Verification of numerical model of water resources and analysis of groundwater balance at present

8.2. 1 model verification and identification

Based on the data of 1990 ~ 2000, such as the actual utilization of water resources, river hydrology, groundwater level dynamics and meteorology, the numerical model of water resources in the middle reaches is verified and identified.

During this period, the actual water resources data are brought into the model to simulate the dynamic process of groundwater level, the flow process of spring water and the flow process of Zhengyixia. Comparing the simulated data with the actual data, adjusting the model structure and model parameters until a good fitting is achieved, that is, the macro simulation of the middle reaches water resources system is realized.

The dynamic data of groundwater level is influenced by random factors such as surface hydrology, mining and irrigation, and has certain random components. The fluctuation of groundwater level in a month, or the trend of water level change in a quarter or even a year, does not necessarily represent the general trend of regional groundwater level. Therefore, the groundwater decline value in a quarter or a year cannot be used as the verification basis of the model. In addition, the actual water resources data is simplified in data processing, and there are also small random fluctuation factors in the simplified and summarized data.

The ideal data used to judge the proofreading model should have the characteristics of long time series and large amplitude change. If the variation amplitude far exceeds the random fluctuation interference, its essence is to "filter" the random interference and improve the reliability of simulation identification.

The study of groundwater in the middle reaches of the plain is relatively high, and a large number of water conservancy data have been accumulated. The accumulated fluctuation value of groundwater level in 1990s and the runoff data of Zhengyixia Hydrological Station from February to February in the past 65438+ years are selected as the basis for model verification and identification (from February to February in 65438+ years, the runoff of Zhengyixia is basically the overflow of spring water in this period, because there is no water diversion along the Heihe River), although the groundwater level dynamics in a certain year have the following characteristics.

By adjusting the structural parameters and formation parameters of the model, the simulated water level and the actual water level decline depth have achieved a good regional fitting, especially the cumulative water level change in 10, and the relative fitting accuracy is close to 90%. This can explain two problems: first, the generalization of the numerical model of water resources is reasonable, and second, the model can better simulate the interaction between various elements of water resources. The identified numerical model can be used for water resources scheduling prediction and simulation analysis, so as to scientifically and reasonably allocate middle-stream water resources.

8.2.2 Analysis of groundwater balance

Through numerical simulation, the results of groundwater resources balance in different periods in the plain area of the middle reaches of Heihe River are obtained (Table 8. 1), and draw the simulation diagram of groundwater level decline in 10 (1990 ~ 1999) (Figure 8. 3).

Figure 8. 3 Simulation diagram of groundwater level decline in the plain area of the middle reaches of Heihe River 10 year (1990 ~ 1999).

Table 8. Table 1 analysis unit of groundwater simulation balance in central plain area: 108m3/ a

According to the analysis in Table 8. 1, and the total groundwater recharge resources in the early 1990s and 2000 (that year) were 16. 627× 108m3/ a and 14. 632× 108m3/ a。 During the period of 10, it decreased by nearly 2× 108m3/ a, in which the leakage of canal system and farmland irrigation decreased by 2. 125× 108m3/a ... According to the rough statistics of the whole middle reaches plain calculation area, the utilization coefficient of main canal, branch canal and bucket canal was about 60% in the early 1990s, and the infiltration coefficient of field irrigation was about 15% ~ 20%. By 2000, due to the strengthening of canal seepage control, the utilization coefficient of main canal, branch canal and bucket canal increased to about 80% on average, which greatly reduced the recharge of groundwater.

From the early 1990s to the year of 10 in 2000, in order to solve the problem of spring drought, the amount of groundwater exploitation increased greatly, from 0. 65× 108 m3/a to 2. 476× 65,438+008m3/a, which led to the readjustment of drainage elements, the overflow of rivers and springs, and the corresponding evaporation of groundwater. The overflow of river spring water has gradually decreased from12.131×108m3/a in the early 1990s to1.537×108m3/a, which is a decrease. 594× 65438. The evaporation of groundwater is from 4. 4 17× 108m3/ a to 2 in the early 1990s. 849× 108m3/ a, decreased by 1. 568× 108m3/a ..

According to Figure 8. 3. The cumulative 10-year decreasing distribution shows that there is no strong groundwater exploitation in the position where the groundwater level drops deeply, which is obviously not caused by groundwater exploitation. The area with large depth drop can exceed 4m, and the maximum value appears in the upper part of alluvial fan in front of Hongshui River and Tongziba River in Minle County, with a depth drop of over 10m. Other places with large depth drop are distributed in the southern piedmont zone without groundwater exploitation (except Camel City groundwater exploitation irrigation area).

