Browsing by Subject "Eddy covariance"

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    The role of Phragmites australis in carbon, water and energy fluxes from a fen in southwest Germany
    (2019) van den Berg, Merit; Streck, Thilo
    The global carbon emission from peat soils adds up to 0.1 Gt-C per year. Under anaerobic conditions, organic material is decomposed to methane (CH4). Over a 100-year cycle, methane is a 28 times stronger greenhouse gas than carbon dioxide and is an important factor for climate change. Therefore, there is a great interest to get a better understanding of the carbon flows in peatlands. Phragmites peatlands are particularly interesting due to the global abundance of this wetland plant (Phragmites australis, common reed) and the highly efficient internal gas transport mechanism. This is a humidity-induced convective flow (HIC) to transport oxygen (O2) to the roots and rhizomes, with the effect that simultaneously soil gases (CH4 and CO2) can be transported to the atmosphere via the plant. Thereby, Phragmites is expected to have a high evapotranspiration (ET) rate due to the large leaf area, open water habitat and high aerodynamic roughness. This ET could highly influence the hydrology of the system. Because he accumulation of organic material occurs because of limiting oxygen levels, hydrological processes are fundamental in the development of peatlands. The research aims were: 1) to clarify the effect of plant-mediated gas transport on CH4 emission, 2) to find out whether Phragmites peatlands are a net source or sink of greenhouse gases, and 3) to evaluate ET in perspective of surface energy partitioning and compare results with FAO’s Penman-Monteith equation. CO2, CH4 and latent and sensible energy fluxes were measured with the eddy covariance (EC) technique within a Phragmites-dominated fen in southwest Germany in 2013, 2014 and 2016. In 2016, a field experiment was set up to quantify the contribution of plant-mediated CH4 transport to the overall CH4 flux and how it influences ebullition. One year of EC flux data (March 2013–February 2014) shows very clear diurnal and seasonal patterns for both CO2 and CH4. The diurnal pattern of CH4 fluxes was only visible when living green reed was present. This diurnal cycle had the highest correlation with global radiation, which suggests a high influence of HIC on CH4 emission. But if the cause were HIC, relative humidity should correlate stronger with CH4 flux. Therefore, we conclude that in addition to HIC at least one other mechanism must have been involved in the creation of the convective flow within the Phragmites plants. We quantified the influence of pressurized flow within Phragmites on total CH4 emission in a field experiment (see chapter 3) and found between 23% and 45% lower total CH4 flux when pressurized flow was excluded (by cutting or cutting and sealing the reed). The gas transport pathways from the soil to the atmosphere changed as well. Relative contribution of ebullition to the total flux increased from 2% in intact Phragmites to 24-37% in cut vegetation. This increase in ebullition in cut vegetation, obviously, did not compensate the excluded pathway via the pressurized air flow at our site. It also means that the effect of CH4 bypassing the oxic water layer by plant transport on CH4 emission is much larger than the effect of O2 transport through the plants on CH4 oxidation and production in the rhizosphere. Overall, the fen was a sink for carbon and greenhouse gases in the measured year, with a total carbon uptake of 221 g C m-2 yr-1 (26% of the total assimilated carbon). The net uptake of greenhouse gases was 52 g CO2 eq.m-2 yr-1, which is obtained from an uptake of CO2 of 894 g CO2 m-2 yr-1 and a release of CH4 of 842 g CO2 eq.m-2 yr-1. Compared to the long term uptake of carbon by northern peatlands (20–50 g C m-2 yr-1) 212 g C yr-1 is therefore very high. One year of measurements is not enough to draw hard conclusions about the climate change impact of this peatland. The measured ET at our site was lower than other Phragmites wetlands in temperate regions. ET was half the amount of precipitation (see chapter 4). Therefore, the risk of the wetland to dry out is not realistic. ET was especially low when there was little plant activity (May and October). Then, the dominant turbulent energy flux was sensible heat not latent heat. This can be explained by the high density of dead reed in these months. the reed heats up causing a high sensible heat flux. Evaporation was low due to the shading of the water layer below the canopy and low wind velocities near the surface. FAO’s Penman-Monteith equation was a good estimator of measured ET with crop factors from the regression model of Zhou and Zhou (2009) (see chapter 4). Especially the day-to-day variation was modeled very well. Their model had air temperature, relative humidity and net radiation as input variables. This is likely related to stomatal resistance, which depends on the same variables. Therefore, the model of Zhou and Zhou (2009) is an interesting tool for calculating daily crop factors and it is probably robust enough to be used also in different regions.
