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  • View in gallery

    Map of study sites at Ji-Paraná River basin.

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    Daily discharge averages between 1999 and 2003, for 12 gauging stations at Ji-Paraná basin rivers. Data from ANA.

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    Rivers’ measured channel widths vs calculated widths based on the relations with drainage basin area (see equations in the text).

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    Average CO2 flux in tributaries and the Ji-Paraná River main stem at low and high water periods. The numbers correspond to rivers: 25 = Comemoração, 26 = Pimenta Bueno, 27 = Urupá, 28 = first site at Ji-Paraná, and 29 = second site at Ji-Paraná.

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    Carbon dioxide outgassing from rivers of the Ji-Paraná River basin during low, rising, high, and falling water periods. The uncertainty bar corresponds to the combined standard uncertainty resulting from contributions of calculated surface area and CO2 fluxes measured with chambers.

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Estimating the Surface Area of Small Rivers in the Southwestern Amazon and Their Role in CO2 Outgassing

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  • 1 Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, São Paulo, Brazil
  • | 2 Departamento de Ciências da Natureza, Campus Universitário Reitor Aulio Gelio Alves de Souza, Universidade Federal do Acre, Rio Branco, Acre, Brazil
  • | 3 Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, São Paulo, Brazil
  • | 4 National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 5 Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, São Paulo, Brazil
  • | 6 School of Oceanography, University of Washington, Seattle, Washington
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Abstract

A recent estimate of CO2 outgassing from Amazonian wetlands suggests that an order of magnitude more CO2 leaves rivers through gas exchange with the atmosphere than is exported to the ocean as organic plus inorganic carbon. However, the contribution of smaller rivers is still poorly understood, mainly because of limitations in mapping their spatial extent. Considering that the largest extension of the Amazon River network is composed of small rivers, the authors’ objective was to elucidate their role in air–water CO2 exchange by developing a geographic information system (GIS)-based model to calculate the surface area covered by rivers with channels less than 100 m wide, combined with estimated CO2 outgassing rates at the Ji-Paraná River basin, in the western Amazon. Estimated CO2 outgassing was the main carbon export pathway for this river basin, totaling 289 Gg C yr−1, about 2.4 times the amount of carbon exported as dissolved inorganic carbon (121 Gg C yr−1) and 1.6 times the dissolved organic carbon export (185 Gg C yr−1). The relationships established here between drainage area and channel width provide a new model for determining small river surface area, allowing regional extrapolations of air–water gas exchange. Applying this model to the entire Amazon River network of channels less than 100 m wide (third to fifth order), the authors calculate that the surface area of small rivers is 0.3 ± 0.05 million km2, and it is potentially evading to the atmosphere 170 ± 42 Tg C yr−1 as CO2. Therefore, these ecosystems play an important role in the regional carbon balance.

* Corresponding author address: Maria de Fátima F. L. Rasera, Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Av. Centenário, 303, 13416-000, Piracicaba, São Paulo, Brazil. mrasera@cena.usp.br

Abstract

A recent estimate of CO2 outgassing from Amazonian wetlands suggests that an order of magnitude more CO2 leaves rivers through gas exchange with the atmosphere than is exported to the ocean as organic plus inorganic carbon. However, the contribution of smaller rivers is still poorly understood, mainly because of limitations in mapping their spatial extent. Considering that the largest extension of the Amazon River network is composed of small rivers, the authors’ objective was to elucidate their role in air–water CO2 exchange by developing a geographic information system (GIS)-based model to calculate the surface area covered by rivers with channels less than 100 m wide, combined with estimated CO2 outgassing rates at the Ji-Paraná River basin, in the western Amazon. Estimated CO2 outgassing was the main carbon export pathway for this river basin, totaling 289 Gg C yr−1, about 2.4 times the amount of carbon exported as dissolved inorganic carbon (121 Gg C yr−1) and 1.6 times the dissolved organic carbon export (185 Gg C yr−1). The relationships established here between drainage area and channel width provide a new model for determining small river surface area, allowing regional extrapolations of air–water gas exchange. Applying this model to the entire Amazon River network of channels less than 100 m wide (third to fifth order), the authors calculate that the surface area of small rivers is 0.3 ± 0.05 million km2, and it is potentially evading to the atmosphere 170 ± 42 Tg C yr−1 as CO2. Therefore, these ecosystems play an important role in the regional carbon balance.

