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1.
X. Jia T. S. Zha J. N. Gong B. Wu Y. Q. Zhang S. G. Qin G. P. Chen W. Feng S. Kellomäki H. Peltola 《水文研究》2016,30(6):972-985
During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy‐covariance measurements made over a semi‐arid shrubland in northwest China in 2012, we examined how surface energy‐balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid‐summer when frequent rainfall co‐occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day?1, and summed to 283 mm yr?1. The evaporative fraction (EF, i.e. λE/Rn), Priestley–Taylor coefficient (α), surface conductance (gs) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of gs. The gs played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi‐arid areas. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
2.
Haiying Liu Xiaolin Meng Zhiming Chen Scott Stephenson Pekka Peltola 《GPS Solutions》2016,20(4):795-805
Current cooperative positioning with global navigation satellite system (GNSS) for connected vehicle application mainly uses pseudorange measurements. However, the positioning accuracy offered cannot meet the requirements for lane-level positioning, collision avoidance and future automatic driving, which needs real-time positioning accuracy of better than 0.5 m. Furthermore, there is an apparent lack of research into the integrity issue for these new applications under emerging driverless vehicle applications. In order to overcome those problems, a new extended Kalman filter (EKF) and a multi-failure diagnosis algorithm are developed to process both GNSS pseudorange and carrier phase measurements. We first introduce a new closed-loop EKF with partial ambiguity resolution as feedback to address the low accuracy issue. Then a multi-failure diagnosis algorithm is proposed to improve integrity and reliability. The core of this new algorithm includes using Carrier phase-based Receiver Autonomous Integrity Monitoring method for failure detection, and the double extended w test detectors to identify failure. A cooperative positioning experiment was carried out to validate the proposed method. The results show that the proposed closed-loop EKF can provide highly accurate positioning, and the multi-failure diagnosis method is effective in detecting and identifying failures for both code and carrier phase measurements. 相似文献
3.
Gabriel Katul Olli Peltola Tiia Grönholm Samuli Launiainen Ivan Mammarella Timo Vesala 《Boundary-Layer Meteorology》2018,169(2):163-184
The three turbulent velocity components, water vapour (\(\text {H}_2\text {O}\)), carbon dioxide (\(\text {CO}_{2}\)), and methane (\(\text {CH}_{4}\)) concentration fluctuations are measured above a boreal peatland and analyzed using conditional sampling and quadrant analysis. The overarching question to be addressed is to what degree lower-order cumulant expansion methods describe transport efficiency and the relative importance of ejections and sweeps to momentum, \(\text {CH}_{4}\), \(\text {CO}_{2}\) and \(\text {H}_2\text {O}\) fluxes across a range of atmospheric flow regimes. The patchy peatland surface creates distinctly different source and sink distributions for the three scalars in space and time thereby adding to the uniqueness of the set-up. The measured and modelled fractional contributions to the momentum flux show that sweep events dominate over ejections in agreement with prior studies conducted in the roughness sublayer. For scalar fluxes, ejections dominate the turbulent fluxes over sweeps. While ejective motions persist longer for momentum transport, sweeping events persist longer for all three scalars. Third-order cumulant expansions describe many of the results detailed above, and the results are surprising given the highly non-Gaussian distribution of \(\text {CH}_{4}\) turbulent fluctuations. Connections between the asymmetric contributions of sweeps and ejections and the flux-transport term arising in scalar turbulent-flux-budget closure are derived and shown to agree reasonably well with measurements. The proposed model derived here is much simpler than prior structural models used to describe laboratory experiments. Implications of such asymmetric contributions on, (i) the usage of the now proliferating relaxed-eddy-accumulation method in turbulent flux measurements, (ii) the constant-flux assumption, and (iii) gradient-diffusion closure models are presented. 相似文献
4.
5.
