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International audienceQuantifying suspended sediment exports from catchments and understanding suspended sediment dynamics within river networks is important, especially in areas draining erodible material that contributes to the siltation of downstream reservoirs and to the degradation of water quality. A one-year continuous monitoring study of water and sediment fluxes was conducted in three upland subcatchments (3.0, 9.3, and 12.0 km2) located within the Cointzio basin, in the central volcanic highlands of Mexico (Michoacán state). Two subcatchments generated high sediment exports (i.e., Huertitas, 900-1500 t km− 2 y− 1 and Potrerillos, 600-800 t km− 2 y− 1), whereas the third subcatchment was characterized by a much lower sediment yield (i.e., La Cortina, 30 t km− 2 y− 1). Such disparities in subcatchment behaviours were associated with the presence of severely gullied areas in Huertitas and Potrerillos rather than with rainfall erosivity indices. An adapted classification of hysteretic patterns between suspended sediment concentration (SSC) and discharge was proposed because 42% of flood events contributing to 70% of sediment export were not discriminated by the classical clockwise/anticlockwise typology. This new classification allowed the identification of relationships in the hydrosedimentary responses of successive floods. A stream transport capacity limit was also detected during hydrograph recession phases. Overall, hydrosedimentary processes proved to be seasonally dependent: sediment export was repeatedly limited by the stream transport capacity during the first part of the rainy season, whereas a channel minimum erosivity threshold was frequently reached at the end of the season

International audienceTropical regions are affected by intense soil erosion associated with deforestation, overgrazing, and cropping intensification. This land degradation leads to important on-site (e.g., decrease in soil fertility) and off-site (e.g., reservoir siltation and water pollution) impacts. This study determined the mean soil particle and sediment residence times in soils and rivers of three subcatchments (3-12 km2) with contrasted land uses (i.e., cropland, forests, and rangelands) draining to a reservoir located in highlands of the transvolcanic Mexican belt. Calculations were based on rainfall amount and river discharges as well as on fallout radionuclide measurements (Be-7, Cs-137, and Pb-210) conducted on rainfall precipitated samples, soil sampled in the catchments, and suspended sediment collected by automatic samplers in the river during most storms recorded throughout the 2009 rainy season. Calculations using a radionuclide two-box balance model showed that the mean residence time of particles in soils ranged between 5000 ± 1500 and 23,300 ± 7000 years. In contrast, sediment residence time in rivers was much shorter, fluctuating between 50 ± 30 and 200 ± 70 days. The shortest mean residence times were measured in a hilly catchment dominated by cropland and rangelands, whereas they were the longest in an undulating catchment dominated by forests and cropland. Calculation of the Be-7/excess-Pb-210 in both rainfall and sediment allowed gaining insight on sediment dynamics throughout the rainy season. The first heavy storms of the year exported the bulk of the sediment stock accumulated in the river channel during the previous year. Then, during the rainy season, the two steeper catchments dominated by cropland and rangelands reacted strongly to rainfall. Sediment was indeed eroded and exported from both catchments during single heavy storms on several occasions in 2009. In contrast, the agro-forested catchment with gentler slopes exported sediment at a constant and low rate throughout the rainy season. Overall, land cover and flood type clearly proved to exert more control on sediment export than slope steepness and rainfall erosivity. Our results show the priority of stabilising old gully systems to prevent their extension by regressive erosion to cropland and to concentrate the implementation of on-site erosion control measures in cropland and rangeland of the most reactive catchments

Carbon dioxide (CO2) evasion from inland waters is an important component of the global carbon cycle. However, it remains unknown how global change affects CO2 emissions over longer time scales. Here, we present seasonal and annual fluxes of CO2 emissions from streams, rivers, lakes, and reservoirs throughout China and quantify their changes over the past three decades. We found that the CO2 emissions declined from 138 ± 31 Tg C yr−1 in the 1980s to 98 ± 19 Tg C yr−1 in the 2010s. Our results suggest that this unexpected decrease was driven by a combination of environmental alterations, including massive conversion of free-flowing rivers to reservoirs and widespread implementation of reforestation programs. Meanwhile, we found increasing CO2 emissions from the Tibetan Plateau inland waters, likely attributable to increased terrestrial deliveries of organic carbon and expanded surface area due to climate change. We suggest that the CO2 emissions from Chinese inland waters have greatly offset the terrestrial carbon sink and are therefore a key component of China’s carbon budget.