* Corresponding author address: Maria de Fátima F. L. Rasera, Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Av. Centenário, 303, 13416-000, Piracicaba, São Paulo, Brazil. mrasera@cena.usp.br

Introduction

Rivers receive and process carbon from their watersheds, reflecting both natural and anthropogenic processes in the drainage basins. While in transit, the composition and concentration of various carbon fractions (organic and inorganic, particulate and dissolved) are modified by metabolic processes within the river channel, and part of the inorganic carbon may be outgassed to the atmosphere as CO2. Recent studies in temperate ecosystems indicate that the magnitude of CO2 outgassing from surface waters is comparable to the amount of carbon exported by fluvial discharge (Hope et al. 2001; Cole and Caraco 2001). In tropical regions, the contribution of CO2 outgassing seems to play an even more significant role in net carbon emissions. Medium to large rivers and wetlands in the Amazon are generally supersaturated in CO2, resulting in large outgassing to the atmosphere. A recent estimate of CO2 outgassing from rivers and wetlands of the central Amazon basin suggests that an order of magnitude more carbon is emitted to the atmosphere than is exported to the ocean in organic and inorganic forms (Richey et al. 2002). In this region, high CO2 concentrations in surface waters and subsequent outgassing result largely from in situ respiration of contemporary organic matter (Mayorga et al. 2005a).

Although CO2 outgassing has been measured in situ for more than two decades in the Amazon River basin (Devol et al. 1987; Richey et al. 1989; Bartlett et al. 1990), only recently has its contribution to the regional carbon balance received significant attention. Advances in remote sensing and geographic information system (GIS) techniques provided data and tools to better estimate the extent of these habitats, which was a major limitation to extrapolate field results. In the Amazon, radar and multispectral images have been used successfully to map floodplain habitats for rivers with channels wider than 100 m (Sippel et al. 1998; Alsdorf et al. 2000; Shimabukur et al. 2002; Hamilton et al. 2002; Hess et al. 2003). However, for small rivers less than 100 m wide, the use of satellite images is limited by the spatial resolution of sensors, cloud cover, and computer processing capacity. Considering that 92% of the total length of the Amazon River network (Mayorga et al. 2005b) consists of channels less than 100 m wide, these areas may play an important role in the outgassing of CO2 to the atmosphere. Hence, the inclusion of these still poorly mapped areas is critical to accurately assess regional net carbon emissions.

Although remote sensing products are available for mapping some of the lower-order river networks in the Amazon, complete coverage of the basin at such resolution would be prohibitively expensive. As an alternative for estimating channel surface area we used GIS techniques and known geomorphological and hydrological relationships. The confluence of tributaries and the increase in drainage area downstream result in higher flows and wider channels, defined to a considerable degree by landscape characteristics, such as topography and channel morphology (Knighton 1999; Church 2002). Since the size of a river channel is determined by streamflow, and runoff is highly correlated with drainage basin area, river channel scaling varies systematically throughout a drainage basin (Leopold 1994; Knighton 1999; Finlayson and Montgomery 2003; Reinfelds et al. 2004). Therefore, hydraulic geometry and power-law equations (Leopold and Maddock 1953) can be used as analytical tools to develop mathematical relationships between drainage basin area and river scaling.

Based on these relations we developed a new methodology to calculate small river surface areas in a mesoscale basin in southwestern Amazon that, combined with field measurements of carbon fluxes, allowed us to calculate the relative importance of CO2 effluxes to the atmosphere. This GIS-based methodology was then used to calculate the surface area covered by rivers with channels less than 100 m wide in the entire Amazon basin and to extrapolate their role in CO2 evasion.

