Hydrochemical Responses of Stormflow to Extreme Rainfall during Dry Season in a Typical Karst Catchment of Northwest Guangxi Province,China

Based onin situobservation and tracing sampling,hydrochemical process in a karst small watershed during a rainstorm(64mm)after 20-d drought in dry season were studied.Furthermore,the influencing factors were analyzed.Results show that the responses of water stage,electrical conductivity,temperature and pH were not obvious during the first rainfall event(8.5mm).The stream did not response to the rainfall until the rainfall amount reached 35.5mm.After that,all hydrochemical indexes,except for the nitrate nitrogen,began to rapidly change.Conductivity,δ18 O,and Ca2+quickly dropped to the lowest values because of the dilution effect.The stream was mainly recharged by the supply of old water during the early and late rainfalls.However,the proportion of new water was up to 34% during the rapidly rising of the water stage.In the two rainfalls,although the water stage increased,the concentration of nitrate nitrogen(NO-3-N)was still higher than its value before the rainfall event.The research provides necessary information for the small watershed water resources utilization and contaminant controlling.