Extending periodic eddy covariance latent heat fluxes through tree sap-flow measurements to estimate long-term total evaporation in a peat swamp forest (original) (raw)
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Evapotranspiration (ET) represents the second largest flux in terrestrial ecosystem water budget. In recent years, much attention has been given to the coherent linkages among hydrological cycle, ecophysiological processes, disturbances, and ecosystem function. However, quantification of ET at various temporal and spatial scales remains challenging (e.g., continuous changes of ET with time of a forest). Large uncertainties and measurement errors exist in fully accounting the ET flux, a process that involves both the physical (atmospheric and soil water control) and biological processes (leaf stomata and stem conductance control). In 2004, we established three research sites to study the climatic and forest management effects on ecosystem carbon and water balances in three contrasting forests: an oak openings in NW Ohio, a recent plantation of loblolly pine in eastern North Carolina, and a 13 year-old loblolly pine stand in eastern NC. The oak-opening ecosystem in a dry, cold environ...
PhD Desertation
This dissertation focused on using eddy covariance technique to investigate a variety of primary hydrologic, radiative and turbulent transport processes driving forest-atmosphere exchanges of heat and water vapor at a subtropical evergreen forest. A total of six chapters was described in this study. A brief literature review on theoretical and applied eddy covariance techniques was introduced in Chapter1. Climatological condition and experimental setup at Lien-Hua-Chih study site were presented in Chapter 2. Determining the adequate averaging periods and reference coordinates for measuring surface heat and water vapor fluxes over the mountainous terrain was devoted in Chapter 3. In sequent Chapter 4, a gap-filling model, combining principle component analysis and the K-nearest neighbor algorithm, for estimating latent heat gaps was developed. In Chapter 5, a surface resistance parameterizing model for the Penman-Monteith equation suitable for application on hourly time scale was proposed. Finally, some thoughts summarizing my findings and future works were given in Chapter 6.
Theoretical and Applied Climatology, 2005
Evapotranspiration of a mixed European beech -Norway spruce forest was measured by means of the eddy covariance technique during a 19 day fine weather period in August 2000. Site conditions were non-ideal as to conventional micrometeorological standards. Sloping terrain and heterogeneously composed, small fetch required a thorough evaluation of the results by (1) the degree of energy balance closure (EBC), (2) footprint analysis and (3) cross-checking against soil water depletion estimates of evapotranspiration. Spatially distributed soil moisture measurements (194 permanent measuring points across a 70 Â 70 m plot) guaranteed extended representativeness of the soil water budget method. Due to the convectively active atmosphere during the observation period and to the low measuring height, the source area of the latent heat flux remained small. Footprint analysis revealed that in 95% of the situations with positive net radiation the cumulative latent heat flux reached 50% of the total flux at a maximum upwind distance of 135 (AE27) m. Fifty five percent of the peak footprints occurred within the soil moisture measuring plot, providing good spatial comparability between the two methods for determining evapotranspiration. They differed only by 0.2 mm for the whole period, showing better agreement in the second half of the period, when water redistribution in the subsoil had ceased. EBC evaluated on a daily basis ranged between 73 and 92%. No clear relation between magnitude of the closure gap and meteorological parameters could be identified. Overall, 46% of the incident net radiation drove evapotranspiration and 28% were released as sensible heat, the Bowen ratio being sensitive to weather conditions and wind direction, though. The results of this study give confidence that the eddy covariance technique is a useful tool to measure forest evapotranspiration also in complex terrain.
A comparison of methods for determining forest evapotranspiration
A multi-year, multi-technique study was conducted to measure evapotranspiration and its components within an uneven-aged mixed deciduous forest in the Southeastern United States. Four different measurement techniques were used, including soil water budget (1 year), sap flow (2 years), eddy covariance (5 years), and catchment water budget (31 years). Annual estimates of evapotranspiration were similar for the eddy covariance and catchment water balance techniques, averaging 571 ± 16 mm (eddy covariance) and 582±28 mm (catchment water balance) per year over a 5-year period. There were qualitative similarities between sap flow and eddy covariance estimates on a daily basis, and sap flow estimates of transpiration were about 50% of annual evapotranspiration estimated from eddy covariance and catchment studies. Soil evaporation was estimated using a second eddy covariance system below the canopy, and these measurements suggest that soil evaporation explains only a small portion of the difference between sap flow estimates of transpiration and eddy covariance and catchment water budget estimates of evapotranspiration. Convergence of the catchment water balance and eddy covariance methods and moderately good energy balance closure suggests that the sap flow estimates could be low, unless evaporation of canopy-intercepted water was especially large. The large species diversity and presence of ring-porous trees at our site may explain the difficulty in extrapolating sap flow measurements to the spatial scales representative of the eddy covariance and catchment water balance methods. Soil water budget estimates were positively correlated with eddy covariance and sap flow measurements, but the data were highly variable and in error under conditions of severe surface dryness and after rainfall events.
