Energy partitioning at treeline forest and tundra sites and its sensitivity to climate change

Abstract

Summertime energy budgets of contiguous wetland tundra and forest near Churchill, Manitoba along the coast of Hudson Bay were measured over a five year period, 1989–1993. An examination of differences in energy budgets between the two sites showed that net radiation was similar in all years. Soil heat flux was greater at the tundra site in most, but not all, years. However, sensible heat flux was always larger at the forest site and latent heat flux was always greater at the tundra site. Mean daily Bowen ratios at both sites were less than unity in all years. Average Bowen ratios for the five years were 0.45 for tundra and 0.66 for forest. Wind direction is used as an analogue for changing climatic conditions where onshore winds are cooler and moister than offshore winds. Sensible and latent heat fluxes at both sites varied significantly between onshore and offshore wind regimes. However, differences between onshore and offshore fluxes at the tundra site were larger than for the forest. Thus, Bowen ratios also varied more at the tundra site. We have plotted the ratio of tundra‐to‐forest Bowen ratios as a measure of the relative sensitivity of energy partitioning to climatic change. The ratio decreases with increasing vapour pressure deficit (and increasing air temperature). We interpret these results as suggesting that energy partitioning over the wetland tundra is more sensitive to changes in climate than the treeline forest environment. Thus, as the climate warms and becomes drier, more additional energy goes into evaporation of water from the wetland tundra than from the forest.     

The Influence of Changes in Vegetation Type on the Surface Energy Budget

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A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest

Sensible and latent heat flux densities (H and E) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and E underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. for/L<0.0, EC favouredH and BREB favoured E. Practical and theoretical limitations of the BREB and EC methods above forests are discussed. The most plausible causes for the failure of EC to close the surface energy balance are a low frequency loss of flux and the failure of a single point measurement to account for the spatial dispersive flux. The most plausible causes of the EC-BREB energy partitioning anomaly are the invalidity of the BREB similarity assumption and the violation of flux-gradient diffusion assumptions in the near-field diffusion region.     

Bowen ratio/energy balance technique for estimating crop net CO2 assimilation,and comparison with a canopy chamber

Summary This paper describes a Bowen ratio/energy balance (BREB) system which, in conjunction with an infra-red gas analyzer (IRGA), is referred to as BREB+ and is used to estimate evapotranspiration (ET) and net CO₂ flux (NCF) over crop canopies. The system is composed of a net radiometer, soil heat flux plates, two psychrometers based on platinum resistance thermometers (PRT), bridge circuits to measure resistances, an IRGA, air pumps and switching valves, and a data logger. The psychrometers are triple shielded and aspirated, and with aspiration also between the two inner shields. High resistance (1 000 ohm) PRT's are used for dry and wet bulbs to minimize errors due to wiring and connector resistances. A high (55 K ohm) fixed resistance serves as one arm of the resistance bridge to ensure linearity in output signals. To minimize gaps in data, to allow measurements at short (e.g., 5 min) intervals, and to simplify operation, the psychrometers were fixed at their upper and lower position over the crop and not alternated. Instead, the PRT's, connected to the bridge circuit and the data logger, were carefully calibrated together. Field tests using a common air source showed appartent effects of the local environment around each psychrometer on the temperatures measured. ET rates estimated with the BREB system were compared to those measured with large lysimeters. Daily totals agreed within 5%. There was a tendency, however, for the lysimeter measurements to lag behind the BREB measurements. Daily patterns ofNCF estimated with the BREB+ system are consistent with expectations from theories and data in the literature. Side-by-side comparisons with a stirred Mylar canopy chamber showed similarNCF patterns. On the other hand, discrepancies between the results of the two methods were quite marked in the morning or afternoon on certain dates. Part of the discrepancies may be attributed to inaccuracies in the psychrometric temperature measurements. Other possible causes include the highly artificial air turbulence in the canopy chamber and possible associated stomatal response. More work is necessary to identify conclusively the causes. In spite of these uncertainties, the BREB+ technique appears well suited for the automated and simultaneous tracking of photosynthetic performance and water economy of crops in their virtually undisturbed natural environment.To whom reprint requests should be sent.With 7 Figures     

Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour

This is the second paper describing a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. Two eddy correlation units were operated in the daytime to measure the fluxes of sensible heat and water vapour and other turbulence statistics at various heights within and above the stand. Net radiation was measured above the overstory using a stationary net radiometer and beneath the overstory using a tram system. Supplementary measurements included soil heat flux, humidity above and beneath the overstory, profiles of wind speed and air temperature, and the spatial variation of sensible heat flux near the forest floor.The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. On some days, energy budget closure was far better than on others. The average value of the Bowen ratio was 2.1 above the stand and 1.4 beneath the overstory. The mid-morning value of the canopy resistance was 150–450 s/m during the experiment and mid-day value of the Omega factor was about 0.20. The daytime mean canopy resistance showed a strong dependence on the mean saturation deficit during the two-week experimental period.The sum of sensible and latent heat fluxes beneath the overstory accounted for 74% of the available energy flux beneath the overstory. One of the reasons for this energy imbalance was that the small number of soil heat flux plates and the short pathway of the radiometer tram system was unable to account for the large horizontal heterogeneity in the available energy flux beneath the overstory. On the other hand, good agreement was obtained among the measurements of sensible heat flux made near the forest floor at four positions 15 m apart.There was a constant flux layer in the trunk space, a large flux divergence in the canopy layer, and a constant flux layer above the stand. Counter-gradient flux of sensible heat constantly occurred at the base of the canopy.The transfer of sensible heat and water vapour was dominated by intermittent cool downdraft and warm updraft events and dry downdraft and moist updraft events, respectively, at all levels. For sensible heat flux, the ratio of the contribution of cool downdrafts to that of warm updrafts was greater than one in the canopy layer and less than one above the stand and near the forest floor.     

内蒙古半干旱草原能量物质交换的微气象方法估算

根据1998年5～8月和1999年8月在中国科学院内蒙古草原生态系统定位试验站进行微气象观测的资料,作者分析了该地区能量平衡及其各分量的基本特征.结果表明:(1)净辐射通量的转化形式有明显的季节性变化,5～6月份,净辐射能大部分用于感热交换,而后期则多用于潜热交换,5～8月份的日波文比值分别为1.26,1.42,0 41和0.20.(2)观测期间,波文比的日变化特征表现为,早晚变化大不稳定,而白天则相对稳定.(3)用涡度相关方法观测的感热和潜热通量之和与同期的净辐射相比较,前者的结果偏小1 5%左右,两种方法观测到的潜热通量的差异达平均35%左右.(4)半干旱草原CO2通量有明显的日变化,在生长旺期,白天CO2通量强度可达到1.5 mg s-1m-2以上,但在生长后期,1998年和1999年8月份的白天CO2通量强度分别为0.38 mg s-1m-2和0.2 mg s-1m-2左右;其差异与草地土壤水分和植物长势有关.     

Applying single and two layer canopy models to derive conductances of a Scots pine plantation from micrometeorological measurements

Summary A single layer (Penman-Monteith) and a two layer (modified Shuttleworth-Wallace) evapotranspiration (ET) model are used alternatively to derive conductances related to the dominant fluxes of water vapor from a semi-closed Scots pine plantation. The derivations are based on micrometeorological measurements of above canopy energy flux densities and a simple resistance network. For a period of consecutive fine weather days, below canopy net radiation and below canopy ET were about 20 percent of the corresponding above canopy values. Resulting conductances for latent heat flux agreed well with porometric measurements of pines and understory scaled to canopy level. The shift from single to two layer modelling reduced the canopy conductance to pine conductance by the fraction of understory ET.However, characteristics of porometer results and micrometeorologically derived conductances were quite different: The porometer estimates of conductance were highly variable due to stomatal response to local environmental conditions or natural variability within the tree canopy and vegetation patches which characterized the forest understory. Micrometeorologically derived conductances integrate spatially resulting in relatively smooth and repetitive daily patterns that lack the information of small scale variability. This is seen as a favorable feature of micrometeorological derived conductances when used for the parameterization of atmospheric models for climate research as long as small scale bio-diversity is irrelevant.With 5 Figures     

半干旱区不同下垫面近地层湍流通量特征分析

本文分析了2003～2005年半干旱区退化草地和农田下垫面近地层湍流通量日、季、年的变化特征, 探讨了不同年份的气候背景和下垫面土地覆被对地气交换过程的影响。结果表明: 半干旱区退化草地和农田近地层湍流通量具有明显的日变化、季变化周期; 地气间通量交换年际间的差异主要受当年的气候背景影响, 尤其是降水的影响; 同时还受到下垫面覆被的影响。潜热通量和感热通量的能量分配比率呈反位相变化, 且农田和退化草地的变化趋势相似, 在夏季潜热通量所占比例均超过感热通量; 两种下垫面的波恩比月均值变化趋势十分相似, 量级接近, 夏季低, 春、秋季高; 夏季退化草地和农田下垫面波恩比均小于或等于1。     

