Cavity Ring Down Spectrometry

Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, υ 18 O and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining nested catchments of 6-5700 ha) from 2000 to 2008 in the semi-arid agricultural Missouri Flat Creek (MFC) watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline υ 18 O value of 14Ð7‰ (VSMOW) year round. Winter precipitation accounted for 67% of total annual precipitation and was found to dominate streamflow, tile drainage, and groundwater recharge. 'Old' and 'new' water partitioning in streamflow were not identifiable using υ 18 O, but seasonal shifts of nitrate-corrected EC suggest that deep soil pathways primarily generated summer streamflow (mean EC 250 µS/cm) while shallow soil pathways dominated streamflow generation during winter (EC declining as low as 100 µS/cm). Using summer isotopic and EC excursions from tile drainage in larger catchment (4700-5700 ha) stream waters, summer in-stream evaporation fractions were estimated to be from 20% to 40%, with the greatest evaporation occurring from August to October. Seasonal watershed and environmental tracer dynamics in the MFC watershed appeared to be similar to those at larger watershed scales in the Palouse River basin. A 0Ð9‰ enrichment, in shallow groundwater drained to streams (tile drainage and soil seepage), of υ 18 O values from 2000 to 2008 may be evidence of altered precipitation conditions due to the Pacific Decadal Oscillation (PDO) in the Inland Northwest.

The characteristics of the CISE-LOCEAN seawater isotope dataset (δ 18 O, δ 2 H, referred to as δD) are presented (https://doi.org/10.17882/71186; Waterisotopes-CISE-LOCEAN, 2021). This dataset covers the time period from 1998 to 2021 and currently includes close to 8000 data entries, all with δ 18 O, three-quarters of them also with δD, associated with a date stamp, space stamp, and usually a salinity measurement. Until 2010, samples were analyzed by isotopic ratio mass spectrometry and since then mostly by cavity ring-down spectroscopy (CRDS). Instrumental uncertainty in this dataset is usually as low as 0.03 ‰ for δ 18 O and 0.15 ‰ for δD. An additional uncertainty is related to the isotopic composition of the in-house standards that are used to convert data to the Vienna Standard Mean Ocean Water (VSMOW) scale. Different comparisons suggest that since 2010 the latter have remained within at most 0.03 ‰ for δ 18 O and 0.20 ‰ for δD. Therefore, combining the two uncertainties suggests a standard deviation of at most 0.05 ‰ for δ 18 O and 0.25 ‰ for δD. For some samples, we find that there has been evaporation during collection and storage, requiring adjustment of the isotopic data produced by CRDS, based on d-excess (δD − 8×δ 18 O). This adjustment adds an uncertainty in the respective data of roughly 0.05 ‰ for δ 18 O and 0.10 ‰ for δD. This issue of conservation of samples is certainly a strong source of quality loss for parts of the database, and "small" effects may have remained undetected.

Induction module cavity ring-down spectroscopy (IM-CRDS) has been proposed as a rapid and cost-effective alternative to cryogenic vacuum distillation (CVD) and isotope ratio mass spectrometry (IRMS) for the mesurement of δ 18 O and δ 2 H values in matrix-bound waters. In the current study, we characterized the performance of IM-CRDS relative to CVD and IRMS and investigated the mechanisms responsible for differences between the methods. METHODS: We collected a set of 75 soil, stem, and leaf water samples, and measured the δ 18 O and δ 2 H values of each sample with four techniques: CVD and IRMS, CVD and CRDS, CVD and IM-CRDS, and IM-CRDS alone. We then calculated the isotopic errors for each of the three CRDS methods relative to CVD and IRMS, and analyzed the relationships among these errors and suites of diagnostic spectral parameters that are indicative of organic contamination. RESULTS: The IM-CRDS technique accurately assessed the δ 18 O and δ 2 H values of pure waters, but exhibited progressively increasing errors for soil waters, stem waters, and leaf waters. For soils, the errors were attributable to subsampling of isotopically heterogeneous source material, whereas for stems and leaves, they were attributable to spectral interference. Unexpectedly, the magnitude of spectral interference was higher for the solid samples analyzed directly via IM-CRDS than for those originally extracted via CVD and then analyzed by IM-CRDS. CONCLUSIONS: There are many types of matrix-bound water samples for which IM-CRDS measurements include significant errors from spectral interference. As a result, spectral analysis and validation should be incorporated into IM-CRDS post-processing procedures. In the future, IM-CRDS performance could be improved through: (i)

