In order to understand the distributions and influences of pCO2 in medium and small karstic rivers, the Sancha River was investigated in February and August 2014. The temperature, EC, DO, pH and dissolved inorganic carbon (DIC) of the river were measured and the partial pressure of CO2 (pCO2) was calculated. The results showed that EC、pH、TDS and DIC in dry season were higher than that in wet season. The pCO2 in the surface water ranged between 300 and 10000μatm, with an average value of 3100μatm. The pCO2 values in the wet season were higher than that in dry season. The analyses of supersaturated CO2 and apparent oxygen utilization demonstrated that pCO2 was controlled by the carbonate system in the dry season. The in-situ aerobic respiration was one of the significantin fluencing factors in the wet season, and the supersaturated CO2 may attribute to the flushing of carbon dioxide from soils. The CO2 emission flux from Sancha River to atmosphere was estimated about 0.9~1.7×109molC/a and 10.8~20.3MgC/(hm2·a). The water-to-air CO2 outgassing flux from Sanchahe River was higher than that from large rivers (i.e. Amazon River、Yangtze River) and lower than that from streams (i.e. Houzhai River、G?ddtj?rn River). The results indicated that the CO2 emission fluxes from surface water systems are influenced by the scale of rivers, and the contribution to the regional carbon cycling brought by the medium and small river may be underestimated for a long time.%为了解喀斯特中小型河流并计算水体CO2分压(pCO2的分布及其影响因素,于2014年2月和8月调查了三岔河的温度、EC、DO、pH值和溶解无机碳(DIC),pCO2).结果表明:EC、pH、TDS和DIC均表现为枯水期>丰水期,水体pCO2在300~10000μatm之间,年平均值3100μatm,枯水期<丰水期.过饱和CO2与表观耗氧量的相关关系分析发现,枯水期pCO2与HCO3-含量呈正相关,表明枯水期主要受碳酸盐岩溶解控制的碳酸体系的影响;而丰水期河流水体高pCO2则主要受有机好氧呼吸作用和高浓度土壤CO2进入到水体的影响.三岔河水-气界面CO2交换速率为10.8~20.3MgC/(hm2·a),CO2释放通量约为0.9~1.7×109molC/a.对比分析发现,喀斯特中小型河流三岔河CO2释放速率高于亚马逊河、长江、西江等大型河流,却低于喀斯特地区以及北欧等小型溪流,可见不同级别河流具有不同的CO2释放通量,长期以来可能低估了中小型河流对区域CO2循环的贡献.
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