Analysis on the reasons for continuous declination of groundwater table in the Ciba-Gangtou village water-rich block in Kunming in recent years

The Ciba-Gangtou village water-rich block，16.07 km2 in area，is located in the Panlongjiang river valley on the east of Sheshan fault in northern suburb of Kunming. There are some famous karst springs such as the Heilongtan，the Lanlongtan and the Bailongtan. The karst aquifer of the water-rich block mainly composed of limestone and dolomite in the Permian and Carboniferous systems covered on the top by quaternary proluvial and alluvial gravel layers of 2.5-173.92 m in thickness. It takes east Heilongtan branch fault and basalt as the impermeable boundary to the east and the Douposi group argillaceous siltstone as the impermeable boundary to the south. Groundwater flows from north to south and from west to east，some of the groundwater discharges by the spring on the gentle slope piedmont edge and some runs deep down，forming the confined area. The rainfall decreased in recent years，leading to drought in Kunming，where the water-rich block is located. In particular for the continuing drought during 2009-2011，the karst groundwater table declines continuously in the water-rich block affecting greatly the production and livelihood there. In order to clarify the reasons for the declining groundwater table，this paper analysis the decrease of rainfall，comparing to the average rainfall，and the decrease of groundwater recharge and extraction，as well as amplitude of groundwater table in the water-rich block during 2009-2011，based on ascertaining hydrogeological condition. The results show that the reduction of the annual rainfall compared to normal amounts leads to the reduction of groundwater recharge，which is the main reason for the declining trend of groundwater table after reducing exploitation continuously and closing of the wells in the Ciba-Gangtou village water-rich block. This understanding to the drought in Yunnan and rational exploitation of karst groundwater is of some guiding significance.