Induction module cavity ring-down spectroscopy (IM-CRDS) has been proposed as a rapid and cost-effective alternative to cryogenic vacuum distillation (CVD) and isotope ratio mass spectrometry (IRMS) for the mesurement of δ 18 O and δ 2 H values in matrix-bound waters. In the current study, we characterized the performance of IM-CRDS relative to CVD and IRMS and investigated the mechanisms responsible for differences between the methods. METHODS: We collected a set of 75 soil, stem, and leaf water samples, and measured the δ 18 O and δ 2 H values of each sample with four techniques: CVD and IRMS, CVD and CRDS, CVD and IM-CRDS, and IM-CRDS alone. We then calculated the isotopic errors for each of the three CRDS methods relative to CVD and IRMS, and analyzed the relationships among these errors and suites of diagnostic spectral parameters that are indicative of organic contamination. RESULTS: The IM-CRDS technique accurately assessed the δ 18 O and δ 2 H values of pure waters, but exhibited progressively increasing errors for soil waters, stem waters, and leaf waters. For soils, the errors were attributable to subsampling of isotopically heterogeneous source material, whereas for stems and leaves, they were attributable to spectral interference. Unexpectedly, the magnitude of spectral interference was higher for the solid samples analyzed directly via IM-CRDS than for those originally extracted via CVD and then analyzed by IM-CRDS. CONCLUSIONS: There are many types of matrix-bound water samples for which IM-CRDS measurements include significant errors from spectral interference. As a result, spectral analysis and validation should be incorporated into IM-CRDS post-processing procedures. In the future, IM-CRDS performance could be improved through: (i)

The Quequén Grande River (QGR) is a large catchment (10 000 km 2) in the Pampa Plain in Argentina. From November 2004 to April 2013, a hydrochemical and stable isotopes monitoring program was conducted, which included three sampling stations of monthly composite precipitation, weekly samples in two sites along the river and several groundwater samples. A standard data interpretation was initially performed applying standard statistics, Piper diagrams and δ 18 O versus δ 2 H diagrams. The time evolution of the values of δ 18 O in precipitation and streamwater were also determined. The integration of hydrogeochemistry and stable isotopes data indicates the existence of three main components of streamflow: (i) baseflow characterized by electrical conductivity (EC) from 1200 to 1800 μs/cm and an isotope composition quite constant around δ 18 O À5.3‰ and δ 2 H À33.8‰. Water age for groundwater contribution is typically around 30 to 40 years using chlorofluorocarbons; (ii) direct runoff composed of channel interception and overland flow, which is of low EC in the order of 50 to 100 μs/cm, and a highly variable isotopic composition; and (iii) translatory flow (pre-event water that is stored within the subsoil) with an intermediate EC and isotopic composition close to that of the weighted average composition of precipitation. The hydrochemical and stable isotopic data allow the differentiation between baseflow and direct runoff. In addition to this, chlorofluorocarbon dating is a useful tool in assessing the dominance of baseflow in a stream. The data lead to a conceptual model in which an intermediate flow system, with mean residence time (MRT) of around 35 years, discharges into the drainage network. A regional flow system (MRT > 50 years) discharges to the ocean. It is concluded that in this large plain catchment streamflow separation, only two components can be applied in: (i) short storm precipitation events having a high sampling frequency and (ii) during long dry periods when pre-event soil water is not released.

Stable isotopic composition (δ 18 O and δD of water, δ 13 C DIC) of the water column in the open ocean is related to the origin of water masses. Due to the recent increase of paleoceanographic studies on continental shelves, it is also important to understand their distribution and variability in those systems. To examine the influence of continental shelves internal processes on isotopic composition of water masses, we present data of stable isotopes and phosphate content from a western boundary upwelling system located on the Southeastern Brazilian coast and compare them with offshore observations. High mixing of the main water masses (SSW, TW and SACW) was observed in the majority of the samples collected during different seasons in 2011 and 2012. A mixing triangle approach was used to separate the water masses contribution and characterize their isotopic composition. In addition, an isotopic three end-member model was established, proposing it as a paleoceanographic tool to reconstruct relative contribution of these water masses in sediment records. Variations of δ 18 O values are linked to oceanographic dynamics, mixing, continental runoff and upwelling processes on the shelf. Differently the δ 13 C DIC variations in the middle and inner parts of the shelf are related to the productivity of the upwelling system. Seasonal variability of the δ 13 C DIC values may be also related to changes in the upwelling intensity.

