Global carbon dioxide efflux from rivers enhanced by high nocturnal emissions

Carbon dioxide (CO2) emissions to the atmosphere from running waters are estimated to be four times greater than the total carbon (C) flux to the oceans. However, these fluxes remain poorly constrained because of substantial spatial and temporal variability in dissolved CO2 concentrations. Using a global compilation of high-frequency CO2 measurements, we demonstrate that nocturnal CO2 emissions are on average 27% (0.9 gC m−2 d−1) greater than those estimated from diurnal concentrations alone. Constraints on light availability due to canopy shading or water colour are the principal controls on observed diel (24 hour) variation, suggesting this nocturnal increase arises from daytime fixation of CO2 by photosynthesis. Because current global estimates of CO2 emissions to the atmosphere from running waters (0.65–1.8 PgC yr−1) rely primarily on discrete measurements of dissolved CO2 obtained during the day, they substantially underestimate the magnitude of this flux. Accounting for night-time CO2 emissions may elevate global estimates from running waters to the atmosphere by 0.20–0.55 PgC yr−1

Correlation and spectral analyses to assess the response of a shallow aquifer to low and high frequency rainfall fluctuations

International audienceSummary With the predicted acceleration of drought and flood frequency worldwide, it is critical to build knowledge on how shallow groundwater systems respond to rainfall variability. In this study, we characterise the hydrodynamic response of an agricultural catchment located in southeast Queensland, Australia, to the low and high frequency fluctuations in precipitation that occurred in the past 25years. Strong interannual variations in the precipitation input affected surface water flow more substantially than groundwater levels (GWLs). There was also a likely influence of groundwater abstraction on stream baseflow and GWLs, although it was difficult to quantify. Several low frequency oscillations were apparent in the GWL records, particularly in the upstream section of the aquifer, which were neither detectable in rainfall nor in discharge records. Statistically significant episodes of coherence were found at the 2\textendash4-year band between the Niño3.4 index and GWLs for those upstream bores, especially during the 1995\textendash2000 interval, which may be related to a strong La Niña event. In the downstream section of the catchment, higher groundwater persistence probably led to higher filtering of these low frequency oscillations. This paper also proposes a methodology for assessing the dynamic response time of GWLs in shallow aquifer systems to the precipitation input. Response time in a downstream borehole was highly variable temporally, ranging from 11 to 121days. Rainfall amount was found to significantly affect the short-term dynamics of response time, with elevated rainfall leading to a decreased response time. Annually averaged response time was correlated with the annual number of days with rainfall \textgreater30mm, which was interpreted as the potential for diffuse recharge. Higher recharge potential induced longer response times, probably because of the larger amplitude in GWL variations

Drivers of erosion and suspended sediment transport in three headwater catchments of the Mexican Central Highlands

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

Estimation of sediment residence times in subtropical highland catchments of central Mexico combining river gauging and fallout radionuclides

International audienceSubtropical regions of the world 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 impacts (e.g. reservoir sedimentation, water pollution). This study determined the mean sediment residence times in soils and rivers of three catchments (3-12 km$^2$) with contrasted land uses (i.e. cropland, forests, rangelands, extended gully networks) located in highlands of the transvolcanic belt of central Mexico. Calculations were based on rainfall and river gauging as well as on fallout radionuclide measurements (Be-7, Cs-137, Pb-210). Atmospheric deposition of Be-7 and Pb-210 was estimated based on the analysis of rainfall precipitated samples. Rainfall samples were collected all throughout the rainy season in order to take account of the temporal variations of the radionuclide fluxes. Furthermore, sampling of suspended sediment was conducted at the outlet of each catchment during most of the storms that occurred throughout the 2009 rainy season. Be-7, Cs-137 and Pb-210 concentrations of this sediment were determined by gamma-spectrometry. A two-box balance model was then used to estimate the sediment residence time and the inventory of radionuclides in the three selected catchments. This model subdivided each catchment into two boxes: (i) a "soil-box" characterised by low transport velocities and hence long radionuclide residence times and (ii) a "river-box" covering the river surface and its surroundings characterised by quicker exchanges and shorter radionuclide residence times. Input and output fluxes of sediment and radionuclides were taken into account in each box. Radioactive decay during the residence time of sediment was also considered. The mean residence time of sediment in soils ranged between 13,300-28,500 years. In contrast, sediment residence time in rivers was much shorter, fluctuating between 28 and 393 days. The shortest residence time (∼ 3 months) was measured in a catchment dominated by rangelands, whereas it was the longest (∼ 13 months) in a catchment dominated by cropland and extensive networks of 'historical' gullies. Our results support the hypothesis of a sediment transfer through a succession of deposition and resuspension steps. They also show the priority of stabilising old gully systems and to implement on-site erosion control measures in subtropical regions

Hydrological processes in tropical Australia: Historical perspective and the need for a catchment observatory network to address future development

