The main stream plain area in the middle reaches is the focus of this study, and the water resources model mainly simulates the plain area where water resources regulation has a direct impact on the discharge of Zhengyixia. The simulation range starts from Qijiadian Reservoir in Shandan County in the east and reaches Qingshui Station in Jiuquan East Basin in the west, which is a complete groundwater basin bounded by the mountain front line in the north and south. Including all the affected areas of Heihe water conservancy project and the distribution area of Quaternary aquifer connected with this area (Figure 8. 1).
The boundary of the whole model calculation area is the inflow boundary of groundwater, which is a complete faulted groundwater basin. The thick loose deposits deposited in this period are the natural places where groundwater occurs, and the Quaternary aquifer is a continuous and unified water-bearing rock aggregate limited by the basin boundary, and the surrounding piedmont faults constitute the natural geological boundary of the aquifer. The whole model calculation area belongs to Zhangye Basin and Jiuquan East Basin respectively, including plain irrigation areas in Zhangye, Linze, Gaotai and Minle County, Shandan in Sunan and individual irrigation areas in Minghua District, with an area of nearly 9000km2.
8. 1.2 simplification of underground water flow system
The general distribution of aquifer structure in the model area is that the top of alluvial fan in the southern mountainous area is a single-layer thick phreatic area, and the aquifer gradually changes from a single phreatic area to a phreatic-confined water area from the north to the downstream fine soil plain. In the confined groundwater area in the north, some areas lack cohesive soil layer and have the characteristics of single-layer diving. In addition, a large number of mechanical wells distributed in this area collude with each aquifer, which makes them have strong hydraulic connection, and the upper and lower water levels are almost the same, so the plain area in the middle reaches can be regarded as a single-layer phreatic aquifer. This reasonable simplification, on the one hand, simplifies the complexity of the simulation system, on the other hand, it also avoids the difficulty for managers to distinguish the layered exploitation of groundwater in each irrigation area.
Figure 8. Scope map of plain area 1 groundwater model in the middle reaches of Heihe River
In the calculation area of the model, the annual variation of groundwater level is less than the thickness of aquifer, so the variation of aquifer permeability coefficient with time can be ignored, and the aquifer permeability coefficient T which does not change with time can be used to approximate the aquifer permeability coefficient.
The boundary of the simulation calculation area can be further divided into two categories: one is the natural geological boundary formed by the south, north and east piedmont fault depression or uplift, and the inflow of groundwater is the lateral inflow of bedrock fissure water and the undercurrent of river valley. The groundwater inflow of this boundary has nothing to do with the groundwater state in the plain area. As a constant flow boundary, its flow takes the annual average of the boundary inflow. When different water resources are adjusted and allocated in the middle reaches of the basin, the boundary flow remains unchanged. The other is the unnatural boundary on the west side, and a small amount of groundwater runoff flows into this area under the current conditions. Because the boundary is far away from the main stream of Heihe River, the groundwater runoff in this area is weak. From qualitative analysis, it can be inferred that the influence of water resources regulation measures on the boundary flow is weak, and the boundary can be approximately regarded as the flow boundary that changes with hydraulic conditions. When the influence of water conservancy measures does not reach the boundary, its flow still takes the current groundwater runoff. When the water conservancy project affects the boundary, the current groundwater runoff will be taken.
The groundwater in the whole model calculation area flows to Heihe River, where it flows to the lower reaches of Heihe River through Zhengyixia.
8. 1.3 Construction of mathematical model in the middle reaches of plain area
The groundwater movement in the calculated area can be described by groundwater model, river and spring overflow model and evaporation model at the same time.
8. 1.3. 1 mathematical model of groundwater
The mathematical expression of groundwater model in the middle reaches of Heihe River Basin is as follows:
Regulation and Optimal Utilization Mode of Water Resources in Typical Inland Watershed in Northwest China —— A Case Study of Heihe River Basin
formula
H—— aquifer water level elevation (m);
T—— permeability coefficient of aquifer (m2/d);
μ —— the water supply of aquifer (dimensionless);
Wb—— sum of intensities of various recharge projects (excluding river water leakage recharge) (m/d);
Wp—— the sum of the intensities of various discharge items (excluding overflow and evaporation discharge) (m/d);
Et-groundwater transpiration and discharge intensity (m/d);
Wr—— leakage intensity of rivers to groundwater (m/d);
Ws—— overflow intensity of spring water (including groundwater discharged by river water) (m/d);
Q2, q3—— the flow rate per unit flow boundary width (m2/ d);
α-boundary outflow coefficient (m/d);
γ2- natural flow boundary;
γ 3 —— the boundary of westward flow;
N- is the outer normal direction on the boundary.