Ecological Modelling, 2004
We used a combination of eddy flux, stem sap flow and environmental measurements with an integrated resistance/energy model to analyse the seasonality of energy and water fluxes in a 50-year-old boreal Scots pine forest (62 • 51 N, 30 • 40 E). The analyses utilised data obtained from three contrasting growing seasons (days 120-270 of 2000-2002). The measured latent heat above the canopy (LE e) accounted for the majority of forest net radiation (R n) from June to mid-August (37.6-42.7%), while sensible heat (H e) was the dominant consumer of net radiation (41.3-52.4%) at other times during the growing season. The seasonal sums of LE e were 362, 395 and 418 MJ m −2 for 2000-2002, respectively. The lower LE e in 2000 than in the other years resulted from the low rainfall and high vapour pressure deficit in July-August of that year. Canopy transpiration (LE c) accounted for 82-87% of LE e , and was generally coupled with seasonal patterns of daily mean temperature and net radiation, but was reduced by the low soil water content (<0.25 m 3 m −3) and a high vapour pressure deficit (>1.1 kPa) during the summer. The modelled flux components at the forest floor were significant throughout the growing season, with daily-averaged rates ranging between 2 and 50 W m −2 for net radiation, 5 and 30 W m −2 for latent heat, and 5 and 40 W m −2 for sensible heat. The Bowen ratio (H e /LE e) was within the range 0.5-3.0 for over 80% of the time during the season, and the ratio between the forest floor sensible and latent heat (H s /LE s) varied from 1.2 to 4.5. Day-today variation in the ratio H s /LE s was greatly controlled by the soil moisture in summer, but by the mean daily temperature in spring and autumn. Although the canopy heat storage and soil heat flux were important on diurnal and seasonal time scales, their seasonal integrations accounted for only a small portion of net radiation.
Biogeosciences, 2010
We examined the water balance components of an 80-year-old Scots pine (Pinus sylvestris L.) forest stand in the Campine region of Belgium over a ten year period using five very different approaches; our methods ranged from data intensive measurements to process model simulations. Specifically, we used the conservative ion method (CI), the Eddy Covariance technique (EC), an empirical model (WATBAL), and two process models that vary greatly in their temporal and spatial scaling, the ORCHIDEE global land-surface model and SECRETS a stand-to ecosystemscale biogeochemical process model. Herein we used the EC technique as a standard for the evapotranspiration (ET) estimates. Using and evaluating process based models with data is extremely useful as models are the primary method for integration of small-scale, process level phenomena into comprehensive description of forest stand or ecosystem function. Results demonstrated that the two process models corresponded well to the seasonal patterns and yearly totals of ET from the EC approach. However, both WATBAL and CI approaches overestimated ET when compared to the EC estimates. We found significant relationships between several meteorological variables (i.e., vapour pressure deficit [VPD], mean air temperature [T air ], and global radiation [Rg]) and ET on monthly basis for all approaches. In contrast, few relationships were significant on annual basis. Independent of the method examined, ET exhibited low inter-annual variability. Consequently, drainage fluxes were highly correlated with annual precipitation for all approaches examined, except CI.
Remote Sensing for Agriculture, Ecosystems, and Hydrology XVIII, 2016
Determining water and carbon fluxes over a vegetated surface is important in a context of global environmental changes and the fluxes help in understanding ecosystem functioning. Pursuant to this, the study measured evapotranspiration (ET) using an eddy covariance (EC) system installed over an intact example of the Albany Thicket (AT) vegetation in the Eastern Cape, South Africa. Environmental constraints to ET were also assessed by examining the response of ET to biotic and abiotic factors. The EC system comprised of an open path Infrared Gas Analyser and Sonic anemometer and an attendant weather station to measure bi-meteorological variables. Post processing of eddy covariance data was conducted using EddyPro software. Quality assessment of fluxes was also performed and rejected and missing data were filled using the method of mean diurnal variations (MDV). Much of the variation in ET was accounted for by the leaf area index (LAI, p < 0.001, 41%) and soil moisture content (SWC, p < 0.001, 32%). Total measured ET during the experiment was greater than total rainfall received owing to the high water storage capacity of the vegetation and the possibility of vegetation accessing ground water. Most of the net radiation was consumed by sensible heat flux and this means that ET in the area is essentially water limited since abundant energy was available to drive turbulent transfers of energy. Understanding the environmental constraints to ET is crucial in predicting the ecosystem response to environmental forces such as climate change.
Water flux estimates from a Belgian Scots pine stand: a comparison of different approaches
Journal of Hydrology, 2003
Four distinct approaches, that vary markedly in the spatial and temporal resolution of their measurement and process-level outputs, are used to investigate the daily and seasonal water vapour exchange in a 70-year-old Belgian Scots pine forest. Transpiration, canopy interception, soil evaporation and evapotranspiration are simulated, using a stand-level process model (SECRETS) and a soil water balance model (WAVE). Simulated transpiration was compared with up-scaled sap flow measurements and simulated evapotranspiration to eddy covariance measurements.