International audienceStudy region: Tropical Australia. Study focus: Streams and rivers of the Australian tropics have been the subject of substantial hydrological process research spanning the last 50 years. In this review, we highlight initial efforts to understand the hydrological response of forested ecosystems in the humid tropics, and how this has been more recently followed by work in savannas of the seasonal tropics. We describe recent findings from modelling and tracer studies and derive a framework of dominant hydrological processes for the region. We also detail five critical knowledge gaps that will require further attention with climate change and ongoing interest in development in the region. New hydrological insights for the region: We outline the diversity of runoff generation mechanisms that prevail in the region and emphasise the role of connected wetlands and floodplains in catchment response. We discuss the prominence of focused, episodic recharge in the replenishment of groundwater stores across the region. We also review how climate change and potential water resource development projects may alter the hydrology of northern Australian catchments. Future research should focus on improving our physical understanding of key hydrological processes, as well as anticipate the likely effects of development and climate change on these processes. Intensive and long-term studies of experimental observatories, which capture the diversity in landscapes and climates of the region, will help frame sustainable water development policies in northern Australia

Land transformation in tropical savannas preferentially decomposes newly added biomass, whether C(3)or C(4)derived

As tropical savannas are undergoing rapid conversion to other land uses, native C-3-C(4)vegetation mixtures are often transformed to C-3- or C-4-dominant systems, resulting in poorly understood changes to the soil carbon (C) cycle. Conventional models of the soil C cycle are based on assumptions that more labile components of the heterogenous soil organic C (SOC) pool decompose at faster rates. Meanwhile, previous work has suggested that the C-4-derived component of SOC is more labile than C-3-derived SOC. Here we report on long-term (18 months) soil incubations from native and transformed tropical savannas of northern Australia. We test the hypothesis that, regardless of the type of land conversion, the C(4)component of SOC will be preferentially decomposed. We measured changes in the SOC and pyrogenic carbon (PyC) pools, as well as the carbon isotope composition of SOC, PyC and respired CO2, from 63 soil cores collected intact from different land use change scenarios. Our results show that land use change had no consistent effect on the size of the SOC pool, but strong effects on SOC decomposition rates, with slower decomposition rates at C-4-invaded sites. While we confirm that native savanna soils preferentially decomposed C-4-derived SOC, we also show that transformed savanna soils preferentially decomposed the newly added pool of labile SOC, regardless of whether it was C-4-derived (grass) or C-3-derived (forestry) biomass. Furthermore, we provide evidence that in these fire-prone landscapes, the nature of the PyC pool can shed light on past vegetation composition: while the PyC pool in C-4-dominant sites was mainly derived from C(3)biomass, PyC in C3-dominant sites and native savannas was mainly derived from C(4)biomass. We develop a framework to systematically assess the effects of recent land use change vs. prior vegetation composition

Spatial and temporal dynamics of sediment in contrasted mountainous watersheds (Mexican transverse volcanic axis and French Southern Alps) combining river gauging, elemental geochemistry and fallout radionuclides

International audienceIn mountainous environments, an excessive sediment supply to the rivers typically leads to an increase in water turbidity, pollution and a rapid filling of reservoirs. This situation is particularly problematic in regions where reservoirs are used to provide drinking water to large cities (e.g. in central Mexico) or where stream water is used to run hydroelectric power plants (e.g. in the French Southern Alps). Sediment source areas need first to be delineated and sediment fluxes between hillslopes and the river system must be better understood to implement efficient erosion control measures. In this context, the STREAMS (« Sediment Transport and Erosion Across MountainS ») project funded by the French National Research Agency (ANR) aims at understanding the spatial and temporal dynamics of sediment at the scale of mountainous watersheds (between 500 – 1000 km2) located in contrasted environments. This 4-years study is carried out simultaneously in a volcanic watershed located in the Mexican transverse volcanic axis undergoing a sub-humid tropical climate, as well as in a sedimentary watershed of the French Southern Alps undergoing a transitional climate with Mediterranean and continental influences. One of the main specificities of this project consists in combining traditional monitoring techniques (i.e. installation of river gauges and sediment samplers in several sub-catchments) and sediment fingerprinting using elemental geochemistry (measured by Instrumental Neutron Activation Analysis – INAA – and Inductively Coupled Plasma - Mass Spectrometry – ICP-MS) and fallout radionuclides (measured by gamma spectrometry). In the French watershed, geochemical analysis allows outlining different sediment sources (e.g. the contribution of calcareous vs. marl-covered sub-catchments). Radionuclide ratios (e.g. Cs-137/Be-7) allow identifying the dominant erosion processes occurring within the watershed. Areas mostly affected by gully erosion, rill or sheet erosion have been delineated. Furthermore, the measurement of radionuclide content in suspended sediment after the snowmelt suggests that most of this sediment consists in resuspended material rather than on newly eroded soil. In the Mexican watershed, a different contribution of andosols and acrisols to erosion is suspected. Overall, the bulk of erosion is generated by rather small areas of the watershed. In this region characterised by a succession of wet and dry seasons, the Be-7 content in rainfall and sediment has been measured at the scale of a 2.5-km2 sub-watershed in order to better understand the erosion transfer between hillslopes and rivers during the wet season. This outlines the contribution of individual storms to seasonal erosion. Overall, this study brings important insights about sediment sources and fluxes within these watersheds located in contrasted environments. A further step consists in comparing experimental results with model outputs, and to evaluate the impact of on-going erosion mitigation measures