8. 1.3.2 Heihe mathematical model
The mathematical expression of the river model in the middle reaches of Heihe River Basin is as follows:
Regulation and Optimal Utilization Mode of Water Resources in Typical Inland Watershed in Northwest China —— A Case Study of Heihe River Basin
formula
QR- river flow (m3/d);
B —— river width (m);
L —— the length of the river from the entrance (m);
Qrb—— tributary flow (m3/d);
Li —— confluence of tributaries (m);
δ(x)- Dirac function;
Regulation and Optimal Utilization Mode of Water Resources in Typical Inland Watershed in Northwest China —— A Case Study of Heihe River Basin
αr—— river bed leakage coefficient (1/d);
Hr—— River water level (m);
Wrl-ultimate seepage strength of river bed (m/d);
8. 1.3.3 Mathematical model of spring
The mathematical expression of spring water model in the middle reaches of Heihe River Basin is as follows:
Regulation and Optimal Utilization Mode of Water Resources in Typical Inland Watershed in Northwest China —— A Case Study of Heihe River Basin
formula
αs—— spring overflow coefficient (1/d);
Hs-overflow height of spring water (m).
8. 1.3.4 Mathematical model of groundwater evaporation
According to the series data of groundwater evaporation of the Second Hydrogeological Team of Gansu Bureau of Geology and Mineral Resources at Dipu infiltration test Station in Zhangye Plain, the piecewise linearization method is selected to approximately describe the highly nonlinear groundwater evaporation process.
Regulation and Optimal Utilization Mode of Water Resources in Typical Inland Watershed in Northwest China —— A Case Study of Heihe River Basin
formula
Et( x, y, t)- groundwater evaporation intensity (m/d);
Hg—— ground elevation (m);
{Et i( t), (Hg-hi)}- evaporation intensity data series under different burial depths;
H (x, y)- groundwater level (m);
Veg (t) —— Correction coefficient of vegetation evaporation and coverage.
8. Solution of1.4 Numerical Model
8. 1.4. 1 model division and solution
The groundwater model, river model, spring model and evaporation model in the middle reaches are combined into a set of nonlinear differential equations, and the joint model of water resources is solved by orthogonal grid finite difference method.
The numerical model adopts different grid spacing (δX =δY = 5km, 2km, 1km, 0. 5km) for comparative numerical calculation. The calculation results show that when the grid spacing is ≥2km, the spatial distribution error of river runoff and spring runoff is large, and it is difficult to describe the spatial distribution characteristics well.
Select Δ x =Δy =1km and directly use the km network in the Gaussian projection topographic map as the subdivision grid. The model area of the middle reaches basin is located in the 6-degree projection zone of 17 Gauss, and the division result is: the east-west direction is 173 (km) grid, from 17500km to17673 km; North-south direction 154 (km), from 4256km to 44 10km. There are 8622 effective computational grids, of which 178 grids are used to describe Heihe River. In addition, the time discrete step is natural month, that is, every year is divided into 12 time periods.
In order to ensure the stability and convergence of the solution, the spatial division of groundwater model adopts the central five-point scheme, and the time dispersion adopts the backward difference scheme (implicit difference scheme). Taking the water exchange intensity between models as the link, different models are iterated at the same time. In order to overcome the fluctuation of the solution, the exchange strength between different models adopts the "exponential damping" iterative technique. Although the iterative convergence speed is slightly slow, it can ensure the convergence and stability of the solution.
After solving the water resources joint model system, the following results can be obtained at the same time: temporal and spatial distribution of groundwater level, temporal and spatial distribution of groundwater depth, river flow process and spring flow.
According to the different simulation stages, the groundwater level in different periods is selected as the initial condition. In the model verification stage (the numerical model was modified by the actual data from1990 to 2000), the water level field and buried depth field of groundwater in the middle reaches of the basin were taken as the initial flow field; In the stage of simulation adjustment and prediction, the simulated flow field and buried depth field in 2000 are selected as initial conditions.
