Changes of groundwater recharge and discharge

I. River-groundwater mutual transformation

Restricted by topography, geomorphology and structural conditions, groundwater and river water are regularly transformed from mountainous areas formed by runoff in the south to dissipative areas in the north. In mountainous areas, precipitation (rain, snow water, glacier melt water) moves from the top of the mountain to the side of the mountain, and most of it gathers in the valley and becomes river water. When entering the southern basin, a large amount of river water leaks into groundwater in the alluvial fan belt, and then overflows with spring water in the fine soil plain area at the fan edge, forming river water (spring-based river). The river flows into the northern basin through the valley between the upstream and downstream basins, and is replenished into groundwater in the downstream alluvial fan until it completely evaporates in the shallow buried area of the tail water level, forming a complete water circulation process (Figure 4-2).

Figure 4-2 Schematic Diagram of Hydrogeological Profile of Water Resources Transformation in Shule River Basin

The working area is located in the transition zone from Qilian Mountain to Beishan Desert. Qilian Mountain has abundant precipitation and dense hydrological network. Glacier meltwater collects underground runoff provided by precipitation, forming a perennial river. When passing through small mountain basins such as Changma and Shibaocheng, part of the surface water is converted into groundwater and then flows into the mountain pass. The groundwater of Shule River system flowing into the river from the northern slope of Qilian Mountain is 584.6 million m3/a (Table 4-4), accounting for 36.9% of the total runoff, and the groundwater of Yulin River accounts for 96% (Mushroom Platform). Therefore, no matter how complicated the transformation between groundwater and surface water is, the general feature is that rivers discharge groundwater. The piedmont zone is generally an aquifuge formed by Cenozoic red beds, which makes the amount of water entering the corridor in the form of underground runoff extremely limited.

Table 4-4 Runoff of Mountain Rivers and Their Groups in Shule River Basin (average for many years)

When entering the southern basin, because the river water level is higher than the groundwater level 10 ~ 200 m, the riverbed and the lower strata are highly permeable, and a large amount of river water seeps down to replenish groundwater. The leakage of Changma River in 42km Gobi is 84.6%, and Yulin River is exhausted in Gobi because of its small flow. Since the development of river basin water conservancy construction in the late 1950s, part of the river water has been transported to the irrigation area through the canal system, and the river water infiltration in alluvial fan areas has decreased year by year. In 1950s, the leakage of rivers and canals was large, accounting for 76.42% of the river outflow. After 1958, the Changma main canal was built, which increased the water diversion of the canal system and reduced the water inflow. 60' s to 80' s is a watershed development period. With the increase of irrigation area, the infiltration rate of canal system increased from 12% to 16.28%, and the total infiltration rate also increased. Later, due to the improvement of the utilization rate of the canal system, the infiltration rate of 1992 decreased by 23,730m3/a compared with the 1950s.

Table 4-5 River Infiltration in Changma Diluvial Fan Area

When the river flows to the front of the alluvial fan, the terrain slows down, the medium becomes thinner, the basement rises, the groundwater runoff is blocked, and the river goes deep into the aquifer, so that the groundwater is transformed into surface water in the form of spring water, which becomes the main component of runoff flowing into the downstream basin. After diversion, the groundwater of Jinchi River accounts for 93. 12% of the runoff, and that of Shule River accounts for 64.24%. With the improvement of the utilization rate of water resources in the middle reaches, the absolute and relative quantities of spring water in the downstream rivers are also increasing.

Table 4-6 Annual Variation Table of Spring Water Quantity in Panjiazhuang (unit: m3/s)

In July, 2004, the amount of spring water in the fan-edge area of Yumen-Zhen Shi Basin was measured, and the amount of groundwater converted into spring water reached 654.38+56.8 million m3/a, of which the overflow amount of spring water in the fan-edge area was 654.38+0.02/kloc-0.00 billion m3/a, and the overflow amount in the riverbed area was 54.7 million m3/a. This area is not only spring water. Infiltration methods are mainly canal irrigation spring water and field leakage.

These rivers, which are dominated by groundwater, enter the downstream basin through Chijinxia and Shuangbatou for the second surface-groundwater circulation. Due to the construction of Shuangta and Jinchi Reservoir, the main source of groundwater recharge in the downstream has changed from river infiltration in natural state to irrigation infiltration. With the improvement of water resources utilization rate, the groundwater recharge is decreasing. Under the present conditions, the river water entering the downstream basin has been completely controlled by human beings, and most of the river water has been brought into the canal system for irrigation. The process of two-water conversion has become an "irrigation water-groundwater" model.

According to the calculation in 2004, the amount of river water entering the downstream basin is 679.8 million m3/a, and the amount converted into groundwater is 327.8 million m3/a, accounting for 48.22% of the inflow and 76.62% of the total recharge of the downstream basin (Table 4-7).