Diel (24 hour) cycles of dissolved inorganic carbon and dissolved metals have rarely been studied in concert in coal mine drainage systems containing high CO2. Diel samples were collected from two locations at a site with elevated CO2; the locations differed in their CO2 concentrations and the amount of vegetation present. Field data and samples were collected from both locations during March, May, and July 2014. To determine if the parameters cycled in a diel fashion, the data were fit using a cosine model and the goodness of fit was determined using an f-test statistic. Overall, 15 of 20 selected parameters could be fit using the cosine model with an f-test statistic p≤0.01. Parameters found to have diel cycling patterns were pH, temperature, dissolved oxygen, CO2, inorganic carbon, δ 13 CDIC, Fe(II), FeTOT, Y, Zn, K, Al, Mn, As, and Ni. More parameters had a diel behavior according to the model fit in the downstream location (which is in a wetland) and as the seasons progressed. When the same model and analysis were applied to data from other sites and studies, similar phasing was observed. Metals concentrations were approximately 200% lower at this study site than in 2007 but stronger diel cycles were present in 2007. Likely mechanisms driving diel behavior at this site are a combination of solaractivated process such as pH; temperature-controlled sorption reactions; the photosynthesiscellular respiration cycle; degassing of CO2; and the residence time of the water. Future studies are needed to further separate and quantify these mechanisms on diel behavior as well as to investigate other possible mechanisms like hyporheic exchange and plant uptake. iii ACKNOWLEDGMENTS This work was performed as part of the National Energy and Technology Laboratory's Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, under RES contract DE-FE0001000. Specifically, I would like to thank my advisor, Dr. Dorothy Vesper, for training me in field techniques and her patience and guidance during the duration of the project. I would also like to thank Drs. Louis McDonald, Helen Lang, Hank Edenborn, and Jon Martin for their input and guidance during this project as well as Dr. Marie Kurz for her statistical instructions.

In the last years great attention has been paid to graphene-based devices for optoelectronic applications such as photodetection. In this work, we report on Graphene Field Effect Transistors (GFETs) photoelectrical response due to the photo-transistor effect. Photoelectrical measurements were performed using a 1.55 μm erbium fiber laser. Optical measurements as a function of both the incident laser power and the DC bias of the fabricated devices have been carried out and show that photocurrent increases with the power of the IR beam illuminating the sample.

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High-resolution hydrochemical data from five spring-fed ponds are presented to demonstrate the effect of different land uses and aquatic biological processes on the carbon cycle at a karst-analog test site. The results show that hydrochemical parameters including pH and the concentrations of HCO 3 − , Ca 2+ , NO 3 − , partial pressures of CO 2 (pCO 2) and dissolved O 2 (DO) as well as carbon isotopic compositions (δ 13 C) of HCO 3 in the pond water all displayed distinct diurnal variations, while those of the spring water itself were rather stable. The coupled dynamic behaviors of pCO 2 , DO and NO 3 − indicate a significant influence from the metabolism of submerged plants in the ponds. In the afternoon, when photosynthesis is the strongest, the pCO 2 of the five pond waters was lower even than that of the ambient atmosphere, demonstrating the existence of a "biological carbon pumping (BCP) effect", similar to that in the oceans. It was determined that, in October (autumn), the BCP fluxes in the five spring-fed ponds were 156 ± 51 t C km −2 a −1 in P1 (Pond 1adjoining a bare rock shore), 239 ± 83 t C km −2 a −1 in P2 (adjoining uncultivated soil), 414 ± 139 t C km −2 a −1 in P3 (adjoining land cultivated with corn), 493 ± 165 t C km −2 a −1 in P4 (adjoining grassland) and 399 ± 124 t C km −2 a −1 of P5 (adjoining brushland), indicating the potentially significant role of aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink. In addition, by comparing the DIC concentrations and fluxes of DIC transformed into autochthonous organic matter (AOC) in the five ponds, the so-called "DIC fertilization effect" was found in which more AOC is produced in pond waters with higher concentrations of DIC. This implies that the carbon cycle driven by aquatic biological processes can be regulated by changing land use and cover, the latter determining the DIC concentrations. Further, the rock weathering-related carbon sink is underestimated if one only considers the DIC component in surface waters instead of both DIC and AOC.

The hydrologic cycle plays an important role in carbon cycling, due to the coupling of vapour release and CO 2 uptake during photosynthesis. This coupling, expressed as water use efficiency of transpiration ratio, can provide an inexpensive alternative for estimating the Net Primary Productivity (NPP) of terrestrial ecosystem. Stable isotopes of hydrogen and oxygen and long-term hydrologic and meteorological data together with stoichiometric relations of water and carbon are used to constrain water and carbon balances for the Volta River watershed. Soils annually respire 0.199 Pg C, and the balance of these fluxes or Net Ecosystem Productivity (NEP) is +0.029 Pg C Yr -1 , implying an annual flux of CO 2 to the atmosphere. Annually, the Volta river watershed receives about 380 km 3 of rainfall. Approximately 50% of this volume of water is returned to the atmosphere through plant transpiration. Associated with annual transpiration flux is a carbon flux of 0.170 × 10 15 g C yr -1 or 428 g C m -2 yr -1 from the terrestrial ecosystem. Modeled estimates of heterotrophic soil respiration exceed slightly the NPP estimate, implying that carbon flux to and from the Volta river watershed is close to being in balance or the watershed is a small annual source of carbon dioxide to the atmosphere. In addition to terrestrial carbon flux, the balance of photosynthesis and respiration in Volta Lake is also examined. The lake was found to release carbon dioxide to the atmosphere, although the magnitude of the flux is smaller than that of the terrestrial ecosystem.