Study area

The study area was the Ji-Paraná River basin, located in the state of Rondônia, western Amazon (Figure 1). Draining an area of approximately 75 400 km2, this basin has undergone extensive land-cover and land-use change in the headwaters and middle sectors, where the river is highly impacted by agroecosystems distributed across the landscape. In the lower reaches, the basin retains a relatively pristine forest, where the landscape is only slightly altered 400 km before the confluence with the Madeira River. The Ji-Paraná River is formed by the confluence of the Comemoração and Pimenta Bueno Rivers and receives the waters of four other main tributaries, the rivers Rolim de Moura, Urupá, Jarú, and Machadinho. Supersaturated pCO2 conditions are typical in the Ji-Paraná River basin, with values ranging between 2 and 23 times atmospheric equilibrium values (∼940–7500 μatm; Rasera 2005). Topography is relatively flat, with altitudes ranging from 75 to 600 m above sea level and an average slope of 0.62°. Throughout the hydrological year, rivers are confined to their channels. Lower-order streams (first to third) are dominant, accounting for a total channel length of 27 497 km. The main stem has a total length of 972 km and varies in width from 150 to 500 m. Channel widths of major tributaries range on average from 10 to 100 m. Sandy and clayey Oxisols and Ultisols are predominant in the basin (Ballester et al. 2003). Average annual precipitation is about 2200 mm, with a rainy season lasting from October to April, and a relatively short dry season from June to August.

Methods

Extent of river water surface

The surface area of small-to-medium tropical rivers can exhibit large variations throughout the hydrological cycle, affecting the size of the air–water interface and outgassing estimates. Therefore, to calculate average channel width we divided the hydrograph into four stages: falling water (16 April–30 June), low water (July–October), rising water (November–14 January), and high water (15 January–15 April). These four stages were established based on a 5-yr time series (1999–2003) of daily river discharge data from 12 monitoring stations of the Brazilian Water Agency [Agência Nacional de Águas (ANA); http://hidroweb.ana.gov.br/] along different tributaries and the main stem of the Ji-Paraná River, ranging from third- to seventh-order rivers (Figure 2).

To calculate the surface area of the Ji-Paraná River network we first derived a series of equations describing the relationships between drainage area and channel width. As observational data, we used discharge, stage, and transverse profiles from 23 stations measured by ANA, 10 of which are located in the Ji-Paraná River basin. The other 13 stations were in nearby basins with similar hydrological characteristics.

Average channel width was calculated from transverse profiles (width versus river stage) for each station, using the historical average discharge (average of 15-yr time series) for each period and the corresponding river stage and transverse profiles obtained in the field during the sampling period (1999–2003). The drainage area of each station within the Ji-Paraná River basin was delineated in ArcGIS software (version 9) using a digital elevation model (DEM) with a vertical resolution of 100 m (Ballester et al. 2003; Krusche et al. 2005) and the hydrological analyst extension (version 2 for ArcGIS). The areas of neighboring basins were obtained from ANA. These results were then used to derive the fitting parameters for the power-law functions describing the scaling between channel width and drainage area for each hydrograph stage as υ = cAd (Dunne and Leopold 1978; Pinet and Souriau 1988; Knighton 1999; Church 2002; Finlayson and Montgomery 2003), where υ is channel width, A is the drainage area, and c and d are fitting parameters.

For drainage basins smaller than 300 km2, a different set of regressions was developed, using data from Thomas et al. (Thomas et al. 2004). Because of the limitations in this dataset (data were collected solely for the high and low water periods), only two equations were calculated for streams of such size, one for each water period. Because the intrinsic linearity of hydraulic geometry changes as a function of scale (Dunne and Leopold 1978), to calculate channel widths and river surface areas we divided the datasets of the 23 stream gauge stations from ANA and Thomas et al. (Thomas et al. 2004) into three groups: 1) channel widths less than 100 m and drainage areas smaller than 300 km2, 2) channel widths less than 100 m and drainage areas between 300 and 23 000 km2, and 3) channel widths greater than 100 m and drainage areas larger than 23 000 km2.

