The results of 14C isotope studies of groundwater show that groundwater in the Heihe River Basin was mainly formed during the periods of 8000-6000 years before present, 3000-2000 years before present, and the corresponding rainy and warm periods in the last 50 years. The isotope data indicate that the changes in groundwater formation, recharge and renewal capacity in the study area are closely related to the changes in regional water cycle conditions.
I. Groundwater formation and evolution process
Analysis of the statistical frequency distribution pattern of 14C content of 42 water samples in the study area shows (Fig. 3-19) that samples with different 14C contents occur with different frequencies, and since the distribution of the sample points covers most of the area of the study area, the influence of the location of the sampling points on the age distribution is ruled out, and therefore the period with a larger frequency number is characterized as the period of major groundwater recharge, and the period with smaller frequency number is characterized as the period of weaker groundwater recharge. According to the statistical results, the high frequency of groundwater 14C content in the aquifer mainly occurs in the ranges of 25-30 pmc, 45-50 pmc, 60-75 pmc, and 90-95 pmc, and the corresponding uncorrected ages are 8,000-6,000, 3,000-2,000, and 7,200-6,000 years before present.
The climate record of the Qilian Mountain ice core also confirms that the study area was in a stable warm phase during the period of 7,200-6,000 years before present. During this period, the study area was in a stable warm phase (Shi Yafeng, 1995), with abundant precipitation, uninterrupted surface runoff, expanding lakes in the downstream area, and adequate recharge of the groundwater system. In the Ejina Basin, not only the deep groundwater has low mineralization and good water quality, but also the 14C age of the groundwater is mostly during the period of 8,000 to 5,000 years before present, which is the main recharge period.
Figure 3-19 Statistical frequency distribution of 14C content of groundwater samples in the Heihe River Basin at 5 pmc intervals
In the Dundee Bingxin records, the warmest period in the past 3000 years was around 800 B.C. Both the Zuozhuan and the I Ching record the fact that subtropical plants such as bamboo and plum moved northward to the temperate zone during this period. Therefore, this period would have been one of the major groundwater recharge periods on a centennial scale. As mentioned earlier, the periods of 1495-1557, 1652-1772, 1850-1890 and 1919-1939 had more precipitation (Shi Yafeng, 1995), which played a role in groundwater recharge and renewal in the plain area of the Heihe River Basin, especially in the renewal of groundwater in the strong infiltration zones of the basins.
From the frequency distribution of 14C content of groundwater samples in Fig. 3-19 and the corresponding relationship of climate change in Fig. 3-2, it can be seen that the stage characteristics of climate change determines that the evolution of groundwater formation, circulation and recharge is characterized by stages.
Relative to the periods of 1428-1532, 1622-1740 and 1797-1865, the total water resources of the modern Heihe River basin decreased by at least about 20%, the river runoff decreased by 14.6%, and the groundwater recharge decreased by (4.38-7.61) × 108 m3/a, of which the ratio of precipitation and glacial meltwater decreased by 1.2% and 2.0%, respectively. The ratio of groundwater (base flow) recharge in mountainous areas increased by 3.2%.
II. Changes in groundwater recharge and renewal capacity
Since groundwater recharge and renewal are characterized by phases, the isotopic information in the groundwater system retains the pulse-like pattern of the recharge process in the geologic history period, which is a reflection of the intensity of groundwater recharge.
Allison et al. (1984) found that the intensity of groundwater recharge is inversely proportional to the square of the deuterium deviation from the atmospheric precipitation line. Using the relationship given by Allison et al. (1984), the evolution of groundwater recharge intensity in the plain area of the Black River Basin is obtained based on the deuterium value of groundwater (Figure 3-20).Allison et al. (1984) have pointed out that the proportionality constant may be a function of soil hydraulic properties, so the absolute value of calculated recharge intensity merely reflects the relative trend of recharge intensity.
The three peak recharge periods reflected in Figure 3-20 correspond to the groundwater recharge periods, indicating that the groundwater evolution in the Black River Basin Plain is closely related to paleoclimatic and paleohydrologic changes, and corresponds to the various rainy and warm climatic periods during the geologic period.
Third, the quantitative mechanism of climate change on the impact of the water cycle in the Heihe River Basin
The study of the sensitivity of the Heihe River runoff out of the mountains to climate change shows that, when the temperature rises by 0.5 ℃, under the condition of the precipitation is unchanged, because of the melting of snow, the runoff increases in May and October, but in July to August due to the increase in the amount of evaporation and the watershed glacier recharge proportionally smaller, so that the annual runoff is reduced by 4%; when the temperature increases by 1 ℃, precipitation is unchanged, except for May to June, the annual runoff is reduced by 4%. precipitation remains unchanged, the annual runoff will decrease by 7.1%, except for an increase in runoff from May to June, and a larger decrease in runoff from July to August; when the temperature increases by 0.5°C and precipitation increases by 10%, the annual runoff will increase by only 1.6% (Kang Ersi, 1999).
Figure 3-20 Trend of recharge intensity in the Heihe River Basin according to the model
In the recharge source area (mountainous area), precipitation change is the main factor to change the total water resources in the basin, which accounts for about 91% of the weighting; temperature change is an important influencing factor, which accounts for about 9% of the weighting. A 1% change in precipitation increases or decreases runoff by 1.25%; a 1°C change in temperature increases or decreases runoff by 3.28% to 3.9%. The increase in temperature realizes the reduction of total water resources in the basin by decreasing the glacier area and increasing the land surface evaporation (Table 3-16 and Figure 3-21).
