Variability in methane emissions from West Siberia's shallow boreal lakes on a regional scale and its environmental controls

Small lakes represent an important source of atmospheric CH4 from northern wetlands. However, spatiotemporal variations in flux magnitudes and the lack of knowledge about their main environmental controls contribute large uncertainty into the global CH4 budget. In this study, we measured methane fluxes from small lakes using chambers and bubble traps. Field investigations were carried out in July–August 2014 within the West Siberian middle and southern taiga zones. The average and median of measured methane chamber fluxes were 0.32 and 0.30 mgCH4 m−2 h−1 for middle taiga lakes and 8.6 and 4.1 mgCH4 m−2 h−1 for southern taiga lakes, respectively. Pronounced flux variability was found during measurements on individual lakes, between individual lakes and between zones. To analyze these differences and the influences of environmental controls, we developed a new dynamic process-based model. It shows good performance with emission rates from the southern taiga lakes and poor performance for individual lakes in the middle taiga region. The model shows that, in addition to wellknown controls such as temperature, pH and lake depth, there are significant variations in the maximal methane production potential between these climatic zones. In addition, the model shows that variations in gas-filled pore space in lake sediments are capable of controlling the total methane emissions from individual lakes. The CH4 emissions exhibited distinct zonal differences not only in absolute values but also in their probability density functions: the middle taiga lake fluxes were best described by a lognormal distribution while the southern taiga lakes followed a power-law distribution. The latter suggests applicability of self-organized criticality theory for methane emissions from the southern taiga zone, which could help to explain the strong variability within individual lakes

Evaluating closed chamber evapotranspiration estimates against eddy covariance measurements in an arctic wetland

Evapotranspiration (ET) is an important hydrological flux with a strong influence on greenhouse gas emissions from thawing permafrost. This study examines the suitability of the closed chamber method in characterising the spatial heterogeneity of ET fluxes in a typical polygonal tundra landscape in the Lena River Delta, northern Siberia. Actual evapotranspiration is compared across scales using: (1) ground-based chamber measurements to observe ET at the microsite scale; and (2) tower-based eddy covariance (EC) measurements which provide spatially averaged ET observations at the ecosystem level. Adopting an upscaling approach using EC estimates as a benchmark, the authors assess the suitability of the closed chamber method in an arctic wetland environment. A short closure time (40 s) and a linear model to describe the change in chamber headspace water vapour concentration were employed to estimate ET rates from the chamber measurements. Using a correction factor chamber fluxes were successfully scaled to the EC data. Yet, findings suggest that the performance of the closed chamber method is highly sensitive to the prevailing hydrometeorological conditions, and it is likely that the sorption and desorption of water molecules to the inside of the chamber and tubing has a strong impact on results. A number of methodological issues are presented in this paper which question the use of closed chamber measurements as a stand-alone tool for measuring ET in arctic wetland environments. However, when paired alongside a trusted benchmark, chambers can provide valuable information on ET at the microsite scale

Friction-Velocity Estimates Using the Trace of a Scalar and the Mean Wind Speed

A semi-empirical approach based on surface-renewal theory for estimating the friction velocity is tested for measurements taken in the inertial sublayer. For unstable cases, the input requirements are the mean wind speed and the high-frequency trace (10 or 20 Hz) of the air or sonic temperature. The method has been extended to traces of water vapour (H2O) and carbon dioxide (CO2) concentrations. For stable cases, the stability parameter must also be considered. The method’s performance, taking the direct friction velocity measured by sonic anemometry as a reference, was tested over a growing cotton field that included bare soil with some crop residues at the beginning of the season. In general, the proposed friction-velocity estimates are reliable. For unstable cases, the method shows the potential to outperform the wind logarithmic-law computation. Discarding cases with low wind speeds (e.g., \u3c 0.3 m s−1 and mean wind shear \u3c 1 Hz), the proposed approach may be recommended as an alternative method to estimating the friction velocity. There is the potential, based on the input requirements, that the proposed formulation may offer significant advantages in the estimation of the friction velocity in some marine environments

A new free-convection form to estimate sensible heat and latent heat fluxes for unstable cases

