer or multi-layer groundwater system; Vertically, coarse and fine particles appear alternately, and the aquifer and weak permeable layer overlap each other.

In the south of Huxi New Village, the surface river is a perennial river. In the north of Huxi New Village, there is a seasonal river. The supply of submersible system mainly comes from the leakage of river water, and the supply varies greatly with the seasons. Infiltration recharge of irrigation water is an important source in agricultural development zones such as Ding Xin. In addition, there are infiltration supplies from surface reservoirs in the lower reaches of Heihe River Basin. Atmospheric precipitation and condensate supply are very small. Because the average annual precipitation in the study area is only 42 ~ 54 mm, and it is extremely rare that the secondary precipitation exceeds 10 mm, the precipitation infiltration recharge is ignored in the simulation. The direction of groundwater runoff moves radially from south to north. The phreatic water is mainly discharged by evaporation, concentrated in the lower reaches of the north-central region and the Lake Gurinai. The phreatic evaporation decreases with the increase of groundwater level. Artificial mining is mainly concentrated in the densely populated areas such as Jinta County, Ding Xin and Dongfeng Field Base, and the mining horizon is mainly confined water system.

Between Jinta-Huahaizi basin and Ejina basin, although the thickness of Quaternary strata changes greatly, the strata are basically continuous and can be considered as a unified system. In Ejina basin, the Langxinshan-Mujihushan uplift zone divides the whole basin into two parts, but it can also be regarded as a unified whole because of the continuity of Quaternary strata.

Single-layer diving system is distributed near Beishan in the southeast and west. The northwest and northeast are multi-layer phreatic-confined water systems. In the distribution area of multi-layer groundwater system, the aquiclude is thin and discontinuous, and there is hydraulic connection between the upper aquifer and the lower aquifer. Therefore, the confined water aquifer group is regarded as a system. There is a thin weak permeable layer between the phreatic water and the confined water system, and the water level difference is 0. 1 ~ 2.2 m, which is hydraulically connected by overflow. In the single-layer diving system zone, there is a thin "weak permeable layer" which is virtual and has a large permeability coefficient. In this way, the groundwater system in the lower reaches of Heihe River Basin is generalized as a two-layer phreatic water-confined water system.

The bottom boundary of the aquifer is Jurassic and Tertiary mudstone and sandy mudstone, which are relatively water-isolated. The horizontal direction is surrounded by the second kind of boundary, in which the southwest boundary is the groundwater watershed of Huahai Farm and Sanjiu Farm, and the southern, northwestern, northern and northeastern plains are in contact with the foothills or faults, and the mountain fissure water and pore water rarely replenish the plains, which is a weakly permeable boundary. The southeast boundary is underground runoff in desert area, which is a strongly permeable boundary. In the south of Huxi New Village, surface water is a perennial flow, which is simplified as one-dimensional unsteady flow.

Therefore, the groundwater system in the lower reaches of Heihe River Basin can be generalized as a two-layer quasi-three-dimensional groundwater system with infiltration and recharge of river water and irrigation water, evaporation and artificial exploitation and discharge, the second kind of boundary conditions and aquifers connected by overflow. The mathematical model can be described by the coupling equation of formula (9- 1) and formula (9-3) (Qian Hui et al.,1999; Abraham et al., 1999).

(c) source and sink treatment

1. River infiltration recharge

To the south of Huxi New Village, the main stream of Heihe River is a perennial river. According to the one-dimensional unsteady flow model of surface water and the multi-layer groundwater system model of groundwater flow, the recharge of river water to groundwater is solved. To the north of Xixi New Village in Heihe Lake is a seasonal river. According to the lithologic distribution of the river and the water quantity of the river, the infiltration recharge of the river is identified and determined according to the single-length infiltration rate in the simulation process.

2. Irrigation water infiltration recharge

In agricultural concentrated development zones such as Jinta and Ding Xin, the irrigation water infiltration recharge is large, which depends on the lithology of vadose zone, the buried depth of groundwater table and the irrigation water quantity. The supplementary quantity is supplemented by area, and its size is identified and determined in the simulation process.

3. Reservoir infiltration recharge

It mainly includes Yingpan Reservoir and Hexi New Lake. According to the reservoir distribution area and reservoir bottom lithology and other factors, the infiltration amount is determined.

4. Groundwater exploitation

There are few underground water exploitation wells in the lower reaches of Heihe River Basin, which are mainly distributed in Jinta County, Dongfeng Base, Ejina Banner population concentrated area and agricultural development zone in Ding Xin, and mainly exploit confined water. In alluvial fine soil plain, the phreatic water quality is not good, mostly brackish water and salt water, so the exploitation amount is small, which is mainly used as greening water. According to the distribution characteristics of production wells, they can be summarized as non-point source wells and point source wells.