The digital river network, classified according to river order (Strahler 1963) and ranging from third to seventh order, was then used to delineate and calculate the drainage area and river length for each river. All these procedures were performed using ArcGIS. Based on the derived equations, we calculated channel widths. Lastly, individual surface areas were pooled together to derive total surface extent of river channels. Model accuracy was independently tested using field-measured channel width data from 36 sites (13 in the high water period, 13 in the low water period, and 10 during rising water).

CO2 outgassing

Carbon dioxide fluxes were measured with floating chambers at 20 sites in third-order rivers, 4 sites in fourth-order rivers, 3 sites in the main tributaries (Comemoração, Pimenta Bueno, and Urupá), and 2 sites at the Ji-Paraná main stem. Field work was conducted between July 2006 and May 2007 (Figure 1), with discharge conditions ranging from low to high flow. In situ flux measurements were performed using a Plexiglas chamber (with a volume of 18.3 L and a water surface of 0.1 m2) equipped with an inner fan to ensure internal air circulation (Sebacher et al. 1983). The chamber, mounted on a floating platform, was connected to a closed air circuit with an air pump (130 mL min−1) and an infrared CO2 analyzer (LI-COR Instruments, LI-820), both powered with a 12-V battery. The pCO2 concentrations inside the chamber were recorded every second during 5 min. Air–water gas exchange fluxes were calculated as
i1087-3562-12-6-1-eq1
where dpCO2/dt is the slope of the CO2 accumulation in the chamber (μatm s−1), V is the chamber volume (L), T is air temperature (K), S is the surface area of the chamber at the water surface (m2), and R is the gas constant (L atm K−1 mol−1; Frankignoulle 1988). In general, r2 values were >0.98. S. R. Alin et al. (2007, unpublished manuscript) provide a detailed evaluation of this method.

Finally, the CO2 exchange across the water–air surface was extrapolated by multiplying the total area of each river by the average CO2 outgassing rate. These calculations were performed separately for rising, high, falling, and low water.

Importance of CO2 evasion relative to fluvial carbon export

To compare the contribution of total CO2 evasion from water surfaces with the export flux of total dissolved inorganic (DIC) and organic (DOC) carbon, we used daily discharge data from ANA for the most downstream gauged station in the main stem Ji-Paraná River (Figure 1) and multiplied it by DIC and DOC concentration data available for the same site (Leite 2005). This station is located 123 km upstream from the mouth and the drainage area is approximately 60 540 km2. For the purpose of comparing fluxes, we considered this to be the exit of the basin.

Extrapolation for the Amazon basin

To extrapolate for the Amazon basin, we assumed that the average channel width and the CO2 flux from Ji-Paraná are representative of the entire basin. First, using ArcGIS we extracted the third- to fifth-order rivers (rivers with channel width <100 m) from the Amazon River network derived by Mayorga et al. (Mayorga et al. 2005b). For each river order, we assigned an average channel width, based on the averages obtained from the respective orders at the Ji-Paraná River network. To calculate the evasion area we multiplied the assigned channel width by channel length. This operation was performed for low, rising, high, and falling water periods. Finally, we assigned average CO2 flux for each channel order using the field data from the Ji-Paraná and multiplied these values by the corresponding area at each river order.

To compare our results with previous estimates of evasive CO2 fluxes in the Amazon (Richey et al. 2002), we calculated the relative importance of larger and smaller rivers in the central lowland quadrant (18° × 8°) described by Hess et al. (Hess et al. 2003) and Richey et al. (Richey et al. 2002). Based on the high and low water inundation values given by Hess et al. (Hess et al. 2003), we calculated an annual weighted mean inundation area and multiplied that by the annual mean flux calculated by Richey et al. (Richey et al. 2002) (8.3 ± 2.4 Mg C ha−1 yr−1). This value, corresponding to fluxes from larger rivers, was then compared with our estimates of fluxes from smaller rivers in this quadrant.

