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.
karst cavecave temperaturetemperature difference inside and outside the cavespatial discrepancytemperature anomaly of the warm season
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