Step 5 flood

According to the lithology and thickness of the weak permeable layer and the difference between the upper and lower groundwater levels, the overflow coefficient of each region is determined by model partition identification.

6. phreatic evaporation

Phreatic evaporation is mainly related to the lithology of vadose zone, the depth of groundwater table, vegetation development and climate change. According to the measured data of Gansu Second Hydrogeological Team, the limit depth of phreatic water evaporation is determined to be 5.0m. The relationship curve between annual evaporation intensity (qe) of phreatic water and buried depth of groundwater (G) is shown in Figure 9- 17, and the relationship is as follows.

Water Cycle and Evolution Model of Groundwater in Heihe River Basin

Figure 9- 17 Relationship between annual evaporation intensity of groundwater and buried depth of groundwater level in the lower reaches of Heihe River Basin

Second, numerical simulation and results

(A) the division of the calculation area

The perennial river to the south of Huxi New Village is divided by one dimension, and * * * is divided into 12 nodes. The triangular grid is selected to divide the groundwater system, so that one side of the dividing unit of the groundwater system coincides with the section of the surface river as much as possible, and the factors such as aquifer abundance, river distribution, groundwater level observation well location, research degree, land development and utilization status, and mining well location are fully considered. * * * It is divided into 142 nodes and 2 16 units in the single-layer structure groundwater system, and 284 nodes and 432 units in the double-layer structure groundwater system, with 6 1 secondary flow boundary in each layer (Figure 9- 18).

Figure 9- 18 Subdivision of Heihe River Basin Downstream

(2) Conditional treatment of definite solution

1. Enter the condition

Select 1 September, 9871as the initial time and adopt kriging method (Wang Jingbo et al.,1999; Chen Li et al., 2002) reasonably interpolate the missing data area, and determine the groundwater level of each node as the initial flow field during model identification (Figure 9- 19 and Figure 9-20). For surface water, 65438+September 1 0987 is also selected as the initial time, and the surface water level, flow rate, section characteristics (riverbed bottom width, riverbed elevation, etc.). ) is interpolated outside the four known road sections south of Huxi New Village.

Fig. 9- 19 initial flow field diagram of groundwater system model fitting stage in the lower reaches of Heihe river basin

Figure 9-20 Initial Flow Field Diagram in the Model Fitting Stage of the Lower Reaches of Heihe River Basin

2. Boundary conditions

The boundary of groundwater system is the second kind of flow boundary. In the southeast desert area with high permeability boundary, the recharge of groundwater runoff is calculated with reference to the measured data of the second hydrogeological team of Gansu Province. In the southwest of Jinta, it is the zero-flow boundary of watershed. The plain areas in the northwest, north, northeast and south are in contact with the foothills or faults, and the mountain fissure water and pore water have little supply to the plain areas, which is a weakly permeable boundary. The upper section of the surface river is the flow boundary; The lower section is the dividing line of groundwater level.

(3) Selection of simulation period

The simulation period is from September 1987 to August 1988. * * It is divided into 12 time periods, and the step size of each time period is 1 month. There are three observation wells in the calculation area of diving system and confined water system respectively, and their groundwater dynamic observation data are used as the basis for model identification.

(4) Parameter division and model identification

Parameter estimation is determined by partition method. That is, according to hydrogeological exploration, field pumping test data and previous achievements, the parameters of phreatic water system and confined water system in the calculation area are preliminarily partitioned, and the parameters of each partition are assumed to be the same. The main parameters are: ① the leakage coefficient of perennial rivers north of Huxi New Village and the single-length leakage rate of seasonal rivers; ② specific yield and permeability coefficient of diving system; ③ Permeability coefficient and water storage coefficient of confined water system; ④ Overflow coefficient of weak permeable layer.

The model identification requirements meet the following criteria: ① The simulated groundwater flow field is basically consistent with the actual groundwater flow field; ② The simulated groundwater dynamic change process is basically similar to the actual groundwater dynamic change process; ③ The simulated groundwater balance changes are consistent with the actual groundwater balance changes; ④ The determined hydrogeological parameters conform to the actual hydrogeological conditions.

The key to solve the coupling simulation model of surface river water and groundwater is to determine the exchange capacity of river water and groundwater system. In the groundwater simulation model, the hydraulic connection between surface rivers and groundwater is represented by vertical overflow intensity. In the simulation model of surface river water, the hydraulic connection between groundwater and river water is also based on the overflow intensity, and is transformed into the exchange capacity expression. Therefore, exchange capacity interacts with river water level and groundwater level. The change of exchange capacity will inevitably lead to the change of river water level and groundwater level. On the contrary, the change of river water level and groundwater level will lead to the change of exchange capacity. According to these characteristics, iterative method is used to solve the coupling simulation model of surface water flow and groundwater flow. In the process of solving, the iterative process of solving is properly constructed and gradually approached until satisfactory results are obtained (Lu Wenxi, 1999).

