Quantitative evaluation of groundwater resources

4.2.2. 1 Evaluation of natural groundwater recharge resources

(1) It is necessary to calculate the average natural recharge resources of groundwater for many years. It is required to extend the atmospheric precipitation series to 2003, and calculate the average value of precipitation series from 1956 to 2003 and its corresponding precipitation infiltration recharge.

(2) According to the characteristics of groundwater circulation and water resources transformation, the natural recharge of groundwater is calculated by recharge method, and the discharge should be calculated and checked by water balance method. In the intermountain basin, when the groundwater recharge in the basin is basically equal to the total groundwater runoff discharged into the river, the natural groundwater recharge resources in the basin can be counted according to the groundwater runoff divided by the flow hydrograph of the rivers flowing out of the basin.

Groundwater recharge projects generally include: atmospheric precipitation recharge, river leakage recharge, channel leakage recharge, reservoir and pond leakage recharge, flood leakage recharge, surface water irrigation leakage recharge, lateral runoff recharge, overflow recharge and artificial recharge.

Groundwater discharge projects generally include: groundwater exploitation, phreatic evaporation, lateral outflow, spring overflow, overflow outflow, discharge to rivers and lakes, etc.

Regression recharge of groundwater irrigation is the reuse of groundwater resources, which is no longer included in natural recharge resources. In view of the fact that the regression recharge of groundwater irrigation should be considered in the calculation of groundwater balance and exploitation resources, it is necessary to list the regression recharge separately.

(3) The evaluation of groundwater resources requires the latest data and information of this survey to be used for calculation. In addition to the above precipitation series, other relevant data, such as mining volume, river runoff, channel water diversion volume, irrigation area, irrigation quota, buried depth of groundwater level, etc. , new data after 2000 must be adopted. TDS classification of groundwater, exploration holes and experimental data should also use the new data in recent years. The geographical base map provided by the project team shall be adopted for administrative boundaries and national boundaries.

(4) For some areas with relatively high research level, rich data and long data series, models are generally established. In this kind of area, we can refer to the groundwater balance formula and sub-item recharge established by simulation calculation, and make appropriate corrections to the recharge items according to the changes of groundwater recharge, runoff and discharge conditions in recent 30 years.

(5) This groundwater resource evaluation should analyze and study the changes of groundwater recharge, runoff and discharge conditions caused by natural and human factors (the influence of large-scale water conservancy project construction and joint operation is becoming more and more obvious) in groundwater systems at all levels from a dynamic perspective. On this basis, the corresponding hydrogeological parameters are modified and supplemented, and the groundwater recharge and discharge projects are evaluated with the modified hydrogeological parameters.

According to the actual situation in various places, the following issues are emphatically analyzed and studied:

A. interannual variation of precipitation and its influence on groundwater recharge. The precipitation data is extended to 2003, and the data of precipitation observation points are calculated and processed, and the annual precipitation of each district, the multi-year variation law of precipitation, whether there is an increasing or decreasing trend, the multi-year variation cycle of precipitation and the state of precipitation in the 1990 s are obtained. The extended precipitation series is used to calculate the average precipitation from 1956 to 2003.

B.20 In 1970s, 1980s and 1990s, due to the influence of large-scale reservoir projects, surface water and groundwater storage and resettlement, what changes have taken place in river runoff and channel diversion, channel lining projects and channel utilization coefficient, and what effects have these changes had on groundwater recharge?

C. What changes have taken place in the area and distribution, irrigation quota, irrigation times and irrigation methods of well irrigation and canal irrigation, and what effects have these changes had on groundwater recharge?

D changes in the thickness of vadose zone and lithology of water level fluctuation zone caused by groundwater exploitation, and how these changes affect the vertical seepage recharge of groundwater.

E. For areas with a large amount of groundwater exploitation and a long history, it is suggested to use the dynamic data of precipitation, groundwater resources, exploitation amount and groundwater level in recent 30 years for comparative analysis.

F effects of agricultural planting structure, planting techniques and their changes on recharge and evaporation of phreatic water (or shallow groundwater).

G. Comparison of groundwater evaporation under different lithology, different burial depth and with or without vegetation.

H summarize and analyze the investigation and research results in the past and recent years, and re-understand the mechanism and quantity of lateral recharge from mountainous areas to plains.