农田蒸散量计算方法的比较研究

根据观测资料,采用波文比法、廓线梯度法和综合阻地等分别计算了农田植被层上的蒸散量,并对比分析了不同方法之间的相对误差和相关程度。结果表明,用波文比法计算的潜热通量存在系统性偏小的现象,而工法和综合阻抗法的计算结果比较一致。文中还讨论了不同方法的优缺点和产生误差的可能原因。     

Surface fluxes estimation over agricultural areas. Comparisonof methods and the effects of land surface inhomogeneity

Summary  The Bowen Ratio-Energy Balance (BREB) and the aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat flux over an irrigated agricultural area (IAA) and over two dry agricultural areas (DAA1 and DAA2). These turbulent fluxes were analysed and particular attention paid to two specific areas. First, a quantitative analysis of sensible and latent heat fluxes obtained by the BREB method was carried out, taking into account different soil type, vegetation and surface conditions. The results showed that in IAA latent heat flux was higher than sensible heat flux, except in summer months, while in DAA1 and DAA2, sensible heat flux was higher except in the months when the vegetation was at the stage of maximum development. Second, sensible and latent heat fluxes estimates from the BREB method were compared with those obtained from the aerodynamic method. In this comparison factors such as soil type, soil vegetation cover, homogeneity or inhomogeneity of terrain and mesoscale effects such as orography and wind patterns were taken into account. The results show that in conditions of light wind, the two methods only concur if the condition of horizontal homogeniety is fulfilled. The influence of inhomogeneity seems to decrease and agreement between methods improves, if the wind is stronger and the effects of meso and synoptic scales are predominant. Received May 18, 1999/Revised March 15, 2000     

Suburban-rural energy balance comparisons in summer for Vancouver,B.C.

Simultaneous energy balance observations at a rural and a suburban site in Vancouver, B.C. during the summer of 1983 are presented. The study is a follow-up to that conducted in 1980. Many of the 1980 results were unexpected and the present study seeks to assess their representativeness. The net radiant, turbulent sensible, and rural soil heat flux densities were measured directly. The suburban heat storage was parameterized and the turbulent latent heat flux densities were resolved as residuals in the energy balances. The 1983 average diurnal energy partitioning for both sites was typical of those quoted in the literature, suggesting that the 1980 results represent an extreme case. Suburban-rural differences showed the suburban area to have a 4% increase in net radiation, a 51% increase in turbulent sensible heat, and a 46% decrease in turbulent latent heat flux density. The values of the average daytime Bowen ratio were 0.46 and 1.28 for the rural and suburban areas, respectively. The sensible heat flux density exhibited relatively large values in the late afternoon and remained directed upward on many summer evenings. Large day-to-day variability in the relative magnitude of the suburban turbulent fluxes may have been due to synoptic influences. In this environment, the turbulent surface and mixed layers are closely coupled because of the low aerodynamic resistance over the rough surface.     

Comparison of the Bowen ratio-energy balance and stability-corrected aerodynamic methods for measurement of evapotranspiration

Summary The Bowen ratio-energy balance (BREB) and the stability-corrected aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat at an irrigated alfalfa site in a semi-arid valley in northern Utah, U.S.A., during August and September of 1991. Despite inclusion of a generalized stability factor, the aerodynamic method underestimated the daytime (sunrise-sunset) sensible and latent heat fluxes by approximately 30% in comparison with the BREB method. The sum of the aerodynamic estimates of sensible and latent heat seldom balanced the energy avaiable from net radiation and change in storage. Wind speed was low during the experiment (averaging 1.6 m s^–1), and so a second analysis was run for data from daytime, non-rainy, turbulent conditions (wind > 1.5 m s^–1). This showed that sensible and latent heat were still underestimated by approximately 30% in comparison with the BREB approach. This suggests that underestimation of sensible and latent heat fluxes by the aerodynamic method was not related to the wind speed conditions during the experiment. These results show that the stability-corrected aerodynamic model did not agree with the Bowen ratio method in this experiment. It appears unlikely that the discrepancies resulted from measurement errors. Perhaps the theoretical foundation of the similarity parameters (stability functions) in the aerodynamic model are not sufficiently generalized. The discrepancies found here confirm the necessity of calibration checks on the validity of aerodynamic estimates of the turbulent fluxes.With 7 Figures     