Table 3-16 Relationship between groundwater recharge, surface runoff, and changes in temperature and precipitation in the plains area of the Heihe River Basin
The study showed that under the condition of 0.5℃ increase in average annual temperature, both surface runoff and groundwater recharge in the plains area of the Heihe River Basin increased, by 9.16% and 7.38%, respectively, and that under the condition of 1.0℃ increase in temperature, surface runoff and groundwater recharge decreased by 3.33% and 2.68%, respectively. Under the condition of 0.5℃ decrease in mean annual temperature and 10% increase in precipitation, surface runoff increases by 4.39% and groundwater recharge in the plain area increases by 3.54%. If the average annual temperature is stable and unchanged and precipitation increases by 10% or 20%, then surface runoff increases by 4.56% and 13.45%, respectively, and groundwater recharge in the plains area increases by 3.61% and 10.38%, respectively (Table 3-16).
Considering the 100-year scale, the main hydrological characteristic values of the water cycle in the Black River basin are shown in Table 3-17. In dry water years, snow and ice melt water recharge to surface runoff decreases by 10.9%, surface runoff decreases by 25.4%, and groundwater recharge decreases by 20.5%; in partial dry water years, snow and ice melt water recharge to surface runoff decreases by 4.5%, surface runoff decreases by 7.9%, and groundwater recharge decreases by 6.4%; and in partial abundant water years, snow and ice melt water recharge increases by 2.1%, surface runoff increases by 12.3%, and groundwater recharge increased by 9.9%; in the abundant water year, snow and ice melt water recharge increased by 13.6%, surface runoff increased by 34.6%, and groundwater recharge increased by 27.8%.
Figure 3-21 Relationship between annual average ice and snowmelt water changes and temperature and precipitation in the Heihe River Basin
Table 3-17 Characteristic values of water cycle evolution in the Heihe River Basin
The results of the study by Zuming Lai (1997) showed that the total runoff volume in the next 25 years would be reduced by about 4% compared with the average of the past 34 years. Ding Yihui, in "Predicting Environmental Changes in Western China," suggested that runoff will decrease if future temperatures increase by 2°C and precipitation increases by only a few percentage points.
The Blue Book of Chinese Science and Technology No. 5 (1990) pointed out that "from the millennium scale, the northwest region is in a dry and cold period, and from the centennial scale it is in a dry and warm period", and that "the fluctuating phenomenon of dry and wet will occur in this region in the next 20-60 years, and from the end of the 20th century to the beginning of the 21st century There may be a slight increase in precipitation in the late 20th and early 21st centuries", but "it will not be enough to change the general trend of aridification in the northwest region".
Shi Yafeng et al. (1996) argued that by about 2030, the temperature in the high mountainous areas of northwest China will rise by 1±0.5℃ compared with that in the 1980s, and the precipitation will increase appropriately. And pointed out that, even if the precipitation increased by 20%, the glacier recharge is still not enough to increase the temperature by 1 ℃ intensified by the ablation. Lai Zuming et al. (1991) on the Urumqi River study results show that when the temperature increases by 2 ℃, precipitation increased by 10%, 20%, glacier meltwater runoff decreased by 55.1% and 31.6%, while the basin of the river runoff increased by 10% and 13.7%.
Li Dongliang et al. (2003b) used harmonic and wavelet analysis methods to predict the precipitation in central Gansu, which showed that the annual precipitation increased significantly before and after 2005 (Figure 3-22).
Figure 3-22 Trend of precipitation change in central Gansu
Shi Yafeng et al. (1995) proved through the results of NCARGCM study that the annual precipitation will increase by 100-200 mm by 2050 in the northwestern region of China, except for Shaanxi and the eastern part of Gansu.Kang Ersi et al. (2003b), based on the HadCM2Gsa climate model, predicted the annual precipitation for the regions of 93.75° to 105°E and 37.5°-42.5°N (96.42°-102.04°E and 39.45°-42.40°N for the Heihe River Basin) area temperature and precipitation simulation results were corrected, and obtained the Heihe River dry runoff prediction information, the results are shown in Table 3-18.
Table 3-18 Characteristics of changes in precipitation, air temperature, surface runoff and groundwater recharge in the Heihe River Basin in the next 50 years
The water resources in the plains of the Heihe River Basin are formed, including groundwater, and they have correlation coefficients of 0.82 with surface runoff in the Qilian Mountain area, 0.51 with precipitation at Qilian Station in the mountainous area, and 0.38 with precipitation at Zhangye Station in the plains area, gray. Correlation analysis results, the correlation between water resources recharge in the plains and precipitation at Qilian station in the mountainous area is 0.97, the correlation with air temperature at Qilian station is 0.79, the correlation with precipitation at Zhangye station in the plains area is 0.43, and the correlation with air temperature at Zhangye station is 0.60. Thus, it can be seen that the total water resources and groundwater recharge in the plains area of the Heihe River Basin are the closest to the Qilian mountainous area, and the changes of temperature in mountainous areas and plains play an important role in influencing the total water resources recharge in the plains area and groundwater recharge. The temperature changes in the mountains and plains have an important influence.
In short, the change of surface runoff in the mountainous areas upstream of the Heihe River Basin in the next 50 years will also cause the change of groundwater recharge from rivers, rainwater floods and seepage of drainage systems in the southern basin (plains), which will lead to an increase in groundwater recharge in the first 30 years of the future, and a decreasing trend in the last 20 years.