Table 4-7 River Water Transformation Scale in the Lower Reaches of Shule River

Groundwater in the downstream basin is mainly transformed by river water leakage, and continues to migrate to the tail water area of the river along the river flow direction and topographic slope. With the increase of discharge, the hydraulic conductivity of aquifer becomes worse and worse, the particles become finer and finer, the hydraulic gradient becomes slower and slower, and the water level becomes shallower and shallower. In the hinterland of the lake plain, the horizontal movement of groundwater has basically stopped and the vertical change tends to be strong. The discharge of groundwater depends entirely on the potential evaporation of a large area except sporadic individual springs.

According to the calculation in 2004, the evaporation in the downstream basin is 413.6 million m3/a, and the total groundwater discharge in the downstream basin is 505.4 million m3/a, accounting for 8 1.84% of the total discharge.

Due to the construction of Changma Reservoir and canal system project, the infiltration of river water in the front of Changma alluvial fan decreased sharply, and the water level dropped, resulting in a decreasing trend of spring water in the downstream river (Table 4-8). According to the data of Panjiazhuang Hydrological Station, the spring water in front of the alluvial fan in Changma changed from groundwater to surface water (spring water), and the amount of water flowing into the downstream decreased from 265.43.8+0.3 million m3/a in 1960s to 83 million m3/a in 2004, which decreased by10.3 million m3/a. After the joint water transfer of Changma, Shuangta and Chijinxia Reservoir, Shule. In 2004, Panjiazhuang Hydrological Station cancelled the upstream water regulation, and the runoff and spring water decreased to 65,438 0.37 and 83 million m3/a, which were 47% and 39% in the 1960s and 64% and 69% in the 1990s respectively.

Table 4-8 Annual Variation Scale of Runoff and Spring in Panjiazhuang

Second, the change of groundwater recharge

The spatial and temporal distribution of water resources in Shule River Basin is uneven. In recent decades, with the enhancement of human activities, the artificial oasis has expanded, the natural oasis has shrunk, and the water demand has increased. The groundwater recharge in different periods has been decreasing, from13.352 million m3 in the 1950s to the current10.044 million m3, with a reduction rate of 32.94% (Table 4-9). With the construction of mountain reservoirs in the basin and the expansion of water conservancy projects such as high seepage prevention diversion channels, the transformation relationship between surface water and groundwater in plain areas has changed obviously, and the strong recharge zone in alluvial fan areas has lost a large number of recharge sources, the recharge conditions and renewal ability of groundwater have weakened, and the recharge volume has dropped sharply. Changma Irrigation District in the middle reaches of the Yangtze River draws a lot of water resources. From the 1950s to the early 20th century, the groundwater recharge increased slightly. The groundwater recharge in the downstream Anxi-Dunhuang basin has been decreasing, and it has increased slightly after the joint water transfer of reservoirs. Groundwater recharge in Huahai Basin increases with the increase of water regulation. This profoundly reflects the strong influence of human engineering on river-groundwater system.

Table 4-9 Variation Table of Groundwater Recharge in the Middle and Lower Reaches of Shule River Basin (unit: 100 million m3)

Groundwater recharge in Shule River basin is mainly composed of river water infiltration, canal system infiltration and field irrigation infiltration, and its infiltration determines the response change of groundwater recharge resources and its reasons.

(a) Changes in river water infiltration

The Shule River enters the Yumen-Zhen Shi Basin in the middle reaches after coming out of the mountain. There is a large amount of groundwater leakage in the alluvial fan Gobi zone of Changma, which is one of the main sources of groundwater resources. In recent 50 years, in order to meet the demand of industrial and agricultural water, surface water resources have been artificially adjusted, controlled and redistributed in time and space. A large number of rivers are introduced into canal irrigation, and the leakage in the Gobi area of Shule River decreased from 572.9 million m3 in 1950s to 250.6 million m3 in 2004. Especially at the end of 2002, the completion of Changma Reservoir controlled water resources, and the joint water transfer of Changma, Shuangta and Jinchi Reservoir further reduced the recharge of groundwater resources in front of the mountain. In 2004, the leakage of discharged water only accounted for 25.25% of the runoff.

Table 4- 10 Leakage Table of Changla Diluvial Fan in Shule River, Yumen-Zhen Shi Basin

Due to the construction of Shuangta Reservoir, Danghe Reservoir and Chijinxia Reservoir in the 1960s and 1970s, the river water entering the downstream basins of Anxi-Dunhuang and Huahai was completely controlled and regulated by manpower, and most of the river water was introduced into the field for irrigation. The upstream inflow determines the leakage of riverbed and alluvial fan. According to the calculation of infiltration in 1977, 1999 and 2004, it can be seen that the infiltration in the downstream alluvial fan area has little change (Table 4- 1 1).