The calculation of loss is vital for design flood estimation models and in order to estimate continuing loss (CL), proportional loss (PL) and volumetric runoff coefficient, the surface runoff has to be separated from the total given in a stream flow hydrograph. To obtain the volume of surface runoff from the streamflow hydrograph, baseflow separation becomes necessary and in this paper a few base flow separation methods are explored and an appropriate method selected to assess to impact of baseflow on loss estimates. The process of separation requires a base flow separation coefficient and this coefficient () is selected from individual study catchments from 3 to 5 rainfall streamflow events of the same catchments based on sensitivity analysis. The selected value of individual catchments is then applied to other rainfall streamflow events of a given catchment. It has been observed that a small degree of error in the selection of value does not seem to affect the estimates of the CL, PL or runoff coefficient. Hence, the more practical base flow separation method used in this paper may be applied to other rural catchments for baseflow separation in design loss studies.

Tropical rivers emit large amounts of carbon dioxide (CO 2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and welldrained ecosystems to rivers. In a first-order sub-catchment (0.6 km 2) of the Nyong watershed (Cameroon 27 800 km 2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO 2 m −2 yr −1) compared to stream 1 (286 ± 228 gC-CO 2 m −2 yr −1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr −1) and Published by Copernicus Publications on behalf of the European Geosciences Union. 138 M. Moustapha et al.: Partitioning carbon sources to a tropical stream wetland (4.0 ± 1.5 MgC yr −1) to the stream represented 3 %-5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr −1) and 73 % (33.0 ± 12.4 MgC yr −1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO 2 degassing from the entire river network was 652 ± 161 GgC-CO 2 yr −1 (23.4 ± 5.8 MgC CO 2 km −2 yr −1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO 2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO 2 m −2 yr −1 , which implied that only ∼ 10 % of the CO 2 degassing at the water-air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr −1 (10.3 ± 5.8 MgC km −2 yr −1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO 2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the riverwetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

Cavity ring-down spectrometers, with automated sampling interfaces, were deployed to allow measurements of water isotopes (δ 18 O, δD) and dissolved inorganic carbon (δ 13 C DIC) stable isotope ratios at high temporal resolution along a transect from New Zealand to the Antarctic continental shelf. Measurements every 10 min for δ 18 O and δD, 15 min for DIC yielded 2499 and 2289 discrete measurements respectively. High resolution data enabled the delineation of water mass boundaries as well as revealing insights into surface hydrological and biological processes. δ 18 O, δD, and δ 13 C DIC decreased southwards, dropping by approximately 1.0‰, 7.0‰, and 0.5‰, respectively. Though the decline in δ 13 C DIC with latitude was generally linear, the drop in δ 18 O and δD was punctuated by areas of rapid, significant change corresponding to the SubTropical , Sub-Antarctic and Polar Fronts. North of the Sub-Antarctic Front (approx. 54.5°S) the dominant control on water and DIC isotopes was the precipitation-evaporation balance and the contribution of upwelling waters, respectively. Further south, in close proximity to the sea ice and on the Antarctic shelf, water isotope values were more variable and predominantly influenced by the melting/freezing of sea-ice coupled to inputs from glacial/snow melt water. Local increases in δ 13 C DIC were likely due to photosynthetic enrichment of the DIC pool. Using this new instrumentation has provided one of the most comprehensive oceanic transect data sets yet achieved and illustrates the potential of these methods to delineate discrete water masses and advance our knowledge of both water and inorganic carbon cycling processes in the ocean. This methodology, combining high-resolution isotopic measurements with hydrographic data, has significant benefits in modelling water mixing in locations with multiple sources and controlling processes.