I. Changes of hydraulic connection between adjacent aquifers under mining conditions and its influence on groundwater resources and water quality.

J. Influence of seawater intrusion, salt water intrusion and serious groundwater pollution on groundwater resources.

(6) Problems that should be paid attention to in the correction of hydrogeological parameters.

The main parameters involved in the calculation of groundwater resources are precipitation infiltration coefficient, water supply of rock stratum in water level fluctuation area, canal leakage coefficient, irrigation water infiltration coefficient, phreatic water evaporation limit buried depth, aquifer hydraulic conductivity coefficient, aquifer and weak permeable layer permeability coefficient, water storage coefficient and overflow coefficient.

Because the development, utilization and research degree of groundwater vary greatly from place to place, the parameters are appropriately adjusted according to the changes of groundwater recharge and discharge. The hydrogeological parameters obtained by different test methods and different calculation methods will be different, so careful analysis is needed to select the data close to the truth.

A. Precipitation infiltration recharge coefficient (a): The value of precipitation infiltration recharge coefficient (a) is related to the annual precipitation and its change characteristics, the change of groundwater depth, the lithology of vadose zone, topography and other factors. In conditional areas, the relationship curves of precipitation (P)- precipitation infiltration recharge coefficient (A) and groundwater depth (h)- precipitation infiltration recharge coefficient (A) are established by using the calculation of observation data, which makes the parameters more reasonable.

B recharge coefficient of surface water irrigation infiltration: the recharge coefficient of surface water irrigation infiltration depends on irrigation quota, irrigation times, lithologic structure and vadose zone thickness, etc. In spring and summer, temporary observation points can be set up in the field to obtain measurement data.

C. specific yield (μ): The specific yield used in the research and application of groundwater resources refers to the amount of water released in the process of water level change, which is different from the data obtained from indoor tests. Therefore, the water supply calculated by field pumping test and dynamic data should be used as much as possible.

D. Permeability coefficient (k) and hydraulic conductivity coefficient (t)

The permeability coefficient calculated from the pumping test data of small diameter exploration holes is generally small for the coarse-grained lithology in front of the mountain, so it can be corrected by referring to the pumping data of water supply holes in similar lithologic areas.

E. Canal leakage recharge coefficient and river leakage coefficient: due to the method of flow difference between upstream and downstream rivers (canals), the error is often large. Where conditions permit, we should pay attention to collecting and analyzing the changes of well water level near the banks of rivers and canals, and calculate the analysis coefficient.

Evaluation of groundwater exploitation resources in 4.2.2.2

4.2.2.2.1Main environmental constraints to be considered in the evaluation of groundwater exploitation resources.

4.2.2.2.1.1Environmental constraints in the process of groundwater exploitation.

(1) Based on the previous work, the mutual constraints among regional water level decline, land subsidence, collapse, ground fissures and soil salinization, seawater intrusion, water quality deterioration and groundwater exploitation are mainly considered.

(2) Taking the groundwater niche depth as the index and as the main constraint condition of diving (or shallow groundwater) exploitation, this index can be determined quantitatively according to the specific conditions of each region. Summarize the research results of some typical areas in the past and put forward the following suggestions:

A. In the area to prevent soil salinization, the buried depth of groundwater ecological water level is limited to 2 ~ 2.5m or more.

B) In the meadow distribution area for preventing and controlling land desertification, the buried depth of groundwater ecological water level is limited to less than or equal to 3 ~ 4m.

C in the distribution area of trees and shrubs to prevent land desertification, the constraint condition of the buried depth of groundwater ecological water level is less than or equal to 8m.

(3) In areas where the original groundwater is deeply buried or the groundwater level has been declining in recent years, the constraint condition of groundwater exploitation is that the water level will not continue to decline, and technically, it is necessary to maintain a new regional water level dynamic balance.

(4) In the fragile ecological environment restoration area, the diving mining should be strictly controlled to restore the ecological water level.

4.2.2.2.1.2 Influence of groundwater exploitation mode and layout of water intake buildings on groundwater exploitation resources.

There are generally two types of groundwater exploitation: centralized exploitation and utilization of water sources and regional decentralized exploitation and utilization. From the overall situation of groundwater development and utilization in China, it is mainly decentralized development and utilization (accounting for 90% of the exploitation). Therefore, the characteristics of decentralized development and utilization of groundwater and its water supply capacity can not be ignored in the evaluation of groundwater exploitation resources.