Results

Channel width and areal extent of the river surface

Table 1 shows the drainage area and the average discharge and channel width obtained from ANA gauging stations datasets and Thomas et al. (Thomas et al. 2004). Table 2 shows the regression coefficients (r2) and equations derived to predict channel width based on drainage basin area. The models were statistically significant (p < 0.05) for all periods (rising, high, falling, and low water). The average channel width calculated for third-order rivers (n = 288) was 14 ± 5 m during high water and 11 ± 4 m in the low water period. For fourth-order rivers (n = 56), channel widths were 31 ± 11 m and 23 ± 8 m during high and low water, respectively. The fifth- to seventh-order rivers in the Ji-Paraná basin are characterized by large variability in channel width. During the low water period, at the headwaters of the fifth-order rivers (n = 13) widths ranged from 22 to 52 m and at their mouths from 26 to 64 m. The headwaters of the sixth-order rivers (n = 3) had widths ranging from 57 to 74 m, and their mouths from 60 to 213 m. Only sixth- and seventh-order rivers had channel widths greater than 100 m. Measured and calculated channel widths were not statistically different (t = 0.25, p = 0.80, n = 36) (Figure 3).

The Ji-Paraná River basin is dominated by small channels (less than 100 m wide). The total length of third- to sixth-order rivers with channel widths smaller than 100 m wide represents 94% (6694 km) of the Ji-Paraná network. These small channels contribute between 48% and 56% of the total river surface area in this basin (Table 3).

CO2 outgassing to the atmosphere

Carbon dioxide fluxes from third- and fourth-order rivers showed large variation, ranging from 0.67 ± 0.08 to 12.63 ± 1.49 μmol CO2 m−2 s−1 at Arenito and Miolo Rivers, respectively, with an average of 5.49 ± 3.16 μmol CO2 m−2 s−1. At the main tributaries and main stem fluxes ranged from 0.60 ± 0.19 to 4.52 ± 0.10 μmol CO2 m−2 s−1 (Table 4). Most rivers showed similar seasonal patterns, with CO2 outgassing increasing during a high water period when compared to those observed at low water; with the exception of the Comemoração River, which had similar fluxes in both periods (Figure 4), with average fluxes of 2.41 ± 1.59, 1.24 ± 0.64, and 1.25 ± 0.35 μmol CO2 m−2 s−1 for high, low, and falling water periods, respectively.

The total river surface area of the Ji-Paraná basin outgassed 308 ± 188 Gg C yr−1. During the high water period, outgassing was 97 ± 63 Gg C, decreasing during low water, when approximately 88 ± 55 Gg C was evaded. Carbon dioxide outgassing during falling and rising water were very similar, 63 ± 37 and 59 ± 33 Gg C, respectively (Figure 5).

Discussion

The rivers of the Ji-Paraná basin are generally confined to their main channels, without major flooding areas (Caldeiron 1993). The discharge of these rivers shows significant seasonality, with well-defined wet and dry periods resulting in substantial changes in water level and surface area. The methodology developed here to calculate the surface area of rivers larger than third order is appropriate for use on fluvial systems with characteristics similar to those of the Ji-Paraná basin, that is, for rivers confined to their channels.

In Amazonian fluvial systems with large areas of inundation, Hamilton et al. (Hamilton et al. 2002) developed a regression model based on satellite and ground observations, Hess et al. (Hess et al. 2003) mapped the extent of wetlands for the central Amazon region using mosaicked L-band SAR imagery, and Coe et al. (Coe et al. 2002; Coe et al. 2008) developed a computer model to estimate inundated areas based on physical properties of water and the land surface. Their models are more appropriate for use in systems where changes in water level imply large changes in the water surface area, due to flooding of adjacent floodplains. Given their data sources, all these models address water bodies >100 m, thereby not including the streams <100 m discussed here.