Radiative surface temperature and energy balance of a wheat canopy

The sensible heat loss from a stand of winter wheat was calculated from radiometric measurements of crop surface temperature, measurements of air temperature, and an atmospheric resistance to momentum transfer; corresponding latent heat flux was obtained through the energy balance equation. These estimates of sensible and latent heat were compared with fluxes from the Bowen Ratio method. When radiative temperature was derived using a measured canopy emissivity of 0.98, calculations of sensible heat flux were systematically 50–100 W m^-2 less than Bowen Ratio values. The two techniques agreed more closely when an apparent emissivity of 0.96 was used with an apparent reflectivity of 0.03. The mean difference between the estimates of latent heat flux was then -16 ± 32 W m^-2.The surface temperature method showed less systematic error in comparison with the Bowen Ratio values than did estimates using the aerodynamic method.On leave from: University of Nottingham, School of Agriculture, Loughborough LE12 5RD.     

盘锦芦苇湿地水热通量计算方法的比较研究

利用2005年7月盘锦芦苇湿地生长旺季的小气候梯度系统30 min观测资料和开放式涡动相关系统10Hz原始观测资料,比较并分析了廓线法、波文比能量平衡法与涡动相关法计算的芦苇湿地生态系统水热通量。结果表明:廓线法与波文比能量平衡法计算的芦苇湿地生态系统水热通量与涡动相关法得到的芦苇湿地生态系统水热通量具有较好的相关性,但是涡动相关法存在能量不平衡。分析盘锦芦苇湿地生态系统水热通量的日变化发现,能量平衡各分量基本上以正午为中心,呈倒“U”型分布。用波文比法计算得到的芦苇湿地生态系统日感热通量最大值为164.25 W.m-2,日潜热通量最大值为294.18 W.m-2。降雨之后,芦苇湿地生态系统水热通量都有所增加,尤其是潜热通量增加显著,且峰值出现时间提前。     

Precipitation projections under GCMs perspective and Turkish Water Foundation (TWF) statistical downscaling model procedures

Insect outbreaks are major disturbances that affect a land area similar to that of forest fires across North America. The recent mountain pine bark beetle (D endroctonus ponderosae) outbreak and its associated blue stain fungi (Grosmannia clavigera) are impacting water partitioning processes of forests in the Rocky Mountain region as the spatially heterogeneous disturbance spreads across the landscape. Water cycling may dramatically change due to increasing spatial heterogeneity from uneven mortality. Water and energy storage within trees and soils may also decrease, due to hydraulic failure and mortality caused by blue stain fungi followed by shifts in the water budget. This forest disturbance was unique in comparison to fire or timber harvesting because water fluxes were altered before significant structural change occurred to the canopy. We investigated the impacts of bark beetles on lodgepole pine (Pinus contorta) stand and ecosystem level hydrologic processes and the resulting vertical and horizontal spatial variability in energy storage. Bark beetle-impacted stands had on average 57 % higher soil moisture, 1.5 °C higher soil temperature, and 0.8 °C higher tree bole temperature over four growing seasons compared to unimpacted stands. Seasonal latent heat flux was highly correlated with soil moisture. Thus, high mortality levels led to an increase in ecosystem level Bowen ratio as sensible heat fluxes increased yearly and latent heat fluxes varied with soil moisture levels. Decline in canopy biomass (leaf, stem, and branch) was not seen, but ground-to-atmosphere longwave radiation flux increased, as the ground surface was a larger component of the longwave radiation. Variability in soil, latent, and sensible heat flux and radiation measurements increased during the disturbance. Accounting for stand level variability in water and energy fluxes will provide a method to quantify potential drivers of ecosystem processes and services as well as lead to greater confidence in measurements for all dynamic disturbances.     

