•We used the water balance method to estimate the evapotranspiration for two basins in Tibetan Plateau.•We evaluated several global evapotranspiration products for the two basins in Tibetan ...Plateau.•We suggested the improvements of the evapotranspiration products.
Evapotranspiration (E) at regional or basin scale is difficult to estimate. This study estimates E with a water balance method for the upper Yellow River and Yangtze River basins on the Tibetan Plateau, where in situ data accessibility is especially insufficient. Results indicate that annual terrestrial water storage change in the two basins is negligible, and basin-scale E can be reliably estimated by the difference between precipitation and runoff. Thus, four E products from Zhang—(Zhang_E), MODIS (MODIS_E), Japanese 25-year reanalysis product (JRA_E), and the newly published Global Land Data Assimilation System with Noah Land Surface Model-2 (GLDAS_E)—are evaluated against E estimated by the water balance method. GLDAS_E and Zhang_E had the best performance for the upper Yellow River basin and Yangtze River basin, respectively, with relatively small underestimation. Further analysis showed that the underestimation of GLDAS_E was mainly caused by its negative bias for precipitation, whereas the overestimation of JRA_E was due to overestimation of downward shortwave radiation. MODIS_E greatly overestimated E in both basins, which was also caused by high downward shortwave radiation flux inputs from the Global Modeling and Assimilation Office. Thus, more accurate forcing data for these products should be a future focus, since they can improve E estimates, at least for the Tibetan Plateau.
Peatlands are globally‐important terrestrial carbon stores as well as regional sources of potable water supply. Water draining from peatlands is rich in dissolved organic carbon (DOC), which can be ...problematic for water treatment. However, it is unclear how future climate and sulfate deposition changes may impact DOC in peatland‐derived potable water. The United Kingdom (UK) is a global hotspot that consumes 79% of all potable water derived directly from peatlands. Here, a physically‐based hydrological model and a biogeochemical organic carbon model were used to predict discharge and DOC concentration in nine hotspots of peatland‐derived potable water use in the UK under a range of 21st century climate and sulfate deposition scenarios. These nine catchments supply 72% of all peatland‐derived water consumed in the UK and 57% of the global total, equivalent to the total domestic consumption of over 14 million people. Our simulations indicate that annual discharges will decrease and that mean annual DOC concentrations will increase under all future scenarios (by as much as 53.4% annually for the highest emissions scenario) in all catchments. Large increases (by as much as a factor of 1.6) in DOC concentration in the 2090s over the baseline period are projected for autumn and winter, seasons when DOC concentrations are already high in the baseline datasets such that water treatment works often reach their capacity to cope. The total DOC flux is largely insensitive to future climate change because the projected increase in DOC concentration is mostly counterbalanced by the projected decrease in discharge.
Plain Language Summary
Peatlands are important sources of potable water in some parts of the world. The UK is a particular hotspot and consumes around 79% of all drinking water provided by peatlands globally. Water draining from peatlands is rich in dissolved organic carbon (DOC). DOC from peatlands represents an important component of the global carbon cycle and is problematic for water treatment. Using physically‐based hydrological and organic carbon models combined with future climate and sulfate deposition scenarios for the UK, we demonstrate that river DOC concentrations are likely to increase under all future scenarios, particularly in autumn and early winter. These changes will create problems for water treatment because many water treatment plants that remove DOC already reach capacity during these seasons. Furthermore, large decreases in river discharge are projected in future summers for these important catchments, creating additional pressure for UK water resources.
Key Points
Mean annual DOC concentrations in nine UK peatland rivers will increase in the future, by as much as 53% for the highest emissions scenario
Large increases in mean DOC concentrations are projected in future autumn and winter, periods when DOC concentrations are already high
Large decreases in mean discharge are projected for April to September, periods when discharge is already low
Ekman pumping induced by horizontally varying wind and sea ice drift is examined as an explanation for observed seasonal variation of the warm layer thickness of circumpolar deep water on the ...Amundsen Sea continental shelf. Spatial and temporal variation of the warm layer thickness in one of the deep troughs on the shelf (Dotson Trough) was measured during two oceanographic surveys and a two-year mooring deployment. A hydrographic transect from the deep ocean, across the shelf break, and into the trough shows a local elevation of the warm layer at the shelf break. On the shelf, the water flows south-east along the trough, gradually becoming colder and fresher due to mixing with cold water masses. A mooring placed in the trough shows a thicker and warmer layer in February and March (late summer/early autumn) and thinner and colder layer in September, October and November (late winter/early spring). The amplitude of this seasonal variation is up to 60m. In order to investigate the effects of Ekman pumping, remotely sensed wind (Antarctic Mesoscale Prediction System wind data) and sea ice velocity and concentration (EASE Polar Pathfinder) were used. From the estimated surface stress field, the Ekman transport and Ekman pumping were calculated. At the shelf break, where the warm layer is elevated, the Ekman pumping shows a seasonal variation correlating with the mooring data. Previous studies have not been able to show a correlation between observed wind and bottom temperature, but it is shown here that when sea ice drift is taken into account the Ekman pumping at the outer shelf correlates with bottom temperature in Dotson Trough. The reason why the Ekman pumping varies seasonally at the shelf break appears to be the migration of the ice edge in the expanding polynya in combination with the wind field which on average is westward south of the shelf break.
