Vulnerability assessment of karst groundwater in the Jinci spring area and the zoning of protected areas for water quality
Jinci spring is a famous karst spring in Shanxi Province, and its backflow is an ecological restoration goal of the Shanxi Provincial government. Composed of Nanlao spring, Shengmu spring, and Shanli spring, Jinci spring is exposed at the foot of Xuanweng mountain in the western mountains of Taiyuan, twenty-five kilometers away from the city center, and it is a concentrated discharge point for karst water in the Jinci spring area. Karst water in the spring area is mainly supplied by atmospheric precipitation infiltration and leakage from rivers and reservoirs. Karst water in the Jinci spring area is an important water source for people's life and industrial and agricultural production in Taiyuan City. In recent years, with the intensification of climate change, large-scale development of karst water, coal mining and other human activities, especially after the construction of the second reservoir of the Fenhe river, the karst hydrogeological conditions in the spring area have undergone fundamental changes, with the spring area expanding from 2,030 km2 to 2,713 km2, which has affected the evaluation, management and protection of karst water resources in the spring area. The original zoning of protected areas can no longer meet the needs of water resource management and protection. Therefore, it is urgent to re-zone the protected areas and formulate corresponding protection measures. In terms of the problems on water quality and quantity, there exist differences in the causes, regions, and protective measures, and thus LIANG Yongping et al. proposed the concepts of "water quantity vulnerability" and "water quality vulnerability", as well as a method of "first classifying, and then grading" for the zoning of karst aquifer protected areas.This study focuses on the hydrogeological conditions of special recharge, runoff, and discharge in the karst water system of North China, as well as the main influencing factors of the antifouling performance of karst aquifer systems. It combines European methods and "water vulnerability assessment" to evaluate the antifouling performance. Based on infiltration duration of karst water in the unsaturated zone, leakage of rivers and reservoirs, precipitation infiltration in different regions, and replacement of karst aquifer media structure with water abundance, this study selects four factors to evaluate the antifouling performance of karst aquifers in North China, including the thickness of unsaturated zone, the amount of infiltration recharge, the lithology of overlying strata, and the water abundance of karst aquifers. Taking the newly divided karst water system in the Jinci spring area as the evaluation object, this study aims to evaluate the vulnerability of karst water system in the spring area and to zone the protected areas.The results show as follows, (1) The protected areas are mainly distributed in Jinci park and its surrounding areas. (2) The first-class protected areas are mainly distributed in the main stream of the Fenhe river, the exposed areas of carbonate rocks in the front of western mountains of Taiyuan, and the leakage sections of carbonate rocks in Tunlanchuan, the Tianchi river and the Liulin river. (3) The second-class protected areas are mainly distributed in the north of the spring area and the north bank of the Fenhe river. (4) The quasi-protected areas are mainly distributed in the south bank of the Fenhe river and the Taiyuan basin in the southeast. The newly zoned protected areas for spring water quality can provide a basis for the rational protection and scientific utilization of karst water resources in the Jinci spring area.
Daily dynamics effects of microclimate of two underlying surfaces in rocky desertification areas
As an ecologically vulnerable area with the strongest karst development in the world, the karst area in Southwest China exceeds 540,000 km2. Long-term human activities and frequent extreme climate have accelerated soil erosion, causing large areas of bedrock to be exposed on the surface; consequently, a rocky desert landscape with severely degraded vegetation came into being. Under the influence of the hot and humid monsoon climate, carbonate rocks underwent dissolution, which formed various heterogeneous underlying surfaces such as earth flatland and stone surface. The local microclimate differences caused by different underlying surfaces play an important role in regional vegetation restoration and ecosystem reconstruction. Current research on environmental factors in rocky desertification areas mainly focuses on water, soil and other aspects, while there is still a lack of research on microclimate of underlying surfaces. Taking two typical underlying surfaces (earth flatland and bare land) as the research objects, this study aims to explore the microclimate effects of heterogeneous underlying surfaces in rocky desertification areas. A long-term quantitative comparison in daily dynamics of near-surface temperature and humidity of these two underlying surfaces were conducted through simulation experiments. In rocky desertification areas, earth flatland is composed of polygonal rock masses exposed on the surface and patches of soil distributed inside, while bare land is normal land with no exposed rocks and no vegetation coverage. This study area is located in Puding county, Anshun City, Guizhou Province, where karst is strongly developed. The karst landform accounts for 84.27% of the county area with 60.55% of rocky desertification. The county has humid monsoon climate on the north subtropical plateau, with an annual average temperature of 15.1 °C and an annual rainfall of 1,378.2 mm. The annual total solar radiation fluctuates from 85.71 to 458.81 MJ·m−2. Preliminary field surveys found that there were large parameter variations in rock mass shape and size, and orientation of earth flatland. In order to improve the reliability and accuracy of the observation results, this study adopted in-situ limestone and concrete