华北地区水位下降是否会减缓气温上升——浅部地温影响的数值模拟分析

华北地区由于长期持续的地下水过量开采,导致了大面积地下水位大幅下降,引发地面塌陷、地下水质污染等一系列地质环境问题,这些现象早已为人们所熟知和关注.然而地下水位下降还会造成百米量级浅部地温及其梯度的变化,因此即使来自地球深部的大地热流密度没有变化,年度平均的从表浅部位通过地表实际传导进入大气的热流密度会减小,这是中外文献中尚未见讨论过的问题.我们通过数值模拟发现假定大地热流密度不变的条件下,华北数万平方公里地下水位下降会造成百米尺度内的地温降低,从而传入大气的热流密度降低40%以上,且会持续数百年以上的时间.这种长时间大范围的传导入大气的热流密度变化对环境会造成什么影响是一个十分值得关注的问题.这一预测在一定程度上得到了气象站地温观测数据的支持,但由于目前气象观测站只有3.2m深度范围内的地温资料,累计不超过5、60年,中间还有10余年的间断,而且表浅深度地温受地表多种因素的影响也较大,这些资料难以对我们关心的地下水位下降引起流入大气的热流密度变化这一问题提供直接确凿的数据来进行分析,因此今后有必要开展对地下数十乃至数百米地温进行持续精确的监测工作.

Persistent drawdown of groundwater table in North China may reduce local climate warming rate: Numerical simulation and analysis of the impacts on shallow ground temperature

Due to long-term and persistent groundwater over-exploitation, the water table has been found significantly declined across an area of approximately 70000 km² in North China. Consequently, a series of geo-environmental problems have emerged and drawn a large amount of public attention. These include land subsidence, groundwater contamination, etc. We found that decline of groundwater level can result in significant variation of temperature and its gradient within depths of hundreds of meters. Therefore, the annual average heat flux conducted from shallow ground to atmosphere can be reduced, even though there is no change in heat flux from the deep Earth. This effect has yet discussed neither in domestic nor in foreign studies. Our numerical simulation shows that assuming the heat flux from deep Earth is constant, the depression of the water table leads to reduction of ground temperature within depths of hundreds of meters, and therefore the reduction of heat flux transferred to atmosphere as much as 40% in groundwater drawdown zone in North China after 50 years over-exploitation. It is small the amount of reduction of heat flux transferred from surface to atmosphere. However, it can last for a long period of hundreds of years, and cover a large area of 70000 km². Its effect on climate may not be negligible. Meteorological temperature (up to 3.2 m depth) records show similar trend. Although air temperatures are influenced by many factors, the decline of heat flux transferred to atmosphere from shallow ground should be one of the factors to be studied. We suggest that more attention be paid to this effect, and systematic monitoring of geotherm at depths of several dozen to several hundred meters should be carried out.