•Ekman pumping induced by horizontally varying wind and sea ice drift is examined as an explanation for observed seasonal variation of the warm layer thickness of circumpolar deep water on the Amundsen Sea continental shelf.•Spatial and temporal variation of the warm layer thickness in Dotson Trough was measured during two oceanographic surveys and a two-year mooring deployment.•The observations were compared to time series of remotely sensed surface stress.
Air–sea gas fluxes are commonly estimated using wind-based parametrizations of the gas transfer velocity. However, neglecting gas exchange forcing mechanisms – other than wind speed – may lead to ...large uncertainties in the flux estimates and the carbon budgets, in particular, in heterogeneous environments such as marginal seas and coastal areas. In this study we investigated the impact of including relevant processes to the air–sea CO2 flux parametrization for the Baltic Sea. We used six parametrizations of the gas transfer velocity to evaluate the effect of precipitation, water-side convection, and surfactants on the net CO2 flux at regional and sub-regional scale. The differences both in the mean CO2 fluxes and the integrated net fluxes were small between the different cases. However, the implications on the seasonal variability were shown to be significant. The inter-annual and spatial variability were also found to be associated with the forcing mechanisms evaluated in the study. In addition to wind, water-side convection was the most relevant parameter controlling the air–sea gas exchange at seasonal and inter-annual scales. The effect of precipitation and surfactants seemed negligible in terms of the inter-annual variability. The effect of water-side convection and surfactants resulted in a reduction of the downward fluxes, while precipitation was the only parameter that resulted in an enhancement of the net uptake in the Baltic Sea.
•Variability of air–sea CO2 fluxes is modulated by mechanisms other than wind speed.•Gas transfer velocity parameterizations are not adequate for coastal/marginal seas.•Water-side convection enhances air–sea fluxes during winter in the Baltic Sea.•Surfactants suppress CO2 downward fluxes during summer.•Precipitation represents a net increase of the carbon uptake.
We study the variability and the evolution of oceanic deep convection in the northern North Atlantic and the Nordic Seas from 1850 to 2100 using an ensemble of 12 climate model simulations with ...EC-Earth. During the historical period, the model shows a realistic localization of the main sites of deep convection, with the Labrador Sea accounting for most of the deep convective mixing in the northern hemisphere. Labrador convection is partly driven by the NAO (correlation of 0.6) and controls part of the variability of the AMOC at the decadal time scale (correlation of 0.6 when convection leads by 3–4 years). Deep convective activity in the Labrador Sea starts to decline and to become shallower in the beginning of the twentieth century. The decline is primarily caused by a decrease of the sensible heat loss to the atmosphere in winter resulting from increasingly warm atmospheric conditions. It occurs stepwise and is mainly the consequence of two severe drops in deep convective activity during the 1920s and the 1990s. These two events can both be linked to the low-frequency variability of the NAO. A warming of the sub-surface, resulting from reduced convective mixing, combines with an increasing influx of freshwater from the Nordic Seas to rapidly strengthen the surface stratification and prevent any possible resurgence of deep convection in the Labrador Sea after the 2020s. Deep convection in the Greenland Sea starts to decline in the 2020s, until complete extinction in 2100. As a response to the extinction of deep convection in the Labrador and Greenland Seas, the AMOC undergoes a linear decline at a rate of about −0.3 Sv per decade during the twenty-first century.
