Modelling the energy exchange between the snowpack and the atmosphere is critical for many hydrological applications. Of the terms present in the snow energy balance, the turbulent fluxes of sensible and latent heat are the most challenging to estimate, particularly within mountain environments where the hydrological importance is great. Many of the flux estimation techniques, such as the bulk transfer method, are poorly adapted for use in complex terrain. In order to characterize the turbulence and to assess the suitability of flux estimation techniques, eddy covariance flux measurements and supporting meteorological data were collected from two mountain valley forest openings in Kananaskis Country, AB. These sites were generally calm, however wind gusts were frequently observed which markedly affected the turbulence characteristics and increased the rates of momentum and heat transfer. In order to successfully apply the bulk transfer technique at these sites, it was necessary to use environment-specific transfer coefficients to account for the effect of the surrounding complex terrain. These observations were compared with data collected on a treeless alpine ridge near Whitehorse, YT, where it was found that many of the turbulence characteristics were similar to flat sites. However, the boundary layer formed over the alpine ridge was very thin and the site was poorly suited for estimating surface fluxes. The mountain results were further contrasted with data collected over a homogeneous and flat prairie site located near Saskatoon, SK. This site included measurement of all of the snow energy terms, permitting an estimate of the energy balance closure obtainable over snow surfaces. The observed energy balance residual was very large, indicating that the eddy covariance technique was unable to capture all of the turbulent energy. It was concluded that an unmeasured transfer of sensible heat was occurring which was strongly correlated with the long-wave radiation balance. Mechanisms for this relationship were hypothesized. Two snow energy balance models were used to investigate the energy imbalance, where it was observed that the flux terms could be suitably simulated if effective parameters were used to augment the sensible heat transfer rate. The results from this thesis contribute to the understanding of heat transfer processes over snow surfaces during mid-winter conditions and improve the ability to model turbulent heat and mass fluxes from snow surfaces in complex environments.

Modelling the energy exchange between the snowpack and the atmosphere is critical for many hydrological applications. Of the terms present in the snow energy balance, the turbulent fluxes of sensible and latent heat are the most challenging to estimate, particularly within mountain environments where the hydrological importance is great. Many of the flux estimation techniques, such as the bulk transfer method, are poorly adapted for use in complex terrain. In order to characterize the turbulence and to assess the suitability of flux estimation techniques, eddy covariance flux measurements and supporting meteorological data were collected from two mountain valley forest openings in Kananaskis Country, AB. These sites were generally calm, however wind gusts were frequently observed which markedly affected the turbulence characteristics and increased the rates of momentum and heat transfer. In order to successfully apply the bulk transfer technique at these sites, it was necessary to use environment-specific transfer coefficients to account for the effect of the surrounding complex terrain. These observations were compared with data collected on a treeless alpine ridge near Whitehorse, YT, where it was found that many of the turbulence characteristics were similar to flat sites. However, the boundary layer formed over the alpine ridge was very thin and the site was poorly suited for estimating surface fluxes. The mountain results were further contrasted with data collected over a homogeneous and flat prairie site located near Saskatoon, SK. This site included measurement of all of the snow energy terms, permitting an estimate of the energy balance closure obtainable over snow surfaces. The observed energy balance residual was very large, indicating that the eddy covariance technique was unable to capture all of the turbulent energy. It was concluded that an unmeasured transfer of sensible heat was occurring which was strongly correlated with the long-wave radiation balance. Mechanisms for this relationship were hypothesized. Two snow energy balance models were used to investigate the energy imbalance, where it was observed that the flux terms could be suitably simulated if effective parameters were used to augment the sensible heat transfer rate. The results from this thesis contribute to the understanding of heat transfer processes over snow surfaces during mid-winter conditions and improve the ability to model turbulent heat and mass fluxes from snow surfaces in complex environments.