Free convection limit (FCL) approaches to estimate surface fluxes are of interest given the evidence that they may extend up to near neutral stability conditions. For measurements taken in the inertial sublayer, the formulation based on surface renewal theory and the analysis of small eddies (SRSE) to estimate the sensible heat flux (H) was extended to latent heat flux (LE) with the aim to derive their FCL approaches. For sensible heat flux (HFCL), the input requirements are traces of the fast-response (such as 10–20 Hz) air temperature and the zero-plane displacement. For latent heat flux (LEFCL), input requirements are fast response traces of water vapor density, mean temperature of the air, the available net surface energy (Rn-G, where Rn and G are the net radiation and soil heat flux, respectively) and the zero-plane displacement. Taking eddy covariance (EC) as a reference method, the performance of the FCL method was tested over a growing cotton field that involved three contrasting surfaces: partly mulched bare soil, a sparse canopy and a homogeneous canopy. Using traces at 10 Hz and 20 Hz, HFCL overestimated and underestimated the EC sensible heat flux (HEC), respectively. In general, LEFCL tended to slightly underestimate LEEC. The surface energy balance closure show that (HEC + LEEC) underestimated (Rn-G) in a range of 19% (homogeneous canopy) and 8% (sparse canopy). Given that, in general (HFCL + LEFCL) was closer to (Rn-G) than (HEC + LEEC), the FCL method may be recommended for field applications, especially when the wind speed is not available

Automated mapping of rice fields using multi-year training sample normalization

Rice agriculture is of great ecological, environmental, and socioeconomic importance in the Lower Mississippi Alluvial Valley, as its distribution and size heavily impact food production and a number of ecosystem services. Long-term rice mapping is challenging as a result of insufficient training data – both in spatial amount and in temporal coverage, the high cost of powerful geospatial data processing platforms, and incomplete image coverage during the critical window to capture the unique rice signals. Here, we developed a simple yet effective method for rice field extraction without heavy reliance on the complete profiles of Landsat time series or repeated training data. The core is a multiple-year training sample normalization that extends the samples obtained in one year for classification in another year. Pseudo-invariant objects and a set of linear regressions were used to predict what the given vegetation index values of training samples would be if they had been acquired under the same conditions in a different mapping year. The generated pseudo training samples were further utilized to classify the mapping image. We experimented with four years’ Landsat Thematic Mapper and Operational Land Imager data and achieved comparable accuracies as the single-year classification. Because of its simplicity and low computational requirements, it can be efficiently implemented on cloud computing platforms, such as Google Earth Engine platform. This technique provides an affordable and effective solution to derive crop distribution information on a large-scale basis

Eddy covariance measurements of carbon dioxide and water fluxes in US mid-south cotton production

An eddy covariance (EC) system was used over a production-scale, US mid-south cotton field in order to improve understanding of carbon and water flux dynamics in cotton production. Measurement outcomes of NEE and ET from agricultural cropping systems in the humid US mid-south may differ from measurements made in arid production regions due to variation in precipitation, relative humidity, and cloud cover. Local measurements are needed for timely irrigation water management decisions and to assess their impact on the carbon cycle. Measurements were made in a 63-ha field in Northeast Arkansas in the 2016 and 2017 growing seasons (May-October). Average daily NEE during the growing season was driven largely by gross primary production (GPP) measured at -1.80 ± 0.26 g C m−2 d-1 ranging between -13.5 and 2.4 g m−2 d-1 during 2016 and -1.97 ± 0.26 g C m−2 d-1 ranging between -11.1 and 5.9 g m−2 d-1 during 2017. Across both years the average daily GPP during the growing season was 7.65 ± 0.45 g C m−2 d-1 during 2016 and 8.33 ± 0.37 g C m−2 d-1 during 2017. GPP was lowest during emergence and post-harvest and highest from the first week of flowering (FF) stage to about 3 weeks after cutout (cessation of vegetative expansion and production of new fruiting sites). Measured ET was lowest early and late in the season, and highest from FF until cutout. Across both seasons, average ET was 3.5 ± 0.1 mm d-1, more specifically 3.3 ± 0.1 mm d-1 during 2016 and 3.6 ± 0.1 mm d-1 during 2017. Peak water use was 7.6 mm d-1 during both growing seasons. Measured ET values were similar to results from lysimeter studies conducted in humid southeastern US climates, but lower than those observed in studies in arid regions. The water use efficiency of harvest yield per evapotranspiration flux was 0.26 kg lint cotton m-3 water and 0.34 kg lint m-3 water in 2016 and 2017, respectively which 1.5–2 times those of other studies, likely due to higher yields and lower evapotranspiration measured in this study. The findings suggest that US mid-south cotton production should continue to be managed differently than other more arid US cotton production regions

Lateral carbon export has low impact on the net ecosystem carbon balance of a polygonal tundra catchment