8. 1.4.2 model input data processing
The conversion between surface water and groundwater is frequent in the middle reaches plain area, and the joint simulation system of water resources is still too complicated after some non-main control factors in the model are properly simplified. Under the premise of satisfying the simulation accuracy of water resources basin-level dispatching, whether the model data structure can be properly and reasonably simplified has become the bottleneck problem of water resources simulation system.
According to its nature, the data (database) needed by water resources model can be divided into two categories: natural factor data and manual control data. All the factors that people's behavior can't interfere with and operate are attributed to the former, such as the hydraulic conductivity and spatial distribution of aquifers, meteorology, hydrology, aquifer boundary conditions and so on. This kind of data does not need to be changed when the simulation model is used in water resources dispatching planning. The latter kind of data contains the content that everyone's behavior can interfere with, which is exactly the problem that water resources management decision makers need to consider carefully. Such as the distribution and scale of water conservancy projects, the water intake of each irrigation area, the spatial distribution of wells, the exploitation of groundwater, irrigation quota and irrigation system, and the utilization rate of canal system.
The rationality of this data classification lies in that natural factor data (database) can be professionally processed by relevant experts (water resources experts, hydrogeologists, meteorologists, etc.) only after careful preparation once. ). After the model identification and proofreading are successful, all the data are "frozen" and generally do not need to be changed in the future. However, in the stage of resource regulation and demonstration or real-time management, the decision-makers of management institutions need to constantly adjust (that is, change the water use scheme) and simulate it to observe the "preview" effect after the change and determine a more reasonable and feasible water resources regulation and utilization scheme. As for water resources management and dispatching personnel, do not seek experts in hydrology, geology and meteorology, as long as they are proficient in water resources management. Therefore, the simplification of model data structure should be mainly oriented to control data to improve the operability of simulation analysis.
Combined with the characteristics of the calculation area, the specific processing methods of master control data are as follows:
(1) distribution of irrigation areas
The model is divided into grids of 1km2, and each irrigation area is composed of several grids, and the boundary of irrigation area is approximately described by stepped broken line boundary. The calculation area of the model is divided into 38 irrigation areas (including Zhangye City).
(2) Distribution of groundwater exploitation
From the perspective of macro resource management, it is not necessary to specify the specific number of wells, the detailed location of wells and the daily output of each irrigation area. When the number of wells in an irrigation area exceeds 1000 eyes or even1000 eyes, managers will be puzzled. Combined with the actual situation in the middle reaches of the plain, the following principles are simplified: according to the buried depth of groundwater level, each irrigation area is divided into two parts, and the area with buried depth greater than 50m and the area with buried depth less than 50m are suitable mining areas, which is approximately considered as uniform mining, that is, wells are evenly distributed in the area with buried depth less than 50m, but groundwater is not mined in the area with large buried depth.
Because the distribution range of each irrigation area is limited, this simplification will not lead to great distortion. Therefore, only a few parameters can be used to approximately describe the mining situation of each irrigation area, such as the annual total mining amount of the irrigation area, the mining distribution ratio within a year 12 months, etc. Which greatly facilitate that regulation and management of water resource.
(3) Distribution of irrigation canal system and cultivated land
Irrigation canal system is the basic vein of irrigation area, and its leakage is also the main component of groundwater resources. Considering the actual spatial distribution of trunk, branch, bucket and agricultural canal, the resource planning is too complicated and must be moderately simplified.
The spatial distribution data of total water diversion, total leakage of canal system and total leakage of field irrigation in each irrigation area are used to approximately describe the hydraulic state of each irrigation area. According to the total water diversion, the total exploitation of agricultural wells, the utilization rate of canal system and the field regression leakage coefficient, the total leakage of canal in irrigation area and the total groundwater infiltration between fields can be calculated. It is approximately considered that the leakage of canal system is evenly distributed in the whole irrigation area, while the regression of field irrigation is only distributed in cultivated land. Special treatment can be carried out for irrigation areas with extremely uneven distribution of individual canal systems.
8. 1.4.3 water resources simulation system and structure
The numerical simulation system of groundwater resources in the middle reaches of the plain consists of groundwater numerical model and three auxiliary numerical models of river water, spring water and evaporation. The system adopts Microsoft Visual Basic 6. 0 is the development platform, adopting modular program structure, which is open and extensible. It is mainly composed of four modules: main control module, simulation system, pre-processing system and post-processing system. The structure of the simulation system is shown in Figure 8. 2.
Figure 8. 2 the structure of groundwater simulation system in the middle reaches of Heihe river basin