Technical and socio-economic constraints

According to GB15218-94 classification standard of groundwater resources, some types are difficult to mine, but they can be mined through technological progress. For example, deep groundwater can be mined through the transformation of deep well springs; Another example is groundwater with poor water quality, which can be used for different purposes through advanced treatment. However, the use of these technologies should take into account that it is economical and reasonable, and it will not cause social problems that users can't bear because of the high water price. Therefore, technical and socio-economic factors must be considered when exploiting groundwater resources.

4.2.2.2.1.4 Influence factors of groundwater quality on groundwater development and utilization.

Water quality evaluation is an important part of groundwater resources evaluation and an important basis for groundwater exploitation resources evaluation. Therefore, we must consider the composition of water quality, especially the water quality problems that can cause water supply safety and environmental impact.

4.2.2.2.2 Evaluation method of groundwater exploitation resources

4.2.2.2.2. 1 Calculation method of groundwater exploitation resources in key areas

In representative key areas, data series of more than ten years can be used, and the constraints of environmental factors (groundwater ecological water level, etc.) can be considered. ), the numerical model of groundwater flow can be used to calculate the exploitation resources.

4.2.2.2.2.2 Calculation method of groundwater exploitation resources in general areas

(1) In plain areas with good aquifer exploitation conditions and high exploitation degree, the recharge minus the consumption that cannot be seized is taken as the exploitation resource.

(2) In the water-rich area, under the condition of the completed selected mining scheme, the amount of mining resources calculated by the model.

(3) In the area with long-term observation data of groundwater dynamics, according to the Q-S curve analysis, using the Q-S observation data, a linear equation set of one variable and a linear equation set of two variables are established to determine the amount of groundwater exploitation resources under different water level drawdown.

(4) In areas with a long history of mining observation, the amount of mining in the stable period of groundwater level fluctuation is taken as the amount of mining resources.

(5) The amount of exploitation resources obtained through group well pumping test or single well long-term pumping test.

(6) In areas with poor research degree and little data, the current exploitation amount and planned water exploitation amount are used for approximate calculation.

(7) Using the mining coefficient obtained in the representative area, and similarly using the similar area to calculate the amount of mining resources.

(8) The evaluation of groundwater exploitation resources in intermountain basin is mainly aimed at local water supply sources. Experimental mining method can be used to determine mining resources; You can also reduce the depth according to a certain design, and use the influence radius method to calculate the cross-section flow as mining resources; According to the mined data, the mining modulus analogy method can also be used to calculate the mining resources.

Evaluation of recoverable reserves of deep confined water in 4.2.2.3

The composition of deep confined water includes small recharge, elastic release, compression release of weak permeable layer and overflow recharge. Due to the great differences in groundwater exploration, research, development and utilization, some theoretical and practical problems still need to be solved. Therefore, the recoverable reserves of deep confined water with different accuracy should be provided according to local conditions, and they should be continuously revised and improved in the future investigation and development and utilization practice.

(1) The assessment shall include all deep freshwater confined aquifers exposed within the exploration depth (drilling and geophysical exploration).

(2) In areas with high level of research and experimental data on unsteady flow pumping, it is required to calculate the volume and storage capacity, lateral recharge, elastic release, compression release and overflow of each deep confined aquifer item by item.

(3) For areas with high degree of research and development and long-term observation data, it is necessary to establish a model to calculate the recoverable reserves of deep confined water.

(4) For areas lacking experimental data of unsteady flow pumping, the recoverable reserves of deep confined water can be calculated by the average well arrangement method according to the unit water inflow of drilling holes. However, when determining well spacing and well density, we must pay attention to hydrogeological conditions and local water demand.

(5) Environmental constraints should be considered in the evaluation of recoverable reserves of deep confined water. According to the experience at home and abroad, land subsidence is generally taken as the constraint condition, that is, the annual land subsidence and the total land subsidence are taken as the constraint conditions of allowable head drop, and then the recoverable reserves are calculated according to the allowable head drop. Because of the differences of hydrogeological conditions in different places, it is unscientific to determine a unified standard of land subsidence. In principle, all localities are required to determine according to the actual situation.