Dewatering associated with mining below water table to achieve dry mining conditions may exert significant pressure on water balance in terms of lowering the water table and change in the dynamics of interactions between surface water and groundwater. The discharge of surplus mine water into ephemeral streams may also affect the water balance, by elevating groundwater levels and altering the exchange rate between streams and underlying aquifers. However, it is unclear whether volumes and recharge processes are within the range of natural variability. Here, we present a case study of an ephemeral creek in the semi-arid Hamersley Basin of northwest Australia that has received continuous mine discharge for more than six years. We used a numerical model coupled with repeated measurements of water levels, chloride concentrations and the hydrogen and oxygen stable isotope composition (δ2H and δ18O) to estimate longitudinal evapotranspiration and recharge rates along a 27 km length of Weeli Wolli Creek. We found that chloride increased from 73 to 120 mg/L across this length, while δ18O increased from-8.2‰ to-7.00‰. Groundwater is directly connected to the creek for the first 13 km and recharge rates are negligible. Below this point, the creek flows over a highly permeable aquifer and water loss by recharge increases to a maximum rate of 4.4 mm/d, which accounts for ~65% of the total water discharged to the creek. Evapotranspiration losses account for the remaining ~35%. The calculated recharge from continuous flow due to surplus water discharge is similar to that measured for rainfall-driven flood events along the creek. Groundwater under the disconnected section of the creek is characterised by a much lower Cl concentration and more depleted δ18O value than mining discharge water but is similar to flood water generated by large episodic rainfall events. Our results suggest that the impact of recharge from continuous flow on the water balance of the creek has not extended beyond 27 km from the discharge point. Our approach using a combination of hydrochemical and isotope methods coupled with classical surface flow hydraulic modelling allowed evaluation of components of the water budget otherwise not possible in a highly dynamic system that is mainly driven by infrequent but large episodic floods.

Dissolved inorganic carbon Carbon isotopes Carbon cycle Dissolved organic carbon Atmospheric CO 2 Biogenic CO 2 The Nyong watershed, with an area of 27 800 km 2 and a mean annual discharge of 390 m 3 s − 1 , is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km 2) to the Nyong at Edea (24 500 km 2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ 13 C DIC) and DOC (δ 13 C DOC). We estimated terrestrial net ecosystem productivity in the Nyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis:~7 920 000 t C year − 1 (300 g C m − 2 year − 1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux from the basin on an annual basis. This includes a minimum CO 2 outgassing of 1487 g C m − 2 year − 1 , comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC.

Dissolved inorganic carbon Carbon isotopes Carbon cycle Dissolved organic carbon Atmospheric CO 2 Biogenic CO 2 The Nyong watershed, with an area of 27 800 km 2 and a mean annual discharge of 390 m 3 s − 1 , is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km 2) to the Nyong at Edea (24 500 km 2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ 13 C DIC) and DOC (δ 13 C DOC). We estimated terrestrial net ecosystem productivity in the Nyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis:~7 920 000 t C year − 1 (300 g C m − 2 year − 1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux from the basin on an annual basis. This includes a minimum CO 2 outgassing of 1487 g C m − 2 year − 1 , comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC.

The science that underpins our knowledge and understanding of Isotope-Based Hydrograph separation (IHS) has gained grounds, over the last few decades, in the identification of streamflow sources. However, challenges still exist in identifying appropriate tracers and the right combination of end-members for the IHS process. In a two-component IHS analysis, the application of the dual isotopes tracers, δ 18 O and (or) δ 2 H, is regarded as the simplest method. We undertook an IHS study within a nested system of eight Prairie watersheds located in South central Manitoba, Canada. The work evaluated about 17,000 results emanating from the application of a combination of two potential tracers (δ 18 O and δ 2 H) and eight each of potential "old" and "new" water end-members in a two-component IHS process. The outcome showed occurrences of many mathematically possible but hydrologically unacceptable IHS results. The observation was particularly predominant within relatively larger perennial sub-catchments of the watershed. It is also shown that inter-site sub-catchment isotopic end-member transferability is possible within watersheds of similar physio-hydrographic characteristics. We suggest that a careful evaluation of the physio-hydrographic characteristics of catchments be considered in IHS studies in addition to the recommended guidelines in the selection of tracers and end-members.