Abstract.A major limitation to the assessment of catchment transit time (TT) stems from the use of stable isotopes or chloride as hydrological tracers, because these tracers are blind to older contributions. Yet, accurately capturing the TT of the old water fraction is essential, as is the assessment of its temporal variations under non-stationary catchment dynamics. In this study we used lumped convolution models to examine time series of tritium, stable isotopes and chloride in rainfall, streamwater and groundwater of a catchment located in subtropical Australia. Our objectives were to determine the different contributions to streamflow and their variations over time, and to understand the relationship between catchment TT and groundwater residence time. Stable isotopes and chloride provided consistent estimates of TT in the upstream part of the catchment. A young component to streamflow was identified that was partitioned into quickflow (mean TT ≈ 2 weeks) and discharge from the fractured igneous rocks forming the headwaters (mean TT ≈ 0.3 years). The use of tritium was beneficial for determining an older contribution to streamflow in the downstream area. The best fits between measured and modelled tritium activities were obtained for a mean TT of 16-25 years for this older groundwater component. This was significantly lower than the residence time calculated for groundwater in the alluvial aquifer feeding the stream downstream (≈ 76-102 years), emphasising the fact that water exiting the catchment and water stored in it had distinctive age distributions. When simulations were run separately on each tritium streamwater sample, the TT of old water fraction varied substantially over time, with values averaging 17 ± 6 years at low flow and 38 ± 15 years after major recharge events. This counterintuitive result was interpreted as the flushing out of deeper, older waters shortly after recharge by the resulting pressure wave propagation. Overall, this study shows the usefulness of collecting tritium data in streamwater to document short-term variations in the older component of the TT distribution. Our results also shed light on the complex relationships between stored water and water in transit, which are highly non-linear and remain poorly understood.

Carbon dioxide (CO2) emissions to the atmosphere from running waters are estimated to be four times larger than the total carbon (C) flux to the oceans. However, these fluxes remain poorly constrained because of substantial temporal variability in dissolved CO2 concentrations. Using a global compilation of high frequency CO2 measurements, we demonstrate that nocturnal CO2 emissions are consistently larger, by an average of 27% (0.9 g C m -2 d -1 ), than those estimated from diurnal concentrations alone. Canopy shading is the principal control on observed diel (24 hr) variation, suggesting this nocturnal increase arises from daytime fixation of dissolved inorganic C by photosynthesis. Because contemporary global estimates of CO2 emissions to the atmosphere from running waters (0.65 -1.8 Pg C yr -1 ) rely primarily on discrete measurements of dissolved CO2 obtained during the day, they substantially underpredict the magnitude of this important flux. Accounting for night-time CO2 elevates global estimates of emissions from running waters to the atmosphere by 0.20-0.55 Pg C yr -1 .Carbon dioxide (CO2) emission from inland waters to the atmosphere is a major flux in the global carbon (C) cycle, and four-fold larger than the lateral C export to oceans 1 . Streams and rivers are hotspots for this flux, accounting for ~85% of inland water CO2 emissions despite covering <20% of the freshwater surface area 2 . Despite this importance, the magnitude of global CO2 emissions from streams and rivers remains highly uncertain with estimates revised upwards over the past decade from 0.6 to 3.48 Pg C yr -1 (3,4) . Changes to this estimate follow improvements in the spatial resolution for upscaling emissions 2,5 , as well as new studies from previously underrepresented areas such as the Congo 6 , Amazon 7 , and global mountains 8 . Further refinements have emerged from considering temporal variability in CO2 emission rates 9 . However, despite recent studies showing dramatic day-night changes in stream and river water CO2 concentrations 10-14 the significance of systematic sub-daily variation on overall CO2 emissions remains unexplored.Diurnal cycles in solar radiation impose a well-known periodicity on stream biogeochemical processes, creating diel (i.e., 24-hr period lengths) patterns for many solutes and gases, including nutrients, dissolved organic matter, and dissolved oxygen (O2) 15 . Indeed, diel variation in O2 arising from photosynthetic activity is the signal from which whole-system metabolic fluxes are estimated 16 . Photosynthetic production of O2 is stoichiometrically linked to the day-time assimilation of dissolved inorganic carbon (principally bicarbonate and dissolved CO2), lowering CO2 concentrations during the day. The resulting diel variation, with higher night-time CO2 concentrations when respiration reactions dominate, implies increased emissions at night. Despite the obvious connection between photosynthesis and CO2 consumption, the implications for total aquatic CO2 emissions has been neglected, most likely ...

Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. a According to FAO (2006). b Derived from the analysis of aerial photographs. FINGERPRINTING SEDIMENT SOURCES IN MEXICO

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet–dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣFaq). Here, we determined the NLCB of a lowland catchment (~140 km2) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire‐related C emissions and ΣFaq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km−2 year−1), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (−56 Mg C km−2 year−1) and ΣFaq (−28 Mg C km−2 year−1) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣFaq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events—up to 39% of annual ΣFaq was exported in one event. We hypothesize that wetter and hotter conditions in the wet–dry tropics in the future will increase ΣFaq and fire emissions, potentially further reducing the current C sink in the region. More long‐term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet–dry tropics where our result indicating a significant C sink may not hold.