pouring technology to conduct simulation construction based on the average parameters of 30 earth flatlands that have been investigated. There were three replicates for each of the two underlying surfaces. To carry out long-term monitoring of temperature and relative humidity, high-resolution iButton DS1923 temperature and humidity recorders were installed at different heights (2 cm, 40 cm and 80 cm) above the surface of the two underlying surfaces. All data analyses were performed in the R version 4.2.3. The functions of tapply and bartlett.test/var.test were used to test data normality and homogeneity of variances. If the data passed the test, one-way ANOVA would be used to conduct multiple comparisons of air temperature and humidity at different spatial heights. An independent sample t-test was used to compare the air temperature and humidity at the same spatial height on earth flatland and bare land. If the test failed, multiple sets of data would be applied for non-parametric testing and multiple comparisons through the kruskal.test function and PMCMRplus package. These two groups of data were applied for the wilcoxon rank sum test by the wilcox.test function. Daily dynamics of microclimate of bare land and earth flatland obviously differed on the spatial and temporal scales. When solar radiation was the strongest in summer, temperature at each spatial height of earth flatland was significantly higher than that of bare land (P<0.05), and the relative humidity was significantly lower than that of bare land (P<0.05), which led to warming and dehumidifying conditions. However, air temperature in the bottom space of earth flatland was significantly lower than that in bare land (P<0.05), and the relative humidity was significantly higher than that of bare land (P<0.05) from 10:00 to 16:00 in winter, which resulted in cooling and humidifying effects. The daily variation degree of microclimate in bare land and earth flatland was significantly different in response to different seasons. The daily temperature range of the upper layer of earth flatland was significantly higher than that of bare land (P<0.05) in summer, indicating that the earth flatland intensified the daily change of air temperature. However, the daily temperature and humidity range of the lower layer of the earth flatland was significantly lower than that of the bare land (P<0.05) in winter, showing that the earth flatland buffered the daily change of the microclimate. At present, bedrock-exposed areas of rocky desertification are under a more severe and changeable microclimate change background. Therefore, based on the differences in the microclimate effects of heterogeneous underlying surfaces, priority should be given to introduce early fast-growing karst plants to accelerate surface vegetation coverage and improve local microclimate conditions, which would play a positive role in buffering microclimate changes in rocky desertification areas and accelerating vegetation restoration.
Variation characteristics of stable hydrogen and oxygen isotopes in precipitation of Longtan trough valley and Maoba syncline in Youyang, Chongqing
This study aims to explore the characteristics and influencing factors of δ18O and δD in precipitation in two adjacent topographic areas of Longtan trough valley and Maoba syncline in Wuling mountain, southeastern Chongqing. Based on the sampling data about δ18O and δD in precipitation of Maoba syncline (altitude of 1,140 m) and Longtan trough valley (altitude of 330 m) in Youyang county of Chongqing in two hydrological years from June 2020 to May 2022, the meteoric water lines of the two studies areas were established, and the variation characteristics and influencing factors of hydrogen and oxygen stable isotopes in precipitation in these areas were also analyzed. The results show as follows. (1) The slopes and intercepts of meteoric water lines in Longtan trough valley and Maoba syncline exceeded those of the global meteoric water lines. The values of δ18O in precipitation in these two study areas show a significant effect of precipitation, while the influence of temperature effect is relatively weak. (2) The slope and intercept of the meteoric water line in Longtan trough valley were smaller than those in the Maoba syncline. The difference between the meteoric water lines in these two study areas was mainly controlled by the temperature and evaporation conditions during water vapor condensation. (3) The average values of δ18O and δD in precipitation in Longtan trough valley were more positive than those in Maoba syncline. The results of d-excess showed that there was a weak sub-cloud secondary evaporation of δ18O and δD in Longtan trough valley. The values of δ18O and δD in precipitation in Longtan trough valley and Maoba syncline exhibited significant seasonal changes, with negative values during the rainy season (May to October) and positive values during the dry season (November to the following April). (3) The values of δ18O in precipitation in Longtan trough valley and Maoba syncline showed a significant precipitation effect, while the influence of temperature effect is weak. During the same precipitation event, the values of δ18O in precipitation in Longtan trough valley were generally more positive than those in Maoba syncline, indicating that δ18O values in precipitation in Longtan trough valley and Maoba syncline were influenced by elevation effects. (4) The interannual variations of δ18O in precipitation in Longtan trough valley and Maoba syncline were mainly affected by the change of the proportion of water vapor transported from Indian Ocean. In the summer of 2020, the proportion of water vapor from the Indian Ocean to the study areas reached 76%, with negative δ18O in precipitation. However, the proportion decreased to 52% in the summer of 2021, with positive δ18O in precipitation. This study is of great significance for the understanding of the water cycle process in the karst trough valley areas of Southwest China and for future studies on the eco-hydrological process of Longtan trough valley and Maoba syncline.