Inland waters, such as lakes, reservoirs and rivers, are important sources of climate forcing trace gases. A key parameter that regulates the gas exchange between water and the atmosphere is the gas ...transfer velocity, which itself is controlled by near‐surface turbulence in the water. While in lakes and reservoirs, near‐surface turbulence is mainly driven by atmospheric forcing, in shallow rivers and streams it is generated by bottom friction of gravity‐forced flow. Large rivers represent a transition between these two cases. Near‐surface turbulence has rarely been measured in rivers and the drivers of turbulence have not been quantified. We analyzed continuous measurements of flow velocity and quantified turbulence as the rate of dissipation of turbulent kinetic energy over the ice‐free season in a large regulated river in Northern Finland. Measured dissipation rates agreed with predictions from bulk parameters, including mean flow velocity, wind speed, surface heat flux, and with a one‐dimensional numerical turbulence model. Values ranged from ∼10−10m2s−3 to 10−5m2s−3. Atmospheric forcing or gravity was the dominant driver of near‐surface turbulence for similar fraction of the time. Large variability in near‐surface dissipation rate occurred at diel time scales, when the flow velocity was strongly affected by downstream dam operation. By combining scaling relations for boundary‐layer turbulence at the river bed and at the air‐water interface, we derived a simple model for estimating the relative contributions of wind speed and bottom friction of river flow as a function of depth.
Plain Language Summary
Inland water bodies such as lakes, reservoirs and rivers are an important source of climate forcing trace gases to the atmosphere. Gas exchange between water and the atmosphere is regulated by the gas transfer velocity and the concentration difference between the water surface and the atmosphere. The gas transfer velocity depends on near‐surface turbulence, but robust formulations have not been developed for river systems. Their surface area is sufficiently large for meteorological forcing to cause turbulence, as in lakes and reservoirs, but turbulence generated from bed and internal friction of gravity‐driven flows is also expected to contribute. Here we quantify near‐surface turbulence using data from continuous air and water side measurements conducted over the ice‐free season in a large subarctic regulated river in Finland. We find that turbulence, quantified as the dissipation rate of turbulent kinetic energy, is well described using equations for predicting turbulence from meteorological data for sufficiently high wind speeds whereas the contribution from bottom shear dominated at higher flow velocities. A one‐dimensional river model successfully captured these processes. We provide a fundamental model for estimating the relative contributions of atmospheric forcing and bottom friction as a function of depth.
Key Points
Wind and river flow make comparable contributions to near‐surface turbulence in a regulated river
Dissipation rates predicted from wind speed and flow velocity are in good agreement with observations
Diel variability in dissipation rates occurs in response to flow regulation and atmospheric forcing
•Seawater intruded about 2 km in the coastal region, affecting water resources.•The Taylor diagram indicated that L1 robust is the best inversion method.•The new uncertainty approach indicated a ...considerable variation in seawater intrusion extent.•The BERT reached lower RMS error in less iteration compared to RES2DINV.•Mesh size did not improve the inversion results significantly.
Seawater intrusion (SWI) is one of the most important phenomena which occurs in shorelines and islands and affects groundwater storage in the region. This study aims to investigate how far seawater has intruded in a coastal area and what is its extent variation. The results can lead to different management plans. To achieve these goals, a geoelectrical method is applied in the coastal aquifer of Chaouia, Morocco. In order to choose the best inversion process, different settings are applied in two different software packages namely RES2DINV and BERT and the results are compared using the Taylor diagram. Also, to determine the minimum and maximum SWI extent in the region, a new approach of uncertainty analysis is applied in the inversion process by the Monte Carlo method. The general results indicate that the seawater has intruded about 2 km into the shore. The obtained results show that by considering uncertainty, there is a sensible difference between the maximum and minimum extent of SWI (maximum 9% variation in the area of SWI) which should be noted when doing water research management studies. The methodology procedure in this study can be applied to different coastal areas around the world.
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•Isolated microalgae from Sicily were tested for urban sewage treatment.•Indigenous microalgae grew in sewage better than collection strains.•A consortium of Dunaliella sp. and ...Chlorella gave rise to the best growth performance in sewage.•Microalgae were found to be effective in removing TN (up to 77 %) and TP (up to 61 %) from urban sewage.