A single compressor station site along a gathering line network was monitored for fugitive methane emissions to quantify long-term emissions in Appalachia Virginia. Continuous monitoring was conducted from January 2021 to April 2021. The compressor station undergoing monitoring operated two CAT3516 Tale and one CAT3516 B engines operating at 80% of max output flow. Data presented on methane emissions during this period was gathered with an eddy covariance monitoring station. This station was equipped with an LI-7700 methane analyzer, LI-7500A - CO_2/H_2 O analyzer as well as a sonic anemometer these sensors could be observed remotely through cellular connection. The data is represented in flux output ((µmol)/(s m^2 )) as well as kg CO_2 equivalence of methane outlined by the EPA greenhouse gas inventory. The average daily emissions for this compressor station are estimated to be 136 kg CO_2 equivalent emissions. This study shows that the site during the observational period the compressor station emitted on average are estimated to be 5.43 kg of CH_4 per day. / Master of Science / There has been an increased interest in quantifying and recording methane (CH_4) emissions among all sectors. A main focus of interest among methane is to understand fugitive gasses and emissions resulting from the natural gas sector. Leaks along pipelines are most likely occurring at connection points between components. This study aimed to continuously monitor a pipeline compressor station in Appalachia Virginia. Compressor stations are just one component of the pipeline network as well as the natural gas production and delivery chain attributed with CH_4 emissions. To monitor methane emissions at the site a stationary eddy covariance monitoring station was installed that was equipped with an open path methane analyzer, open path CO_2 and H_2 O analyzer, and a sonic anemometer. The data gathered was used to calculate the flux of methane which is the amount of methane being generated or absorbed by the area of interest. The goal of this study was to continuously monitor methane emissions of a natural gas compressor station. Data presented in this study was collected from January 2021 to April 2021. Data was presented in the flux output ((µmol)/(s m^2 )) as well as kg CO_2 equivalence of methane outlined by the EPA greenhouse gas inventory.

Cultivos de extensa área de produção e manejo intensivo, como a indústria canavieira, demandam maior responsabilidade em relação ao conhecimento da sua contribuição na diminuição ou aumento dos gases de efeito estufa. Dentro do manejo do sistema de produção de cana-de-açúcar existe uma etapa de renovação do canavial, período pós última colheita que antecipa a implantação de um novo cultivo de cana. O estudo tem por objetivo avaliar a contribuição do período de renovação de um canavial em relação aos fluxos de massa e energia dentro do sistema produtivo de cana-de-açúcar, considerando a inserção de um cultivo de soja após período de pousio sobre solo com cobertura de palha e sem cobertura, em um canavial localizado em Piracicaba - SP, Brasil. A obtenção dos fluxos foi realizada com o método \"Eddy Covariance\". Com o solo em pousio e cobertura de palha, o fluxo líquido de CO2 (NEE) médio foi de 2,51 µmolCO2 m-2 s-1 e evapotranspiração média do período de 0,72 mm d-1. O período de pousio com solo sem cobertura teve um NEE de 3,10 µmol CO2 m-2 s-1 e evapotranspiração média do período de 2,04 mm d-1. Com a inserção de um cultivo de soja, a área passou a apresentar um comportamento de consumo no lugar da emissão de CO2, com um fluxo médio diário de NEE (da semeadura à maturação de colheita) de -1,47 µmolCO2 m-2 s-1 e evapotranspiração média de 4,52 mm d-1. Para o balanço energético da renovação do canavial, 84,6% da energia disponível no período de pousio sob os dois manejos de cobertura foi utilizada pelo fluxo de calor latente e fluxo de calor sensível, e para o cultivo de soja esta relação foi de 73,4%. Considerando o balanço da troca líquida de CO2 em relação às médias apresentadas, o saldo é positivo, ou seja, a renovação deste canavial foi responsável por maior emissão que consumo de CO2, de acordo com o manejo realizado. O cultivo da soja não compensou a emissão do período em pousio, mas tem potencial de amenizar no total do período. / Crops with extensive production areas and intensive management such as the sugar cane industry demand greater responsibility in relation to the knowledge of their contribution in the reduction or increase of greenhouse gases. Within the management of the sugar cane production system there is a stage of renewal of the cane field, a period after the last harvest that anticipates the implantation of a new sugarcane crop. The objective of this study is to evaluate