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    Towards a better understanding of land surface exchange processes over agricultural crop stands
    (2020) Bohm, Kristina; Streck, Thilo
    Weather and climate models are useful tools for projecting the influence of global climate change on the regional scale. These models are critically dependent on an accurate representation of soil-plant-atmosphere interactions, which are simulated by Land Surface Models (LSMs). The present PhD thesis was designed to improve the representation of land surface exchange processes of croplands in the Noah-MP land surface model. This thesis aims: a) to elucidate the nature of the energy imbalance over a winter wheat stand and to identify the appropriate post-closure method for the study region Kraichgau, southwest Germany; b) to improve the representation of the green vegetation fraction (GVF) dynamics of croplands in the Noah-MP for a more accurate computation of surface energy and water fluxes; and c) to determine the effect of aggregating different crop types with various shares into a single generic cropland class on the simulation of water and energy exchange between land surface and atmosphere.
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    Turbulent exchange of energy, water and carbon between crop canopies and the atmosphere : an evaluation of multi-year, multi-site eddy covariance data
    (2019) Eshonkulov, Ravshan; Streck, Thilo
    The increase of anthropogenic CO2 emissions and other greenhouse gases has raised concern about climate change. Climate change has manifold impacts on yield and yield quality, crop rotations, carbon and nitrogen cycling, water regime and agricultural production systems. To understand its consequences on environmental systems, measuring the matter and energy exchange at the land surface provides data to help validate and inform a wide range of process models. Such flux measurements at the land-surface provide an opportunity to test simulations of processes in the soil-plant-atmosphere continuum. Currently, such measurements are mainly based on the eddy covariance (EC) method, for the quality of which the energy balance closure (EBC) is a problem. The EBC significantly influences the calibration and validity of land-surface models, especially in regard to the energy and water balance at the Earth’s surface. The EBC quantifies the deviation between turbulent fluxes and available energy. It is crucial to obtain high-quality EC measurements to determine the reasons for the EBC. The research aims of this dissertation were: 1) to clarify the role of minor storage and flux terms in the energy balance, 2) to determine the possible reasons for the energy imbalance using a long-term dataset (2010-2017) from agricultural croplands, and 3) to investigate the effects of region, site, year and crop type on carbon fluxes and budgets. In the first study (Chapter 2) the contribution of minor storage terms to the EBC were investigated. I also determined the contribution of ground heat fluxes calculated by different methods. A harmonic analysis method was used to calculate ground heat fluxes from measurements of heat flux plates and soil temperature sensors. Soil heat storage and enthalpy change in the plant canopy were determined at different locations within the EC footprint. Considering minor storage terms improved the energy balance closure on average by 5.0 % in 2015 and by 6.8 % in 2016. The greatest energy balance closure improvement occurred in May of both study years. The dominant fraction of minor energy storage was energy uptake and release through photosynthesis and respiration. Additionally, the energy fluxes related to soil temperature change were also observed. The ground heat flux calculated by harmonic analysis from soil heat flux plates narrowed the EBC by 3 % compared to the calorimetric method. The results indicated that the typical correction approach to achieve energy balance closure, i.e. the Bowen-ratio method, overestimated the turbulent fluxes. The second study (Chapter 3) investigated the effects of crop type, site characteristics, wind directions, atmospheric conditions and footprint on the EBC. The long-term evaluation of EC measurements showed that, with the EC method, 25 % of the available energy could not be detected. Decreasing the flux footprint area increases the chance of a more homogeneous area. Homogeneity plays an important role in achieving a better energy balance closure. The synthesis of long-term EC data indicated that the sonic anemometer is very sensitive to orientation, not allowing accurate measurements from all wind directions. Discarding the measurements from wind directions 0° and 90° at EC4 improved the EBC from 80 to 84 %. In the third study, presented in Chapter 4, a long-term and multi-site experiment was evaluated to clarify the effects of site, year and region on the CO2 fluxes and budgets in agroecosystems. The net ecosystem exchange of CO2 fluxes – measured on six sites during eight years – was comprehensively examined. Winter rapeseed had the lowest CO2 uptake, cropping of silage maize resulted in the highest C losses. The management of harvest residues was the most effective means of controlling the C budgets. Comparing the CO2 fluxes processed with the recently developed ogive optimization method versus the conventional calculation showed that eliminating low-frequency contributions had a considerable effect. On average, the ogive optimization method delivered 6.9 % higher net ecosystem exchange rates than the conventional method. This dissertation provides new insights into how to obtain better measurements of matter and energy fluxes from EC measurements by a) considering storage terms otherwise neglected, b) using harmonic analysis for calculating ground heat fluxes, c) discarding fluxes from behind the anemometer and d) applying the ogive optimization method.