Spatial characteristics of surface and atmospheric properties during HartX

Summary Remote sensing and ground measurements were used to evaluate the homogeneity of the Scots pine plantation of Hartheim and the interactions between the forest and its surroundings during HartX.Remote sensing data contribute to characterization of the Scots pine plantation of Hartheim in terms of surface properties and thermal and biological characteristics of the forest. The surroundings of the main experiment site in the forest has homogenous normalized difference vegetation index, microwave specle and brightness temperatures to all directions for at least 500 m. Local sensible heat flux estimates using satellite measured brightness temperatures andin situ measured wind and air temperatures showed good agreement.On some days during HartX the test area was influenced by advection of dry air from the northern Upper Rhine Valley. Agricultural surroundings close to the forest are influenced by breezes flowing out of the forest during radiative nights with weak large-scale motion.With 7 Figures     

Flux agreement above a Scots pine plantation

Summary The surface energy exchange of 12m high Scots pine plantation at Hartheim, Germany, was measured with a variety of methods during a 11-day period of fine weather in mid-May 1992. Net radiation and rate of thermal storage were measured with conventional net radiometers, soil heat flux discs and temperature-based storage models. The turbulent fluxes discussed in this report were obtained with an interchanging Bowen ratio energy budget system (BREB, at 14 m), two one-propeller eddy correlation systems (OPEC systems 1 and 2 at 17m), a 1-dimensional sonic eddy correlation system (SEC system 3) at 15 m, all on one low tower, and a 3-dimensional sonic eddy correlation system (SEC system 22) at 22 m on the high tower that was about 46 m distant. All systems measured sensible and latent heat (H and LE) directly, except for OPEC systems 1 and 2 which estimated LE as a residual term in the surface energy balance. Closure of turbulent fluxes from the two SEC systems was around 80% for daytime and 30% for night, with closure of 1-dimensional SEC system 3 exceeding that of 3-dimensional SEC system 22. The night measurements of turbulent fluxes contained considerable uncertainty, especially with the BREB system where measured gradients often yielded erroneous fluxes due to problems inherent in the method (i.e., computational instability as Bowen's ratio approaches –1). Also, both eddy correlation system designs (OPEC and SEC) appeared to underestimate |H| during stable conditions at night. In addition, both sonic systems (1- and 3-dimensional) underestimated |LE| during stable conditions. The underestimate of |H| at night generated residual estimates of OPEC LE containing a phantom dew error that erroneously decreased daily LE totals by about 10 percent. These special night problems are circumvented here by comparing results for daytime periods only, rather than for full days. To summarize, turbulent fluxes on the low tower from OPEC system 2 and the adjacent SEC system 3 were in reasonable agreement, while the BREB system appeared to overestimate H and underestimate LE; H and LE measured by SEC system 22 on the high tower were lower than from OPEC and SEC3 on the low tower. The turbulent flux measurements tended to converge, but the data exhibit unexplained differences between days, between systems, and between locations.With 7 Figures     

梯度法观测大气与森林生态系统间羰基硫(COS)的交换通量

本工作用梯度法测定了大气与其下垫的森林间ＣＯＳ的交换通量。测量在德国哥廷根大学的一座５０ｍ高的森林观测塔上进行，该塔坐落在德国中部的Ｓｏｌｌｉｎｇ自然保护区的森林中。观测现场生长着树龄分别为１２０ａ和８０ａ的山毛榉和云杉。树冠线约２８ｍ高。在塔上离地面３２ｍ，３８ｍ和５０ｍ的地方用冷以法同时采集了空气样品。样品用气相色谱－火焰光度检测法测定。ＣＯＳ通量由其梯度及扩散系数求出。扩散系数由与ＣＯＳ一起测得的感热和水蒸气通量导出。在稳定边界层条件下共获得２０条廓线。每条廓线都显示ＣＯＳ浓度随高度下降而降低的趋势，说明森林吸收ＣＯＳ。总的结果表明，ＣＯＳ向森林中的平均输送通量为（１４３±５４）ｎｇＣＯＳｍ－２·ｓ－１．     

利用变分法计算西北典型干旱区地表通量的研究

利用2000年5～6月在敦煌进行的陆面过程野外观测试验加强期的观测资料,采用变分法计算了西北干旱区荒漠戈壁的地表感热通量和潜热通量,并与Bowen比能量平衡方法(简称BREB法)的结果进行了对比.结果表明:变分法由于同时应用了地表能量平衡方程和相似性廓线方程,充分利用了边界层观测资料的信息,避免了BREB法中出现在Bowen比接近于-1时产生的计算不稳定和虚假的峰值,使计算的通量结果更趋合理和稳定.同时也计算了动量和感热的总体输送系数,与已有结果比较表明也具有合理性.     

Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest

Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.