Variations in temperature, photosynthesis and respiration can force daily variations in pH, DO and DIC in surface water, potentially driving calcite precipitation or dissolution of calcium carbonate. Diel cycles of hydrochemistry and d 13 C DIC were measured at high time-resolution (1 h) to assess the relative magnitudes of biological and geochemical controls on carbonate chemistry and carbon cycling in a springfed pool with flourishing submerged plants in Chongqing, SW China under sunny weather. Results show that there were no diurnal variations in the physical and chemical parameters of the Shuifang spring water. However, during the daytime periods, SC, Ca 2+ , alkalinity, NO À 3 and pCO 2 in the pool water decreased to less than those in the spring water, while pH, DO and d 13 C DIC in the pool water became greater than those in the spring water. Conversely, during nighttime periods, pool water SC, Ca 2+ , alkalinity, NO À 3 and pCO 2 returned to or even became greater than the spring water, while pH, DO and d 13 C DIC decreased to less than the spring water. This work shows that photosynthesis and respiration of subaquatic communities are the dominant processes influencing the observed diel variations of hydrochemistry in karst spring-fed pool water. During the daytime, a simultaneous increase of d 13 C DIC and DO, and decrease in DIC indicates that photosynthesis was the primary control on hydrochemistry of the pool water. Conversely, the water remained saturated with respect to calcite (SIc ranging from 0.04 to 0.15) and d 13 C DIC values decreased at nighttime, indicating that respiration of the subaquatic community had a dominant influence over calcite dissolution and outgassing in the pool water. The total amount of DIC loss was estimated to be about 110,785 mmol/day which represented about 1.33 kg C/ day. More specifically, the amount of DIC loss through carbonate precipitation was about 38,775 mmol/day (0.47 kg C/day), whereas photosynthetic uptake was about 60,700 mmol/day (0.86 kg C/day) in the pool water. On one hand, the loss of C through consumption of CO 2 during carbonate precipitation was balanced by carbonate dissolution. On the other hand, the loss of C by photosynthetic uptake was diverted into organic C and constitutes a relatively long-term natural carbon sink in the karst system. In addition, the diurnal variations of NO À 3 and pH in the pool water, caused by photosynthesis and respiration, indicated that biogeochemical processes impact water quality on a daily timescale.

An understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the endmember mixing analysis (EMMA) technique to investigate source water contributions and hydrologic flow paths of the 264 km 2 Boulder Creek Watershed, which drains the Colorado Front Range, USA. Four conservative hydrochemical tracers were used to describe this watershed as a three end-member system, and tracer concentration reconstruction suggested that the application of EMMA was robust. On average from 2009 to 2011, snowmelt and rain water from the subalpine zone and groundwater sampled from the upper montane zone contributed 54%, 22%, and 24% of the annual streamflow, respectively. These values demonstrate increased rain water and decreased snow water contributions to streamflow relative to area-weighted mean values derived from previous work at the headwater scale. Young water (2.3 ± 0.8 months) fractions of streamflow decreased from 18-22% in the alpine catchment to 8-10% in the lower elevation catchments and the watershed outlet with implications for subsurface storage and hydrological connectivity. These results contribute to a process-based understanding of the seasonal source water composition of a mesoscale watershed that can be used to extrapolate headwater streamflow generation predictions to larger spatial scales. 50

Water is one of the critical resources in the agenda of promoting sustainability. Yet, among all the observed natural and anthropogenic adversities, waterrelated disasters appear as the most recurrent which hinder sustainable socioeconomic development goals. Climate change, urban expansion, deforestation and increase in population are thought to be the main factors of water-related disasters. Considering those factors, understanding the hydrologic balance of watersheds and the impact that nature and humanity impose on regional ecosystem becomes one of the important research priorities. This paper reviews the applications of stable isotopes in hydrological studies for catchment management. Advancements in isotope research (origin, flow paths, residence times and water budget), has led to new frontiers in palaeoclimatology and palaeohydrology studies. Future applications hold promise to recognise the patterns of modern and ancient isotopic signatures within ecosystems and provide useful information in understanding potential natural hazards, thus complementing sustainable watershed management.

Study region: Robit-Bata watershed, Upper Blue Nile basin, Ethiopia.
Study focus: Stable isotopes of water (Oxygen-18 and Deuterium) were used as tracers to estimate the contribution of groundwater in shallow hillslope aquifers to streamflow in the Robit-Bata watershed. To assess the spatiotemporal variability of shallow groundwater and develop a hydrograph separation technique, we collected rainfall, shallow groundwater, and streamflow samples and analyzed their δ18O and δ2H isotopic compositions. The local meteoric water line (LMWL) and local evaporative line (LEL) of the study area were determined and compared with the global meteoric water line (GMWL). A standard unweighted two-component isotope-based hydrograph separation model was used to determine the percentage contribution of pre-event water to streamflow.
New hydrological insights for the region: The LMWL (δ2H = 8.63·δ18O + 18.2) mostly showed heavy isotopic enrichment relative to GMWL, and the LEL (δ2H = 5.45·δ18O + 6.96) indicated isotopic enrichment compared to Ethiopian lakes. Shallow groundwater responded rapidly to rainfall, with good spatial correlation depending on topographic positions of wells. pre-event water contributed <50% to peak discharge in July, but >90% when the watershed reached maximum storage. This finding gives insight towards the predominant runoff generation process and has significant implications for sustainable dry season irrigation expansion in the area as the sub-surface flow drains out of the watershed from October onwards reducing water tables in the shallow wells.