Microalgae have promising applications in wastewater treatment because of their ability to use inorganic compounds such as nitrates and phosphates as nutrients for their growth. Microalgae are applied to the secondary and tertiary bio-treatment with two benefits: i) pollutants removal from wastewater; ii) production of microalgal biomass, that can be exploited as a source of biomass and biomolecules. In the present work, four different microalgal strains (two from culture collections and two isolated from Sicilian littoral) were tested in municipal sewage bioremediation. The sewage of a municipal plant, already processed with primary treatment, was used for the cultivation of microalgal strains in order to test their potential on performing the secondary treatment. Microalgal cells were cultivated in growth medium and in sewage with the aim to compare their growth and biomass composition in different conditions. The efficiency of wastewater treatment was established through assessment of biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN) and total phosphorous (TP) of sewage before and after algal growth. Results showed that microalgal treatment alone is not effective in reducing COD and BOD, while all the tested strains were able to significantly reduce wastewater TN (up to 77 %) and TP (up to 61 %) concentrations. Amongst the tested strains, Chlorella genus can be considered the best candidate for wastewater treatment.
Submesoscale flows are energetic processes in the upper ocean that occur at scales of a few kilometers. These phenomena are important to climate because they can strongly impact vertical motions of ...heat, carbon, and biogeochemical properties between the surface and deep ocean. Their presence and magnitude are poorly understood in the Antarctic marginal ice zone (MIZ) and under sea ice due to a lack of observations. We present tagged southern elephant seal hydrographic data from the eastern Weddell Sea, over the austral late summer to spring in 2008, to assess the magnitude and seasonality of submesoscale processes in the Antarctic MIZ. We consider the linkages between sea ice cover, ocean stress variability, and mixed layer depth and how these influence the submesoscale fluxes. We find that there is a strong seasonal cycle in the magnitude of the equivalent heat flux from submesoscale flows (240 Wm−2), with peak activity occurring in midwinter. Periods of increased variability in sea ice cover, likely associated with leads, are also linked to restratification through submesoscale fluxes in midwinter. These findings have implications for understanding the magnitude and variability of upper ocean stratification and therefore highlights the importance of developing parameterizations for submesoscale processes in the Antarctic MIZ within global climate models.
Plain Language Summary
Changes in the mixed layer depth can affect vertical transport of heat and carbon from the atmosphere into the deep ocean. Submesoscale processes, occurring on small time and space scales of hours–days and a few kilometers, can directly impact these mixed layer depth changes. Understanding the magnitude and activity of submesoscale flows is critical to improve our knowledge of how the climate may change in the future, yet there are currently no reports of them in the Southern Ocean under sea ice. Using instrumented seals, we present the first known observations of submesoscale fluxes from late austral summer through to spring in and near Antarctic sea ice. Our findings show that these small‐scale processes can have a strong effect on the upper ocean and can cause the mixed layer depth to shoal even in the middle of winter. There is also a clear seasonal cycle in the magnitude of the submesoscale activity. This study indicates that submesoscale processes should not be overlooked as a critical component of upper ocean processes in the Antarctic sea ice zone.
Key Points
Hydrographic data from tagged seals are used to observe submesoscale processes in the Antarctic Marginal Ice Zone
We find a strong seasonal cycle in the magnitude of submesoscale flows under/within sea ice, with a peak in midwinter
Mixed layer instabilities show the largest seasonality, while wind‐driven frontal flows are dampened by the presence of sea ice
Recently, groundwater in Sweden has attracted media attention due to supply shortages caused by dry periods and low groundwater levels. About half of Swedish drinking water stems from groundwater. ...The small Swedish aquifers are highly dependent on frequent and sufficient recharge. Groundwater recharge forms the link between meteorological and groundwater drought and thus the main link between climate change and sustainable water supply. This study evaluated whether the current knowledge on groundwater recharge and groundwater drought is sufficient to mitigate the impacts of climate change. A review of international literature on groundwater recharge in regions with conditions similar to Sweden was performed. National literature was compared and links to international studies evaluated. A survey among Swedish groundwater experts complemented the study. Findings are that research on groundwater recharge has been scarce in Sweden over the last decades and Swedish experts seem to not have taken much notice of international literature. It is concluded that Sweden is not well prepared to predict the impact of climate change on groundwater resources, as the most crucial process linking climate to groundwater is not well understood. The study has a strong focus on Sweden, however, the results are also relevant for countries with similar geology and climate. One main conclusion is that groundwater recharge studies often remain unique, and thus hardly transferable and comparable. Two or more independent studies are rarely applied in parallel, verification based on direct groundwater observations is not common. This raises concerns about the reliability of climate change impact predictions on groundwater.
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