近30年来降水量变化和人类活动对北京潮白河冲洪积扇地下水动态的影响

对北京潮白河冲洪积扇分布区30年来降水量、地表径流量、地下水水位和地下水储存量进行了时间序列分析。结果表明: 过去30年中，区域地下水动态发生了明显变化，特点是1998年以来，地下水位和地下水储存量迅速下降与减少。1998年以来，年降水量为以往多年平均值的76％左右。在储存量变化的影响因素中，降水量减少导致的补给量减少约占24％，人类活动，如工农业地下水开采、应急水源地地下水开采和地表水体入渗减少等因素约占76％。由于未来气候变化的不确定性，在南水北调的水进京后仍可能出现连续枯水年，因此，以丰水年降水进行水源涵养存在较大风险。对于已经处于严重超采状况的潮白河冲洪积扇来说，为了满足未来供水的需要，应急水源地从现在起应减少开采量或停采以逐步恢复地下水储存量。

IMPACT OF PRECIPITATION CHANGE AND HUMAN ACTIVITIES ON GROUNDWATER REGIME IN THE CHAOBAIHE ALLUVIAL AND DILUVIAL FAN IN BEIJING IN THE LAST 30 YEARS

The Chaobaihe alluvial-diluvial fan is located in the northeast of Beijing as an important water supply region for Beijing urban area.Over the last 30 years,especially since 1998,groundwater system has changed significantly under the dual impacts of climate change and human activities in the form of precipitation decrease and groundwater exploitation.Results from series analysis of changes in precipitation,surface runoff,groundwater regime and groundwater storage show that the region has undergone a marked change in groundwater.Among the reasons for water table decline and storage reduction,decrease of precipitation accounts for approximately 24% while human activities,such as industrial and agricultural use of ground water,use of wells designated for emergency water supply and reduced infiltration of surface water are responsible for the other 76%.Series analysis of precipitation from 1724 to 2007 shows that average precipitation amount from 2000 to 2007 is only three quarters of the total average value,which is the reason for the decreasing of surface water and direct recharge.Analysis of the relationship between precipitation and groundwater recharge shows that when precipitation decreases by 10%,groundwater recharges decreases 10% accordingly.The impact of human activities on groundwater is remarkable based on groundwater monitoring.Changes in groundwater storage has been calculated hased on the monitoring data of groundwater table and specific yield.According to the mass balance equation,deficit of precipitation led to a decrease of 0.75×10~8m~3/a in the groundwater recharge,accounting for 24% to the total storage which is 3.1×10~8m~3/a,while the human activities,like increased groundwater extraction,reduces the groundwater recharge of 0.09×10~8m~3/a which contributes about 3% to the total decreased groundwater storage,and the exploitation of emergency water sources decreases the groundwater recharge of 1.60×10~8m~3/a which contributes about 52% and cutoff of recharge from surface water reservoir decreases 0.66×10~8m~3/a which contributes about 21%.In one word,the human activities contribute 76% in total to the decreased groundwater storage,as main driving factors controlling groundwater system in the study area.The study gives a potential method based on water mass balance to differentiate the impact of climate change from human activities on groundwater system to promote the sustainable groundwater management and utilization.The south-to-north water transfer project is underway to alleviate the water shortage in the North China Plain and it is expected that the diverted water will come to Beijing in 2014.Before that date,it is necessary to continue mining of groundwater.It is with hope that groundwater table can be restored after the water diversion is implemented.However,due to high uncertainty of climate change,there is still a possibility that consecutive dry years may occur after the water transfer project.As a result,there is a high risk for the plan of water conservation in wet years.Therefore,it is recommended that the use of emergency wells should be reduced or even stopped in order to restore the groundwater in Chaobai alluviall-diluvial fan so it will be possible to extract water under emergency situations.