the contribution of the renewal period of a sugarcane field in relation to the mass and energy flows within the sugarcane production system, considering the insertion of a soybean crop after fallow period on soil with cover of straw and without cover, in a field located in Piracicaba - SP, Brazil. Flows were obtained using the \"Eddy Covariance\" method. In the fallow soil with straw cover, the mean net CO2 flux (NEE) was of 2,51 µmol CO2 m-2 s-1 and average evapotranspiration of the period was 0,72 mm day-1. The fallow period without cover had an mean NEE of 3,10 µmol CO2 m-2 s-1 and average evapotranspiration of 2,04 mm day-1. With the insertion of a soybean crop, the area presented assimilation in place of CO2 emission, with a daily mean of NEE (from sowing to harvest maturation) of -1,47 µmol CO2 m-2 s-1 and average evapotranspiration of 4,52 mm day-1. For the energy balance of the sugarcane renewal, 84,6% of the available energy in the fallow period under the two cover treatments was used by the latent heat flux and sensible heat flux. For soybean cultivation this relation was 73,4%. Considering the balance of the net CO2 exchange in relation to the averages presented, the balance is positive, that is, the renovation of this sugarcane field was responsible for higher emissions than CO2 assimilation, according to the management implemented. The soybean crop did not compensate for the fallow period, but it has the potential to reduce the total balance.

The interactions between vegetation and climate are complex and critical to our ability to predict and mitigate climate change. Savanna ecosystems, unique in their structure and composition, are particularly dynamic and their carbon cycling has been identified as highly significant to the global carbon budget. Understanding the responses of these dynamic ecosystems to environmental conditions is therefore central to both ecosystem management and scientific knowledge. Longleaf pine ecosystems are highly biodiverse and unique savanna ecosystems located in the south-eastern USA – an important current carbon sink and key area identified for future carbon sequestration. These ecosystems depend on fire to maintain their structure and function, and the longleaf pine tree itself (Pinus palustris Mill.) has been noted for its resilience to drought, fire, pests and storms and is thus becoming increasingly attractive as both a commercial forestry species and a provider of other ecosystem services. Previous process-based models tested in the south-eastern USA have been shown to fail in conditions of drought or rapid disturbance. Consequently, in order to inform management and understand better the physiology of these ecosystems, there is a need for a process-based model capable of upscaling leaf-level processes to the stand scale to predict GPP of longleaf pine savannas. P.palustris exists across a wide range of soil moisture conditions, from dry sandy well-drained soils (xeric) to claypan areas with higher moisture content (mesic). Previous work has demonstrated that this species adjusts many aspects of its physiology in response to these differing soil conditions, even under identical climate. The research in this thesis supports these previous findings, but additionally explores, with the assistance of the Soil Plant Atmosphere model (SPA), the productivity response of P. palustris across the soil moisture gradient. Contrary to expectations, measurements, field observations and modelling suggest that P. palustris trees growing in already water-limited conditions cope better with exceptional drought than their mesic counterparts. At the leaf-level, xeric P. palustris trees were found to have higher measured net photosynthesis, but the lower stand density and leaf area at this site meant that in non-drought conditions mesic P. palustris annual gross primary productivity (GPP) was 23% greater than xeric annual GPP. Initial upscaling of leaf-level processes to the canopy scale using the SPA model found that, during the growing season when other components of longleaf pine ecosystems are active, the longleaf pine may only be responsible for around 65% of the total productivity. Other important components of longleaf pine savannas are oaks and grasses which, with pine, constitute 95% of longleaf pine ecosystem biomass. Each of these groups, however, responds differently to fire and water availability. Despite this, the other components of longleaf pine savannas have received limited research attention and have never been modelled using a process-based model such as SPA. As integral components of longleaf pine carbon budgets, it is essential that the physiology and productivity of oaks and grasses in this system are better understood. The research in this thesis studied the