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 70000km~2 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 70000km~2.Its effect on climate may not be negligible.Meteorological temperature(up to 3.2m 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.