The active Ni(II) centre of metalloenzyme urease forms a coordination with substrate urea prior to the hydrolysis of urea. The present study provided the direct experimental evidences of isotope preferential coordination between Ni(II) metal centre of urease enzyme and substrate urea, which eventually alters the product yield. Furthermore, in-vitro experiments revealed that the specific 12 CO 2 isotopic species bridges between heavier isotope of substrate urea [ 13 CO(NH 2 ) 2 ] and Ni(II) centre of urease for more effective coordination, which ultimately enhances the yield of the reaction. Finally, the in-vitro observations have been validated under in-vivo physiological conditions exploiting the urease activity of the gastric pathogen Helicobacter pylori present in human stomach. Hence, the present study paves the way for better understanding of the isotope-selective catalytic activity of urease enzyme and exemplifies the link between 12 CO 2 and urease enzyme.

A portable Wavelength Scanned-Cavity Ring-Down Spectrometer (Picarro L2120) fitted with a diffusion sampler (DS-CRDS) was used for the first time to continuously measure δ 18 O and δ 2 H of stream water. The experiment took place during a storm event in a wet tropical agricultural catchment in northeastern Australia. At a temporal resolution of one minute, the DS-CRDS measured 2160 δ 18 O and δ 2 H values continuously over a period of 36 h with a precision of ±0.08 and 0.5‰ for δ 18 O and δ 2 H, respectively. Four main advantages in using high temporal resolution stream δ 18 O and δ 2 H data during a storm event are highlighted from this study. First, they enabled us to separate components of the hydrograph, which was not possible using high temporal resolution electrical conductivity data that represented changes in solute transfers during the storm event rather than physical hydrological processes. The results from the hydrograph separation confirm fast groundwater contribution to the stream, with the first 5 h of increases in stream discharge comprising over 70% pre-event water. Second, the high temporal resolution stream δ 18 O and δ 2 H data allowed us to detect a short-lived reversal in stream isotopic values (δ 18 O increase by 0.4‰ over 9 min), which was observed immediately after the heavy rainfall period. Third, δ 18 O values were used to calculate a time lag of 20 min between the physical and chemical stream responses during the storm event. Finally, the hydrograph separation highlights the role of event waters in the runoff transfers of herbicides and nutrients from this heavily cultivated catchment to the Great Barrier Reef.

Knowing the dominant flow paths within a hydrological system is challenging and crucial to assess the relevant discharge generating processes and the fate of water and solutes in the system. However especially the interpretation of those path ways seems controversy within our study area of the Rio San Francisco in the outskirts of the Amazon basin in a tropical mountainous region of South Ecuador. E.g. the recorded flashiness of the hydrograph contravenes the long mean residence time and hydrogeochemical signature of the event water marking it as old water. Even though theories exist which could reveal these contradictions (e.g. the concept of transmissivity feedback which could be used to explain the rapid mobilization of old water) proof is currently missing to support those concepts. To further study the fate of the water and water pound solutes we installed along two hillslopes (length about 500m each and decline 230m under forest and 157m under pasture) three wick samplers collecting weekly bulk samples of soil water in 10, 25, 40 cm depths for 2 years. The isotopic signature (δ18O and δ2H) of the soil water as well as the incoming rainfall was analyzed using an isotope laser spectrometer (Picarro). We propose the usage of stable water isotopes as conservative tracers to validate a 2D setup of the Catchment Modeling Framework (CMF) simulating the water flow and fate of solutes along the hillslopes. The usage of conservative tracers, such as δ18O and δ2H, to validate hydrological models, bears the advantage that not only the amount of transported solute needs to be correctly simulated but also the concentration of the modeled tracer needs to be correctly accounted for. Model structures derived by such a tracer driven calibration procedure thereby are more likely to represent the actual nature of the hydrological system. Results proving the suitability of the model setup to reproduce the collected isotope data will be shown and the discharge generating processes at hand will be presented. The results of this study will be most helpful while implementing the correct model routines for further assessments on a catchment scale and to ease the interpretation of previous studies conducted within the same study area.