The development of our model was constrained by 1) limited amount of available data from hydrological stations in low-order rivers and 2) limited resolution of the digital elevation model. Given the inaccuracy of the DEM at this scale, it was necessary to manually adjust drainage areas by following elevation lines across maps for approximately 60% of third-order rivers and 40% of fourth-order rivers. For this reason, the application of this method becomes essentially impossible for first- and second-order rivers, particularly because hydrological stations with historical data series and digital elevation models with sufficient resolution are both lacking. Furthermore, seasonality in water level in low-order rivers is not well defined, as water level and discharge respond rapidly and transiently to precipitation events.

River waters of the Ji-Paraná basin present evasive CO2 fluxes within the ranges observed in other Amazonian rivers (S. R. Alin et al. 2007, unpublished manuscript). Carbon dioxide evasion from river systems is a function of pCO2 gradient between water and air and the gas exchange coefficient (k). Factors that affect the pCO2 levels in these ecosystems appear to yield pronounced differences among watersheds. For the Ji-Paraná basin, Rasera (Rasera 2005) observed that the spatial distribution of pCO2 was heterogeneous and strongly related to regional geology; rivers draining eutrophic soils had higher values of pCO2. The higher values and larger variations in fluxes in third- and fourth-order rivers than in main tributaries and the main stem can be related with higher variability in physical factors that influence the magnitude of the gas k between water and air in these systems. According to Borges et al. (Borges et al. 2004) and (S. R. Alin et al. 2007, unpublished manuscript), current velocity and water depth in streams should result in higher values and variability of k, since turbulence generated by bed traction declines with increasing channel depth. Because of this large variation, uncertainties in flux extrapolations for these systems are higher than in larger rivers.

The increase in the fluxes during high water at the main tributaries and main stem are likely related to the highest values of pCO2 at this period. The Ji-Paraná Rivers showed high water pCO2 ranging between 1.5 to 8 times low water period values (Rasera 2005). During high water, pCO2 increases can result from one or the combination of several factors, such as 1) larger inputs of CO2-rich soil water, as described by Johnson et al. (Johnson et al. 2006) for the headwaters of a lowland stream in Amazonia; 2) increased inputs of more acidic rainwater (Germer et al. 2007); 3) additional sources of carbon and nutrients for respiration brought into channels adsorbed onto fine suspended sediments (Aufdenkampe et al. 2001; Cogo 2005); or 4) increases in depth-to-surface area ratios (sensu; Devol et al. 1995). Although the highest fluxes occurred during high water, the net flux over this period was not higher than for other periods, because of the different durations of each hydrograph period [high (91 days), falling (76 days), low (123 days), and rising water (75 days)]. Therefore, while the low water period had the lowest fluxes, the longer period of this hydrograph stage produced values similar to the high water for the entire period (Figure 5).

Our results showed an important contribution of small streams to the total carbon dioxide outgassing from riverine systems at regional scales. Rivers less than 100 m wide contributed on average with 82% of the total outgassing from the Ji-Paraná basin, with a larger contribution at the low water period, when they comprised 88% of total CO2 evasion. Moreover, this estimate is probably conservative, since first- and second-order rivers were not considered in this balance, and these small streams may play a substantial role in local and regional CO2 outgassing, since the majority of the pCO2 in groundwater evades to the atmosphere within meters of emerging from springs into rivers (Lehmann et al. 2004).

To assess the importance of CO2 outgassing in fluvial carbon export, we calculated an annual net carbon balance for CO2, DOC, and DIC in the Ji-Paraná main stem up to the last ANA hydrological station (8°56′S, 62°3′W; near site 4 in the Ji-Paraná River). Our results show that, from this portion of the basin, CO2 outgassing was the main carbon export pathway, releasing 289 ± 178 Gg C yr−1, 2.4 times the amount of carbon exported as DIC (121 ± 13 Gg C yr−1) and 1.6 times the export of DOC (185 ± 35 Gg C yr−1).