Table 4- 1 1 Leakage table of alluvial fans in the lower reaches of major rivers in Shule River Basin (unit: 100 million m3)

(B) changes in channel penetration

Agricultural irrigation is carried out after a large number of rivers in Shule River basin are introduced into the canal system, and canal system leakage is one of the important recharge projects of groundwater resources. With the improvement of water conservancy projects and supporting projects in Shule River Basin, the infiltration of canal system generally shows a decreasing trend (Table 4- 12). From 65438 to 0958, Changma Dam and main canal were built until the end of 1970s. Except for the main canal system, most of them are earth canals, with low utilization rate of 0.46 ~ 0.72, and the Shuangta irrigation area and Danghe irrigation area are 0.53 and 0.46 respectively, with large leakage. From 1980s to the middle and late 1990s, main canal system and branch canal system were built in irrigation area, which improved the utilization rate of canal system and greatly reduced the water seepage of canal system compared with 1970s. After the implementation of the "Shule River Basin Comprehensive Development Project", the Changma main canal, Gan Xi, Shuangta north-south main canal, Huaxi main canal and branch canal in the irrigation area were rebuilt and expanded. Combined water transfer from upstream and downstream reservoirs, land expansion, water intake from various channels and infiltration in the three major river basins have increased, but they are still lower than those in the 1970s.

Table 4- 12 Table of Leakage Changes of Drainage System in Shule River Basin (unit: 100 million m3)

(3) Infiltration change of field irrigation

In the 1970s, although there was little arable land, water resources were relatively abundant, flooding was serious, and field infiltration was large. In the late 1990 s, the cultivated land area in the basin increased, the water intake increased accordingly, and the awareness of water saving increased. Infiltration of field irrigation in Changma irrigation area in the middle reaches increased slightly, while it decreased in the lower reaches. With the implementation of the "Shule River Basin Comprehensive Development Project", the water transfer in the downstream irrigation area has increased correspondingly, the water consumption in Changma irrigation area has decreased relatively, and the infiltration has also decreased; Downstream Shuangta and Huahai irrigation areas have expanded cultivated land and increased water diversion, and Danghe irrigation area has also expanded cultivated land and increased infiltration. Generally speaking, the infiltration decreased slightly (Table 4- 13).

Table 4- 13 Table of Field Irrigation Infiltration Change in Shule River Basin (unit: 100 million m3)

Three, the main discharge of groundwater changes

Variation of evapotranspiration

Evapotranspiration is the largest groundwater discharge term in each basin, and its change indirectly reflects the dynamic change of regional groundwater level. According to the evapotranspiration calculated in different periods, it can be seen that there has been a decreasing trend since 1970s, which is consistent with the overall decline of regional groundwater level (Table 4- 14).

Table 4- 14 Table of Changes of Evapotranspiration in Shule River Basin (unit: 100 million m3)

(B) changes in the amount of artificial exploitation

The artificial exploitation of groundwater in Shule River basin is mainly concentrated in plain oasis agricultural areas, and most of them are agricultural irrigation wells. With the increase of population and land area in Shule River area, especially the implementation of the "Shule River Basin Comprehensive Development Project", immigrants moved to the project area, the land development area increased rapidly, the water consumption increased, and the groundwater exploitation doubled (Table 4- 15), which has now become one of the main groundwater discharge projects in this area. In recent years, due to the increase of water transfer and the decrease of pure well irrigation area, the exploitation of Huahai River Basin has decreased. Groundwater exploitation in Danghe irrigation area is over-exploited, and groundwater exploitation is restricted.

Table 4- 15 Change Table of Artificial Mining in Shule River Basin (unit: 100 million m3)

Fourth, the change of spring water quantity.

In recent 50 years, the amount of spring water in Yumen-Zhen Shi basin in the middle reaches of Shule River basin has been decreasing continuously. In the 1960s, the spring overflow in the alluvial fan belt of Changma was 335 million m3/a, which decreased to 253 million m3/a in the 1970s and reached 65.438+56.8 million m3/a in 2004, which was 53.2% lower than that in the 1960s (Table 4-65.438+06).

Table 4- 16 Annual Spring Overflow Table of Changma Alluvial Fan

The change of spring overflow is the inevitable response change of output system caused by the change of input system of groundwater system. The recharge of groundwater in the upper reaches of Yumen-Zhen Shi Basin continues to decrease, the groundwater circulation weakens, and the regional groundwater level drops, which becomes the main factor for the attenuation of spring flow. On the other hand, the exploitation of groundwater accelerates the decline of water level in the spring overflow zone, and also plays a certain role in the attenuation of spring water.