In the process of model updating, in order to seek the influence mechanism of regional groundwater level decline, a large number of combined simulation analysis are carried out on various possible mechanisms. After comprehensive analysis, it is considered that there are two main reasons for the formation of this shape of falling field, the most important of which is the reduction of "area distribution" leakage or irrigation return recharge in various irrigation areas, that is, the strengthening of canal lining seepage control and the gradual popularization of water-saving irrigation methods in recent ten years; Another main reason is that the interception of floods in mountainous areas reduces the recharge of groundwater in front of mountains.

At the same time, the simulation results show that the subsequent impact of the flood impounded in front of the mountain on groundwater will last for decades or even hundreds of years before reaching a new balance.

Heihe River is the lowest drainage datum in the middle reaches of the plain. Under this specific condition, compared with spring water and evaporation drainage, river overflow and drainage are relatively stable, that is, the amount of groundwater exploitation is increased, or the amount of groundwater recharge is reduced due to hydraulic engineering measures. The first impact should be the overflow of the upstream source area of the spring and the evaporation of groundwater in the shallow buried area. Therefore, the attenuation of river overflow has obvious lag, and the response lag period is long, while the response lag period of spring water and shallow groundwater located in the upstream is short, that is, the spring flow attenuation is fast. Many years of practical data and model simulation conclusions have proved this point, which accords with the circulation law of groundwater. River overflow mainly depends on the local hydraulic gradient near the river. As long as the groundwater flow field (or gradient) near Heihe River does not change greatly, the river overflow will not be greatly reduced. When the local groundwater depth is small, the evaporation intensity of groundwater changes sharply with the groundwater depth because of the nonlinear relationship between evaporation and groundwater depth. Although the groundwater level in shallow-buried areas has not decreased significantly in recent ten years, the evaporation of groundwater has changed obviously, especially in areas with a buried depth of less than 2 m. When the buried depth of groundwater is more than 3m, the evaporation of groundwater can be limited by lowering the groundwater level.

From the point of view of resource balance, the groundwater balance in the middle reaches of the plain is negative. Although the whole calculation area is negative, the negative balance mainly occurs in mountainous areas, spring areas and shallow evaporation areas far away from Heihe River, which shows that the groundwater level in the piedmont plain drops more, while the water level in Heihe River, spring areas and shallow evaporation areas drops less. Taking the balance in 2000 as an example, in the southeast (Minle County), due to the continuous decline of groundwater level, the aquifer in the upper reaches was gradually drained, making the negative average groundwater in this local area close to 1. 5× 108 m3/a； However, due to the "adaptive" regulation of groundwater discharge (when the recharge decreases, the discharge will automatically decrease), the negative average value of groundwater is small, that is, in some areas of the discharge area, the recharge and discharge of groundwater are roughly balanced.

The hysteresis characteristics of overflow response of rivers and springs are easy to give people an illusion. The operation of some water conservancy projects has increased the utilization of some surface water resources and reduced the recharge of groundwater. Due to the lag response of rivers and/or springs, the overflow did not decrease immediately, but the total water resources available on the surface (surface water+groundwater) seemed to increase. This is only a short-term performance. The actual situation is that the aquifer "underground reservoir" is gradually consumed. After a long period of time, the amount of groundwater overflow will gradually decrease, and in severe cases, groundwater resources will be exhausted.

Taking the southeast plain in the middle reaches of Heihe River (Minle County) as an example, the groundwater level is more than 200 meters higher than that of Heihe River near Zhangye. When the groundwater level in the irrigation area drops slightly, such as 10m, compared with the whole groundwater level drop, the overall hydraulic gradient change is less than 10%, that is, in a short time, the groundwater transported from the upstream area to the downstream through the aquifer will not decrease significantly (almost "constant" in a short time), but at the expense of continuously draining the upstream aquifer, according to the simulation equilibrium calculation results, under the current conditions, From the perspective of sustainable development, the consumption of the upstream aquifer by long-term drainage is not allowed by the great changes in the ecological environment, and it will also lead to the depletion of groundwater resources. This way of development and utilization can be described as "spending money to buy grain" and cannot last long.

With the improvement of canal seepage control project and the popularization of water-saving technology, the available water volume has increased. We should use the "surplus" water resources in rainy years or seasons to supplement the upstream areas, so as to prevent or slow down the evolution of groundwater resources to exhaustion, instead of blindly expanding the cultivated land area and transforming the water resources cycle into a reasonable and sustainable state.