To assess the potential impact of small rivers in the regional carbon balance, we performed an extrapolation exercise, assuming that the average channel width and the CO2 fluxes from Ji-Paraná Rivers are representative of the entire Amazon basin. For the central portion of the Amazon basin, Hess et al. (Hess et al. 2003) computed a wetland extent of approximately 2.9 × 105 and 0.8 × 105 km2 for the high and low water periods, respectively. Our calculation adds an area inundated by third- to fifth-order rivers (<100 m wide) of 2600 ± 900 and 2000 ± 700 km2, respectively. Computed over an entire year, this central portion outgases 145 ± 40 Tg C yr−1, 3% of which comes from third- to fifth-order rivers. Integrating for the entire Amazon basin over time and space (annual mean), the surface area of small rivers is 0.3 ± 0.05 × 106 km2, and these rivers alone are potentially evading to the atmosphere 170 ± 40 Tg C yr−1 as CO2.

The relationships established here between drainage basin area and channel width provide a new model for determining surface area of rivers less than 100 m wide, and the results point to the fact that these ecosystems play an important role in the regional carbon balance and are still poorly understood.

Acknowledgments

This is a publication of the international collaboration between Centro de Energia Nuclear na Agricultura–Universidade de São Paulo and University of Washington, as part of the Large Scale Biosphere–Atmosphere Experiment in Amazônia (LBA). Research was sponsored by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP Grants 99/01159-4 and 03/13172-2), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Institutos do Milênio) in Brazil, and by the National Science Foundation (NSF, CAMREX Project) and the National Aeronautics and Space Administration (NASA, LBA-ECO, CD 06) in the United States. We also would like to acknowledge the scholarship granted by Coordenação de Aperfeiçomaneto de Pessoal de Nível Superior (CAPES) to Maria de Fátima F. L. Rasera. We thank Nei K. Leite, Alexandra Ayres Montebello, and Beatriz Machado Gomes for helping with the field work.

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Figure 1.
Figure 1.

Map of study sites at Ji-Paraná River basin.

Citation: Earth Interactions 12, 6; 10.1175/2008EI257.1

Figure 2.
Figure 2.

Daily discharge averages between 1999 and 2003, for 12 gauging stations at Ji-Paraná basin rivers. Data from ANA.

Citation: Earth Interactions 12, 6; 10.1175/2008EI257.1

Figure 3.
Figure 3.

Rivers’ measured channel widths vs calculated widths based on the relations with drainage basin area (see equations in the text).

Citation: Earth Interactions 12, 6; 10.1175/2008EI257.1

Figure 4.
Figure 4.

Average CO2 flux in tributaries and the Ji-Paraná River main stem at low and high water periods. The numbers correspond to rivers: 25 = Comemoração, 26 = Pimenta Bueno, 27 = Urupá, 28 = first site at Ji-Paraná, and 29 = second site at Ji-Paraná.

Citation: Earth Interactions 12, 6; 10.1175/2008EI257.1

Figure 5.
Figure 5.

Carbon dioxide outgassing from rivers of the Ji-Paraná River basin during low, rising, high, and falling water periods. The uncertainty bar corresponds to the combined standard uncertainty resulting from contributions of calculated surface area and CO2 fluxes measured with chambers.

Citation: Earth Interactions 12, 6; 10.1175/2008EI257.1

Table 1.

Average discharges and channel widths of gauging stations from ANA and from Thomas et al. (Thomas et al. 2004).

Table 1.
Table 2.

Equations used to derive river channel width (W) from drainage basin area (A) for 1) channel width less than 100 m and drainage area smaller than 300 km2, 2) channel width less than 100 m and drainage area between 300 and 23 000 km2, and 3) channel width greater than 100 m and drainage area larger than 23 000 km2.

Table 2.
Table 3.

River channel length and surface area of the Ji-Paraná River network.

Table 3.
Table 4.

Average values of CO2 flux measured with the floating chamber at Ji-Paraná basin rivers.

Table 4.
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