productivity response of these groups during drought across a soil moisture gradient, and found that oak and pines at each site appear to fill separate ecohydrological niches depending on whether or not they are growing in a xeric or mesic habitat. As expected, the highest drought tolerance was found in the C4 grass, wiregrass (Aristida stricta), at both xeric and mesic sites. In order to further explore the contributions of the different functional groups in longleaf pine savannas, the SPA model was adapted to run with concurrent functional groups and to represent the different photosynthetic pathways of the understorey grasses (C4) and the canopy trees (C3). The aim of this part of the thesis was to represent better a savanna ecosystem in a process-based model and explore and quantify the contributions of each functional group diurnally, seasonally, annually and interannually. Modelling results suggest that accurately representing the phenology not only of trees but of grasses, is critical to capturing ecosystem GPP and its variability. This phenology may not only be seasonally controlled, but also dictated by fire. Overall, this research highlights the importance of continued research into savanna and savanna-like ecosystems. Additionally, it provides an insight into the responses of multiple ecosystem components to an extreme drought, and how these responses differ at leaf, stand and landscape scales. The thesis also employs a little-used method of combining eddy-covariance data with a process-based model to separate out different ecosystem components, a method becoming more common but not yet widely tested.

Soil carbon (C) is important because even small changes in soil C can affect atmospheric concentrations of CO₂, which in turn can influence global climate. Adequate soil carbon is also required to maintain soil quality, which is important to if agricultural production is to be sustained. The soil carbon balance of New Zealand's pastoral soils is poorly understood, with recent research showing that soils under dairy pasture have lost large amounts of C during the past few decades. The main objective of this research was to determine an annual farm scale C budget for an intensively grazed dairy farm, with a second objective being to determine the amount of CO₂-C lost following cultivation for pasture renewal, and soil pugging by dairy cattle. A third objective was to investigate the environmental controls of CO₂ exchange in a dairy farm pasture system. Net ecosystem exchange (NEE) of CO₂ was measured using an eddy covariance (EC) system from 15 December 2007 to 14 December 2008. Closed chamber techniques were used to measure CO₂ emissions from three cultivated paddocks and three adjacent pasture paddocks between 26 January 2008 and 5 March 2008. CO₂ emissions were also measured using chambers from pugged and control plots between 25 June and 5 August. Coincidentally this research was carried out in a year with a severe summer/autumn drought and a wetter than usual winter. Annual NEE measured with the eddy covariance system was -1,843 kg C ha⁻¹ (a C gain by the land surface). Accounting for C in supplement import, milk export, pasture export and losses in methane, the dairy pasture system was a net sink of -880±500 kg C ha⁻¹. This C sequestration occurred despite severe drought during the study, which was in contrast to other studies of grasslands during drought. Cultivation under dry conditions did not increase cumulative CO₂-C emissions compared to adjacent pasture paddocks. However, when C inputs to pasture paddocks via photosynthesis were included in calculations, net C loss from the cultivated paddocks (during the 39 day study) was estimated to be 622 kg C ha⁻¹ more than the pasture paddocks. CO₂ emissions were lower from pugged plots compared to control plots, probably caused by decreased microbial and root respiration due to wetter soil conditions, and lowered root respiration as a result of lower pasture production. Volumetric soil moisture content (soil moisture) had a dominant effect on CO₂ exchange at a range of temporal scales. Respiration and photosynthesis were both reduced when soil moisture was below 43% (~the lower limit of readily available water) and photosynthesis virtually ceased when soil moisture declined below 24% (~wilting point). Soil moisture also influenced the relationship between temperature and respiration and photosynthetic flux density (PPFD) and NEE. These results suggest that management related soil disturbances of occasional cultivation for pasture renewal and soil pugging, are unlikely to cause large losses of soil C. Further, a severe drought also did not cause CO₂-C losses from the land surface to the atmosphere on an annual scale, in contrast to previous studies.