In the coupling equation, the river water level Z and water table H 1 in each river reach are replaced by the river water level and water table in the middle of each river reach. That is to say, the water level difference between the surface water level and the groundwater level at the middle point of the reach represents the water level difference of the whole reach. In the identification of groundwater flow model, the parameters are obtained by inversion method. The specific steps are (Deninger,1970; Nield et al., 1994):

(1) Firstly, the groundwater level in the exchange volume e between surface water and groundwater in the coupling model is replaced by the initial groundwater level H 10 at the middle point of the river, and the surface river water level z and the river water surface width b are replaced by the initial values.

(2) Given the initial value Cps of the river seepage recharge coefficient in each reach, calculate the initial value E0 of the exchange capacity between river water and phreatic water, so that Ei=E0.

(3) Set E=Ei, run the surface water simulation model (9- 1), and use the Preissmann implicit solution method mentioned above to calculate the river water level, river cross-section flow and water surface width of each divided node of the river.

(4) using the calculated river water level Zi and the river water level at the initial moment to get the average value; The exchange capacity Ei+ 1 between phreatic water and groundwater is recalculated by using the average river water level and the known groundwater level.

(5) check | ei+1-ei | ≤Δ no, if this condition is met, go to step (6), otherwise, let Ei=Ei+ 1 and go to step (3).

And (6) substituting the calculated exchange water into the multi-layer structure groundwater flow model (9-3), adopting the method of simulating the mathematical model of the multi-layer structure groundwater flow, repeatedly adjusting parameters through trial estimation, identifying different parameters according to different time periods, and fitting the groundwater level.

(7) If it is the first time to run the multi-layer groundwater flow model iteratively, directly enter step (8). Starting from the second iteration operation of the mathematical model of multi-layer groundwater flow, it is necessary to check whether the groundwater level at the river division node calculated in the previous two iterations is |Hi-Hi- 1|≤ε, and if this condition is not met, proceed to step (8); Otherwise, replace the initial value and calculate the next time period.

(8) In the coupling equation, replace the groundwater level at the middle point of the river with the average value of the initial time and the last time, and proceed to step (4).

Using the above method, the coupling simulation of surface water flow and multi-layer groundwater system model in the lower reaches of Heihe River Basin is carried out.

See table 9-9 ~ table 9- 13 for the determined partition parameter values, and see figure 9-2 1 ~ figure 9-25 for the parameter partition.

Table 9-9 Zoning Parameters of Permeability Coefficient of Groundwater System in the Lower Reaches of Heihe River Basin

Table 9- 10 zoning parameters of water supply of phreatic system in the lower reaches of Heihe River Basin

Table 9- 1 1 Zoning Parameters of Overflow Coefficient of Weak Permeable Layer in the Lower Reaches of Heihe River Basin

Table 9- 12 Partition parameters of permeability coefficient of confined water system in the lower reaches of Heihe River Basin

Table 9- 13 zoning parameters of water storage coefficient of confined water system in the lower reaches of Heihe river basin

Figure 9-2 1 Division of Permeability Coefficient of Groundwater System in the Lower Reaches of Heihe River Basin

Figure 9-22 specific yield Division of Groundwater System in the Lower Reaches of Heihe River Basin

Figure 9-23 Division of Overflow Coefficient of Weak Permeable Layer in the Lower Reaches of Heihe River Basin

Figure 9-24 Division of Permeability Coefficient of Confined Water System in the Lower Reaches of Heihe River Basin

Figure 9-25 Division of Water Storage Coefficient of Confined Water System in Lower Reaches of Heihe River Basin

It can be seen from Table 9- 14 and Table 9- 15 that during the model fitting, both phreatic water and confined water are in negative balance. Because the numerical simulation is carried out on the basis of small and medium-scale hydrogeological investigation, there are some errors in the basic data, some blank areas and few fitting positions to choose from, so there are some errors in the numerical simulation. However, from the fitting results, the calculated groundwater level of observation wells in the main control area is close to the observed value (Figure 9-26). No matter from the whole fitting correction period or the whole flow field, the fitting effect is ideal, and the model well reflects the mutual transformation relationship between surface water and groundwater.

Table 9- 14 Water Balance Table in the Fitting Stage of Groundwater System in the Lower Reaches of Heihe River Basin (103m3)

Table 9- 15 Water Balance Table for the Fitting Stage of the Confined Water System in the Lower Reaches of Heihe River Basin (103m3)

Figure 9-26 Fitting curve of groundwater level of observation wells in the lower reaches of Heihe River Basin.