Relationship between temperatures of karst caves and local average temperatures: Taking Pailong cave of Puzhehei, Yunnan as an example
Generally speaking, temperatures of karst caves are comparable to local annual average temperatures. However, the actual monitoring has found that the relationship between cave temperatures and local annual average temperatures varies in different regions. To understand the relationship between these two kinds of temperature, we monitored the temperatures of Pailong cave in Puzhehei, Yunnan Province, Southwest China. Our high-resolution monitoring discovered higher temperatures within the cave compared to the local annual average temperatures and the temperatures at the cave entrance. Temperatures at cave entrance were primarily affected by ventilation, while temperatures inside the cave were influenced by seasonal temperatures and precipitation. The monitoring revealed a stronger effect of ventilation in winter and a weaker one in summer at the cave entrance. Additionally, monthly temperatures exceeded local annual temperatures from April to October, and fell below them from November to March, indicating that longer and effective heat import and less heat loss led to higher temperatures inside the cave. Rainfall during the rainy season (May to October) formed a fast flow into the cave and transfers heat, further increasing cave temperatures. Thus, longer and effective heat import, less heat loss, and rainfall-induced heat import resulted in higher cave temperatures than local annual average temperatures.To validate our hypothesis, we collected 48 published data on cave temperatures from China and analyzed the temperature differences inside and outside the cave. Generally, the difference is positive in the north of the Yangtze River and negative in the south. It is positive in the east of longitude 110°E and negative in the west. Correlation analysis between cave temperatures and other influencing factors such as latitude, longitude, altitude, bedrock depth, and overlying vegetation index shows that there is a significant negative correlation between cave temperatures and factors of latitude, altitude, and overlying vegetation index. The same is true of the correlation between local annual average temperatures and these factors, suggesting that cave temperatures are primarily influenced by local annual average temperatures. Weak ventilation effects can be attributed to the large depth of caves, and thus resulting in a negative correlation between the temperature difference inside and outside the cave and the cave length. Interestingly, the temperature differences inside and outside the cave correlate positively with latitude and longitude. To explain this, we introduce the concept of temperatures in the warm season (April to October), when the monthly temperature minus the annual temperature is above zero. We found significantly positive correlations of temperatures in the warm season with latitude as well as with temperature difference inside and outside the cave, indicating a longer warm season and anomaly of higher temperatures in the warm season may contribute to the large temperature difference inside and outside the cave. Furthermore, the rainy season may also influence cave temperatures via heat that is generated by drip water and transported into the cave. Hence, the temperature difference inside and outside the cave is primarily influenced by heat distribution. For instance, temperatures in the warm season increase from Guilin of South China to Beijing of North China. Similarly, temperatures in the warm season also increase with longitude. For example, temperatures in the warm season increase from Wenshan of Southwest China to Guilin of South China. Although the overlying vegetation index negatively correlates with cave temperatures and local annual average temperatures, there is no significant correlation between the overlying vegetation index and the temperature difference inside and outside the cave, probably because the dense vegetation cover can reduce the heat transported to the cave and lower cave temperatures. However, the vegetation cover does not vary significantly with latitude. Additionally, differences in cave structure and environment also affect the temperature difference inside and outside the cave. For example, temperatures in the caves of Guilin are respectively equal to (e.g. Panlong cave), above (e.g. Shuinan cave), or below (e.g. Maomaotou Big cave) the local annual average temperatures. Thus, selecting appropriate caves will ensure the accuracy of statistical results in understanding the difference between cave temperatures and local annual average temperatures. Though some individual results may influence statistical results, the positive correlation between the temperature difference inside and outside the cave and latitude indicated by the 48 cave data will not change. Finally, the long-term monitoring in some closed cave systems at different latitudes can increase the precision of the results. Our study highlights the local seasonal heat distribution and heat transport in the cave as the primary factors that influence cave temperatures, and our study will contribute to a better understanding and protection of the karst cave environment.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
2024, 43(4): 766-779, 809.
doi: 10.11932/karst20240403
2024, 43(4): 810-821, 853.
doi: 10.11932/karst2024y023
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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