RATIONALE: Quantifying the processes that control dissolved inorganic carbon (DIC) dynamics in aquatic systems is
essential for progress in ecosystem carbon budgeting. The development of a methodology that allows high-resolution
temporal data collection over prolonged periods is essential and is described in this study.
METHODS: A novel sampling instrument that sequentially acidifies aliquots of water and utilises gas-permeable ePTFE  tubing to measure the dissolved inorganic carbon (DIC) concentration and d13CDIC values at sub-hourly intervals by Cavity Ring-down spectrometry (CRDS) is described.
RESULTS: The minimum sensitivity of the isotopic, continuous, automated dissolved inorganic carbon analyser (ISOCADICA) system is 0.01 mM with an accuracy of 0.008 mM. The analytical uncertainty in d13CDIC values is proportional to the concentration of DIC in the sample. Where the DIC concentration is greater than 0.3 mM the analytical uncertainty is  0.1 % and below 0.2 mM stability is < _ 0.3 %. The isotopic effects of air temperature, water temperature and CO2 concentrations were found to either be negligible or correctable. Field trials measuring diel variation in d13CDIC values of coral reef associated sea water revealed significant, short-term temporal changes and illustrated the necessity of this technique.
CONCLUSIONS: Currently, collecting and analysing large numbers of samples for d13CDIC measurements is not trivial, but essential for accurate carbon models, particularly on small scales. The ISO-CADICA enables on-site, high-resolution determination of DIC concentration and d13CDIC values with no need for sample storage and laboratory analysis. The initial tests indicate that this system can offer accuracy approaching that of traditional IRMS analysis.

Globally, Dissolved Inorganic Carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon (DOC), particulate organic carbon (POC) and DIC concentrations and stable isotope ratios of DIC (13CDIC) at two time scales;
seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser (ISO-CADICA) was used to measure diel DIC concentration and 13CDIC changes at a 15 minute temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55-3.82 mg/L, and 13CDIC values ranging from -19.7±0.31 to -17.1±0.08 ‰. In contrast, at the seasonal scale we observe wet season DIC
variations predominantly from mixing processes, and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/L) and 13CDIC values of river water (by 5.4 ‰) largely result from proportional increases in subsurface inflows from the savanna plains (C4 vegetation) region relative to inflows from the rainforest (C3 vegetation) highlands. The high DIC river load during the wet season results in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial savanna carbon sink in wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere.

Cavity ring-down spectrometers,with automated sampling interfaces, were deployed to allowmeasurements of water isotopes (δ18O, δD) and dissolved inorganic carbon (δ13CDIC) stable isotope ratios at high temporal resolution along a transect from New Zealand to the Antarctic continental shelf. Measurements every 10 min for δ18O and δD, 15 min for DIC yielded 2499 and 2289 discrete measurements respectively. High resolution data enabled the delineation of water mass boundaries as well as revealing insights into surface hydrological and biological processes. δ18O, δD, and δ13CDIC decreased southwards, dropping by approximately 1.0‰, 7.0‰, and 0.5‰, respectively. Though the decline in δ13CDIC with latitude was generally linear, the drop in δ18O and δD was punctuated by areas of rapid, significant change corresponding to the Sub-Tropical, Sub-Antarctic and Polar Fronts. North of the Sub-Antarctic Front (approx. 54.5°S) the dominant control on water and DIC isotopes was the precipitation–evaporation balance and the contribution of upwelling waters, respectively. Further south, in close proximity to the sea ice and on the Antarctic shelf, water isotope values were more variable and predominantly influenced by the melting/freezing of sea-ice coupled to inputs from glacial/snow melt water. Local increases in δ13CDICwere likely due to photosynthetic enrichment of the DIC pool. Using this newinstrumentation has provided one of the most comprehensive oceanic transect data sets yet achieved and illustrates the potential of these methods to delineate discrete water masses and advance our knowledge of both water and inorganic carbon cycling processes in the ocean. This methodology, combining high-resolution isotopic measurements with hydrographic data, has significant benefits in modelling water mixing in locations with multiple sources and controlling processes.

Stable isotopic composition (δ 18 O and δD of water, δ 13 C DIC ) of the water column in the open ocean is related to the origin of water masses. Due to the recent increase of paleoceanographic studies on continental shelves, it is also important to understand their distribution and variability in those systems. To examine the influence of continental shelves internal processes on isotopic composition of water masses, we present data of stable isotopes and phosphate content from a western boundary upwelling system located on the Southeastern Brazilian coast and compare them with offshore observations. High mixing of the main water masses (SSW, TW and SACW) was observed in the majority of the samples collected during different seasons in 2011 and 2012. A mixing triangle approach was used to separate the water masses contribution and characterize their isotopic composition. In addition, an isotopic three end-member model was established, proposing it as a paleoceanographic tool to reconstruct relative contribution of these water masses in sediment records. Variations of δ 18 O values are linked to oceanographic dynamics, mixing, continental runoff and upwelling processes on the shelf. Differently the δ 13 C DIC variations in the middle and inner parts of the shelf are related to the productivity of the upwelling system. Seasonal variability of the δ 13 C DIC values may be also related to changes in the upwelling intensity.