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Abstract. Carbon transport in river systems is an important component of the global carbon cycle. Most rivers of the world act as atmospheric CO2 sources due to high riverine CO2 partial pressure (pCO2). By determining the pCO2 from alkalinity and pH, we investigated its spatial and temporal variation in the Yellow River watershed using historical water chemistry records (1950s–1984) and recent sampling along the mainstem (2011–2012). Except the headwater region where the pCO2 was lower than the atmospheric equilibrium (i.e. 380 μatm), river waters in the remaining watershed were supersaturated with CO2. The average pCO2 for the watershed was estimated at 2810 ± 1985 μatm, which is 7-fold the atmospheric equilibrium. As a result of severe soil erosion and dry climate, waters from the Loess Plateau in the middle reaches had higher pCO2 than that from the upper and lower reaches. From a seasonal perspective, the pCO2 varied from about 200 μatm to > 30 000 μatm with higher pCO2 usually occurring in the dry season and lower pCO2 in the wet season (at 73% of the sampling sites), suggesting the dilution effect of water. While the pCO2 responded exponentially to total suspended solids (TSS) export when the TSS concentration was less than 100 kg m−3, it decreased slightly and remained stable if the TSS concentration exceeded 100 kg m−3. This stable pCO2 is largely due to gully erosion that mobilizes subsoils characterized by low organic carbon for decomposition. In addition, human activities have changed the pCO2 dynamics. Particularly, flow regulation by dams can diversely affect the temporal changes of pCO2, depending on the physiochemical properties of the regulated waters and adopted operation scheme. Given the high pCO2 in the Yellow River waters, large potential for CO2 evasion is expected and warrants further investigation.

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[1] The temporal and spatial variations of major ions in the Zhujiang (Pearl River) were analyzed using long-term water chemistry data of major dissolved ions (Ca 2+ . The total dissolved solids (TDS) within the Zhujiang basin varies from 34.0 mg/l to 416.1 mg/l generally decreasing from upstream to downstream along the main stem of the Zhujiang. Rock weathering is the dominant controlling factor for the water chemistry of the Zhujiang, and more specifically, on average, 68% (22-92%) of total dissolved load comes from carbonate weathering, 22% (2-68%) from silicate weathering, and 10% (3-24%) from evaporite weathering, respectively. The flux calculations indicate that in total about 41.8 Â 106 tonnes/year of TDS are transported out of the Zhujiang (excluding the Delta Region), averaged for the period 1958-2002. Changes in water chemistry can be observed from long-term trend analysis, notably for SO 4 2À and Cl À, as a result of anthropogenic influences, such as acid deposition, domestic and industrial wastewater discharge, and basin water resource development. An intense reforestation policy coupled with rapid reservoir development in the Zhujiang Basin would trigger more significant anthropogenic impacts on water chemistry in the future.

CO 2 outgassing across water-air interface is an important, but poorly quantified, component of riverine carbon cycle, largely because the data needed for flux calculations are spatially and temporally sparse. Based on compiled data sets measured throughout the Yellow River watershed and chamber measurements on the main stem, this study investigates CO 2 evasion and assesses its implications for riverine carbon cycle. Fluxes of CO 2 evasion present significant spatial and seasonal variations. High effluxes are estimated in regions with intense rock weathering or severe soil erosion that mobilizes organic carbon into the river network. By integrating seasonal changes of water surface area and gas transfer velocity (k), the CO 2 efflux is estimated at 7.9 ± 1.2 Tg C yr À1 with a mean k of 42.1 ± 16.9 cm h À1. Unlike in lake and estuarine environments where wind is the main generator of turbulence, k is more correlated with flow velocity changes. CO 2 evasion in the Yellow River network constitutes an important pathway in its riverine carbon cycling. Analyzing the watershed-scale carbon budget indicates that 35% of the carbon exported into the Yellow River network from land is degassed during fluvial transport. The CO 2 efflux is comparable to the carbon burial rate, while both larger than the fluvial export to the ocean. Comparing CO 2 evasion with ecosystem productivity in the Yellow River watershed shows that its ecosystem carbon sink has previously been overestimated by >50%. Present efflux estimates are associated with uncertainty, and future work is needed to mechanistically understand CO 2 evasion from the highly turbid waters.

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