Permafrost-affected soils contain large quantities of soil organic carbon (SOC). Changes in the SOC pool of a particular ecosystem can be related to its net ecosystem carbon balance (NECB) in which the balance of carbon (C) influxes and effluxes is expressed. For polygonal tundra landscapes, accounts of ecosystem carbon balances in the literature are often solely based on estimates of vertical carbon fluxes. To fill this gap, we present data regarding the lateral export rates of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from a polygonal tundra site in the north Siberian Lena River delta, Russia. We use water discharge observations in combination with concentration measurements of waterborne carbon to derive the lateral carbon fluxes from one growing season (2 June–8 September 2014 for DOC, 8 June–8 September 2014 for DIC). To put the lateral C fluxes into context, we furthermore present the surface–atmosphere eddy covariance fluxes of carbon dioxide (CO2) and methane (CH4) from this study site. The results show cumulative lateral DIC and DOC fluxes of 0.31–0.38 and 0.06–0.08 g m−2, respectively, during the 93 d observation period (8 June–8 September 2014). Vertical turbulent fluxes of CO2-C and CH4-C accumulated to −19.0 ± 1.2 and 1.0 ± 0.02 g m−2 in the same period. Thus, the lateral C export represented about 2 % of the net ecosystem exchange of (NEE) CO2. However, the relationship between lateral and surface–atmosphere fluxes changed over the observation period. At the beginning of the growing season (early June), the lateral C flux outpaced the surface-directed net vertical turbulent CO2 flux, causing the polygonal tundra landscape to be a net carbon source during this time of the year. Later in the growing season, the vertical turbulent CO2 flux dominated the NECB

Variability in methane emissions from West Siberia\u27s shallow boreal lakes on a regional scale and its environmental controls

Small lakes represent an important source of atmospheric CH4 from northern wetlands. However, spatiotemporal variations in flux magnitudes and the lack of knowledge about their main environmental controls contribute large uncertainty into the global CH4 budget. In this study, we measured methane fluxes from small lakes using chambers and bubble traps. Field investigations were carried out in July–August 2014 within the West Siberian middle and southern taiga zones. The average and median of measured methane chamber fluxes were 0.32 and 0.30 mgCH4 m−2 h−1 for middle taiga lakes and 8.6 and 4.1 mgCH4 m−2 h−1 for southern taiga lakes, respectively. Pronounced flux variability was found during measurements on individual lakes, between individual lakes and between zones. To analyze these differences and the influences of environmental controls, we developed a new dynamic process-based model. It shows good performance with emission rates from the southern taiga lakes and poor performance for individual lakes in the middle taiga region. The model shows that, in addition to wellknown controls such as temperature, pH and lake depth, there are significant variations in the maximal methane production potential between these climatic zones. In addition, the model shows that variations in gas-filled pore space in lake sediments are capable of controlling the total methane emissions from individual lakes. The CH4 emissions exhibited distinct zonal differences not only in absolute values but also in their probability density functions: the middle taiga lake fluxes were best described by a lognormal distribution while the southern taiga lakes followed a power-law distribution. The latter suggests applicability of self-organized criticality theory for methane emissions from the southern taiga zone, which could help to explain the strong variability within individual lakes

Greenhouse Gas Emissions and Management Practices that Affect Emissions in US Rice Systems

Previous reviews have quantified factors affecting greenhouse gas (GHG) emissions from Asian rice (Oryza sativa L.) systems, but not from rice systems typical for the United States, which often vary considerably particularly in practices (i.e., water and carbon management) that affect emissions. Using meta‐analytic and regression approaches, existing data from the United States were examined to quantify GHG emissions and major practices affecting emissions. Due to different production practices, major rice production regions were defined as the mid‐South (Arkansas, Texas, Louisiana, Mississippi, and Missouri) and California, with emissions being evaluated separately. Average growing season CH4 emissions for the mid‐South and California were 194 (95% confidence interval [CI] = 129–260) and 218 kg CH4 ha−1 season−1 (95% CI = 153–284), respectively. Growing season N2O emissions were similar between regions (0.14 kg N2O ha−1 season−1). Ratoon cropping (allowing an additional harvestable crop to grow from stubble after the initial harvest), common along the Gulf Coast of the mid‐South, had average CH4 emissions of 540 kg CH4 ha−1 season−1 (95% CI = 465–614). Water and residue management practices such as alternate wetting and drying, and stand establishment method (water vs. dry seeding), and the amount of residue from the previous crop had the largest effect on growing season CH4 emissions. However, soil texture, sulfate additions, and cultivar selection also affected growing season CH4 emissions. This analysis can be used for the development of tools to estimate and mitigate GHG emissions from US rice systems and other similarly mechanized systems in temperate regions