Characteristics and identification of change stages of groundwater flow field in overexploitation areas

1. Characteristics of the initial stage of groundwater overexploitation

Judging from the evolution characteristics of the groundwater flow field in the severely overexploited area of ??Shijiazhuang, before the 1970s, the amount of groundwater extraction in the area was small. The multi-year average mining volume in the entire Shijiazhuang Plain area is less than 1 billion m3/a, the groundwater recharge is greater than the mining volume, Re>1.0, the groundwater flow field direction is SEE (Figure 4-16a), and the groundwater level gradually increases from the plain to the mountainous area, as 30~80m, water level burial depth is 2~10m.

After the worst drought in the basin occurred in 1972, agricultural exploitation in the Shijiazhuang Plain increased significantly. In the early stages of groundwater overexploitation, the average annual exploitation amount reached more than 2 billion m3/a, resulting in years with Re <1.0. It happens that the annual recharge of groundwater is less than the annual extraction.

The iconic characteristics of the groundwater flow field changes in the serious overexploitation area in the Shijiazhuang Plain in the early stage of groundwater overexploitation are: the groundwater level drop funnel expanded significantly, the average groundwater level in the area dropped at a rate of 0.38m per year, and the center of the funnel The water level decreases at a rate of 1.01m per year, and the funnel area expands at a rate of 6.37km2 per year (Table 4-1).

II. Characteristics of the formation stage of groundwater funnels

During the period from 1981 to 1995, after long-term and large-scale over-exploitation, the shallow groundwater level in Shijiazhuang area dropped and the funnel was caused by the initial industrial mining. The local funnel evolved into a regional groundwater funnel dominated by agricultural exploitation (Figure 4-16 b). The annual extraction volume of groundwater reaches more than 2.5 billion m3, and the average over-exploitation volume for many years is more than 1 billion m3/a, Re? 1.0, that is, the groundwater recharge volume is long-term and far less than the extraction volume.

The identifying characteristics of the groundwater flow field variability in the serious overexploitation area of ??the Shijiazhuang Plain during the formation stage of the groundwater funnel are: the characteristic values ??of the groundwater flow field variability have increased significantly compared with the initial stage of overexploitation, and the average groundwater level has dropped by as much as 0.69m/a, the central water level in the funnel area decreases at an average annual rate of 1.35m/a, and the funnel area expands by an average of 11.39km2 every year, which are 0.31m/a, 0.24m/a and 4.98km2/a respectively compared with the initial over-exploitation period (Table 4-1).

3. Characteristics of the Severe Overexploitation Stage of Groundwater

A huge flood occurred in the Shijiazhuang area in August 1996. The shallow groundwater level in the Shijiazhuang Plain generally rose sharply. The groundwater along the Hutuo River and in the flood storage depressions The area rose by more than 5m, and the area with the largest increase reached 11.72m. Since 1998, frequent agricultural droughts have occurred in the Shijiazhuang area. In addition, the auxiliary dam of the upstream Huangbizhuang Reservoir has been strengthened to eliminate risks and prevent seepage. This has greatly reduced the directional inflow recharge in the area, causing the groundwater flow field to be seriously over-exploited, with an average annual over-exploitation of more than 1.5 billion m3/a, Re is still far less than 1.0, the water level in the center of the funnel exceeds 50m, reaching 53.65m in the maximum year, and the funnel area further expands (Figure 4-16c).

The defining characteristic of the groundwater flow field variation in the serious overexploitation area in the Shijiazhuang Plain during the formation stage of the groundwater funnel is: the groundwater level drops in the funnel. Driven by the increasing overexploitation, the regional average groundwater level further declines. increased to 1.03m/a, and the funnel area expansion rate further accelerated to 12.39km2/a (Table 4-1).

IV. Characteristics of the Strict Management Stage of Groundwater Confinement

Since 2005, while increasing the use of surface water, the Shijiazhuang area has implemented well closures and the most stringent water control measures. Under the resource management system, the downward trend of the water level in the center of the funnel has stabilized, and the funnel area expansion rate has been alleviated (Figure 4-16d). The water level in the center of the funnel gradually rises at a rate of 0.5m/a, and the area of ??the falling funnel gradually decreases at a rate of 8.20km2/a (Table 4-1).

Figure 4-16 Variation stage characteristics of groundwater flow field in the serious over-exploitation area in Shijiazhuang Plain from 1961 to 2010

a—Flow field in natural stage (1959); b—Funnel formation Stage flow field (1990); c—Flow field during severe overexploitation stage (2005); d—Flow field during pressure mining and strict management stage (2010)

Table 4-1 Groundwater in Shijiazhuang overexploitation area Evolution characteristics of funnel center water level, area and regional average water level

Continued table

Note: The evolution rate is the change rate of groundwater flow field variability identification indicators in each evolution stage.

The evolution amplitude is the change amplitude of the groundwater flow field variability indicator in each evolution stage relative to the initial year of the stage. The data from 1968 to 1974 in the table comes from the "Hebei Province Groundwater Level Statistical Yearbook (Shijiazhuang City)"; the groundwater dynamic data from 1976 to 2010 comes from the "Shijiazhuang Plain Area Groundwater Environment Monitoring Report", in which the regional average water level is determined by the study area Multiple observation points are averaged, and the central water level and area of ??the funnel area are measured from groundwater level depth and elevation contour maps over the years. The unit of regional groundwater level and funnel center water level evolution rate is m/a, and the unit of funnel area evolution rate is km2/a.