Thirdly, the dynamic trend of groundwater level.

On the basis of the above research, the groundwater level dynamics in the lower reaches of Heihe river basin are predicted by using the obtained hydrogeological parameters and adopting three schemes: A, B and C.

Scheme A does not change the existing mining conditions, and Heihe maintains the flow level of September1987 ~ August 1988, that is, the annual runoff of Zhengyixia is 8.2 1× 108 m3/a, and the flow of Langxin Shanshui Station is 4.14×/kloc.

In Scheme B, the annual discharge of Zhengyixia is 10.5× 108 m3/a, the annual water consumption of Jinta and Ding Xin irrigation areas is 2.5× 108 m3/a, and the leakage from Liangdong in Diwan to Langxinshan is1.0×108m3/.

In Scheme C, the annual runoff of Zhengyixia is 9.5× 108 m3/a, the annual consumption of Jinta and Ding Xin irrigation areas is 2.5× 108 m3/a, the leakage from Liangdong in Diwan to Langxinshan is 1.0× 108 m3/a, and the discharge of Langxinshan Station is 6.0.

Figure 9-27 Initial Isohydrograph Predicted by Groundwater System in the Lower Reaches of Heihe River Basin

Figure 9-28 Prediction of Initial Isohydrograph of Confined Water System in the Lower Reaches of Heihe River Basin

The following characteristics can be drawn from the prediction results:

1) In the case of Scheme A, the phreatic water system and confined water system in the lower reaches of Heihe River Basin are negatively balanced in each period. In Ding Xin Basin, near Nanshan and Jiashan, which are near Heihe River and Beida River, the groundwater level is rising. In Ding Xin and Huxi Xincun area, the groundwater level near Heihe River is rising by 0.2 ~ 0.4m every year ... The groundwater level in other areas is decreasing by 0.05 ~ 0.25 m every year, and the confined water level in the northwest of Jinta County is decreasing greatly, with the maximum depth of 7.0 m at the beginning of September 2009. In the state-owned and surrounding areas of Ejina county under construction, the water level of confined water has dropped greatly, and the cumulative buried depth is 2.0 ~ 4.5 m. ..

2) In the case of Scheme B, the phreatic water system in the lower reaches of Heihe River Basin is in a positive balance in all periods. The groundwater level near the river continues to rise, and the groundwater level near Heihe River in Ding Xin and Huxi New Village rises by 0.08 ~ 0.50m every year ... The groundwater level near the east and west river beds in Laoximiao-Ejina area rises by 0.03 ~ 0. 14m every year. The groundwater level drops by 0.12 ~ 0.33m annually near the northeast border, and the confined water level drops by 0.30 ~ 0.59m annually in Jinta County ... In Gulinai area, the confined water level drops by 0.18 ~ 0.68m ~ 0.68m annually, and near Ejina Banner, the confined water level drops by 0./kloc-0.68m ~ 0.68m annually.

Figure 9-29 Scheme A simulates the isobar of phreatic system in the lower reaches of Heihe River Basin.

Figure 9-30 Scheme A simulates the isobar of the confined water system in the lower reaches of Heihe River Basin.

Figure 9-3 1 Scheme B simulates the isobar of the phreatic system in the lower reaches of Heihe River Basin.

Figure 9-32 Scheme B simulates the isobar of the confined water system in the lower reaches of Heihe River Basin.

3) In the case of Scheme C, the phreatic water system in the lower reaches of Heihe River Basin is in a positive balance in all periods. The groundwater level near the river continues to rise, and the groundwater level near the Heihe River in Ding Xin-Huxi New Village rises by 0.08 ~ 0.50m annually ... The groundwater level near the east and west riverbeds in Laoximiao-Ejina Banner rises slightly, with an average annual increase of 0.02 ~ 0.1/m ... Near the northeast border, the groundwater level drops, with an average annual decrease of 0./kloc-0. In the vicinity of Jinta County, the confined water level drops by 0.3 ~ 0.67 meters annually ... In Gurinai area, it drops by 0.30 ~ 0.78 meters annually, and in the vicinity of Ejina Banner, it drops by 0.39 meters annually ... The confined water level in Laoximiao area drops by 0.30 ~ 0.92 meters annually. ..

Comparing the forecast results of Scheme A, Scheme B and C3, it can be seen that Scheme B, that is, when the downstream flow of Langxin Shanshuiwen Station reaches 7.0× 108m3/a, can basically ensure that the groundwater level in the desert plain of Ejina Basin will not drop, thus ensuring the benign development of the ecological environment.

Figure 9-33 Scheme C simulates the isobar of phreatic system in the lower reaches of Heihe River Basin.

Figure 9-34 Scheme C simulates the isobar of the confined water system in the lower reaches of Heihe River Basin.