Phenological events in temperate forests, such as bud burst and senescence, exert strong control over seasonal fluxes of water, energy and carbon. The timing of these transitions is influenced primarily by air temperature and photoperiod, although the exact nature and magnitude of these controls is poorly understood. In this dissertation, I use in situ and remotely sensed observations of phenology in combination with surface meteorological data and measurements of biosphere-atmosphere carbon exchanges to improve understanding and develop models of canopy phenology in temperate forest ecosystems. In the first element of this research I use surface air temperatures and eddy covariance measurements of carbon dioxide fluxes to evaluate and refine widely used approaches for predicting the onset of photosynthesis in spring that account for geographic variation in thermal and photoperiod constraints on phenology. Results from this analysis show that the refined models predict the onset of spring photosynthetic activity with significantly higher accuracy than existing models. A key challenge in developing and testing these models, however, is lack of adequate data sets that characterize phenology over large areas at multi-decadal time scales. To address this need, I develop a new method for estimating long-term average and interannual dynamics in the phenology of temperate forests using time series of Landsat TM/ETM+ images. Results show that estimated spring and autumn transition dates agree closely with in-situ measurements and that Landsat-derived estimates for the start and end of the growing season in Southern New England varied by as much as 4 weeks over the 30-year record of Landsat images. In the final element of this dissertation, I use meteorological data, species composition maps, satellite remote sensing, and ground observations to develop models of springtime leaf onset in temperate deciduous forests that account for geographic differences in how forest communities respond to springtime climate forcing. Results demonstrate important differences in cumulative heating requirements and photoperiod cues among forest types and that regional differences in species composition explain substantial geographic variation in springtime phenology of temperate forests. Together, the results from this dissertation provide an improved basis for observing and modeling springtime phenology in temperate forests.

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No description available.

<p>Forest ecosystems are a significant component of the global carbon (C) cycle. Afforestation is considered a cost-effective and ecologically viable means to sequester atmospheric carbon. However, afforestation requires intensive management practices, including thinning, to maintain and enhance the carbon sequestration capability of the forest. This study examines thinning effects on forest carbon dynamics using eddy covariance (EC) methods. In January 2012, a 74-year-old white pine (<em>Pinus strobus</em>) plantation located in southern Ontario was selectively thinned. Approximately 30% of trees, equating to 2308 m³ of wood (sawlogs and pulpwood), were removed to improve light, water and nutrient availability for remaining trees. Fluxes of energy, water, carbon dioxide (CO₂) as well as meteorological variables were measured throughout the year following thinning and compared to data from the previous 9 years to evaluate effects of thinning on forest carbon dynamics. Mean annual net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) from the 9 years prior to thinning were 290, 1413 and 1118g C m^-2<strong>,</strong> respectively. Post-thinning NEP, GEP and RE were 154, 1509 and 1350 g C m<strong>^-</strong>²<strong> </strong>year^-1, respectively. Post-thinning NEP was significantly less than pre-thinning at the annual time scale due to higher RE, however post-thinning fluxes were still within the range of interannual variability. At this site, approximately 20% of interannual variability in NEP, GEP and RE was explained by environmental conditions. Effects of extreme weather events, particularly heat and drought stress, were demonstrated to negatively impact NEP. Biotic responses to environmental drivers explained the remaining 80% of interannual variability in fluxes. Thinning did not significantly impact these responses. Further, results suggest that thinning may improve tolerance to drought stress by improving water availability for remaining trees. Therefore, thinning has the potential to effectively reduce resource competition and stimulate growth and carbon sequestration in temperate coniferous forests.</p> / Master of Science (MSc)