Adaptive management to climate change for Norway spruce forests along a regional gradient in Finland
Zhen-Ming Ge Seppo Kellomäki Heli Peltola Xiao Zhou Hannu Väisänen 《Climatic change》2013,118(2):275-289
We hypothesized that the responses of boreal Norway spruce (Picea abies) forests to climate change would be region-specific due to regional differences in temperature and water availability. In this context, we analyzed the adaptive effects of varied thinning intensities on the gross primary production (GPP), total stem wood growth, and timber yield over a 100-year period using a process-based ecosystem model. Our simulations represented Norway spruce forests for five different bioclimatic zones spanning southern to northern Finland (61–67oN). Ten thinning regimes with thinning intensities ranging from 5 to 50 %, as well as an unthinned regime, were included in the calculations. The results showed that at the southern sites without thinning, the cumulative GPP and total stem wood growth were lower under the changing climate than in the current climate over the simulation period due to greater water depletion via evapotranspiration and reduced soil water availability. At the central and the northern sites, the climate changes increasingly enhanced the GPP and total stem wood growth due to the mitigation of low-temperature limitation and the improved soil water availability. Thinning generally mitigated the soil water deficit by reducing water evaporation and led to a reduction of the natural mortality. At the southern sites, light and moderate thinning intensities increased the GPP and total stem wood growth relative to sites with a changing climate that experienced no thinning. Moreover, moderate thinning resulted in the greatest timber yield. Heavy thinning, in which a large proportion of standing trees were removed, reduced the GPP and total stem wood growth despite allowing increased soil water availability. At the northern sites, all levels of thinning, including light thinning, decreased the GPP and stem wood growth, indicating that soil water availability was not a limiting factor for growth prior to thinning. 相似文献
6.
This paper explores the influence of a fluorine-rich granite on fluoride concentration in a small boreal catchment in northern
Europe. The materials include stream water and shallow groundwater sampled in spatial and temporal dimensions, and analytical
data on fluoride and a number of ancillary variables. Fluoride increased strongly towards the lower reaches of the catchment—at
the stream outlet the concentrations were up to 4.2 mg L−1 and 1.6–4.7 times higher than upstream. Additionally, fluoride concentrations were particularly high in groundwater and small
surface-water bodies (including quarries) above or in direct contact with the granite and showed a strong inverse correlation
with water discharge in the stream. Taken together, these data and patterns pin-point the granite intrusion as the ultimate
source, explaining the abundance and distribution of dissolved fluoride within the catchment. The granite most likely deliver
fluoride to the stream by three mechanisms: (1) weathering of the fine fraction of glacial deposits, derived from the granite
and associated fluorine-rich greisen alterations, (2) large relative input of baseflow, partially originating in the granite
and greisen, into the lower reaches during low flow in particular, and (3) water-conducting fractures or fracture zones running
through the fluorine-rich granite and greisen. 相似文献
7.
Anna Augustsson Pasi Peltola Bo Bergbäck Timo Saarinen Eeva Haltia-Hovi 《Journal of Paleolimnology》2010,44(4):1025-1038
A high-resolution geochemical profile from a 5,500-year-old sediment core of Lake Lehmilampi in eastern Finland was analyzed
to study long-term trends and variability in element concentrations and accumulation rates. The accumulation rates of all
studied elements followed the same trend, responding to changes in the total sedimentation rate. Concentration profiles differed
among elements and showed considerable variation over time. Principal components analysis (PCA) was used on the concentration
data to identify groups of elements that have similar geochemical controls. The first principal component was influenced by
changes in mineral matter accumulation, and it incorporated elements that are associated with stable allochthonous minerals
(such as Mg, K, Cs, Rb, Li, Ti and Ga), as well as elements in forms that become diluted when mineral matter increases (e.g.,
S, Fe and Mn). The second and third principal components showed that a large proportion of the variance was accounted for
by elements with continuously increasing or decreasing concentrations related to pedogenetical development of the catchment
soil. In the case of Hg, Pb and Cd, however, accumulation rates increased faster at the surface than is simply accounted for
by changes in total sedimentation rates. For Cu, Cr, Ni and Zn, concentrations increased over the past 150 years, but there
were no indications of a significant addition due to atmospheric deposition. These elements had more variable concentrations
before the mid nineteenth century than after, as did elements that are often used for normalization. These findings suggest
that lake sediments may not properly reflect the history of atmospheric metal deposition in remote areas. 相似文献
8.
Antti Kilpeläinen Hilppa Gregow Harri Strandman Seppo Kellomäki Ari Venäläinen Heli Peltola 《Climatic change》2010,99(1-2):193-209
This work studied the temporal and spatial variability of the risk of snow-induced forest damage in Finland under current and changing climatic conditions until the end of this century. The study was based on a snow accumulation model in which cumulative precipitation, air temperature and wind speed were used as input variables. The risk was analyzed in terms of the number of days per year when the accumulated amount of snow exceeded 20 kg m???2. Based on the risk, the forest area and mean carbon stock of seedling, young thinning and advanced thinning stands at risk were calculated. Furthermore, the number of 5-day periods, when the accumulated amount of snow exceeded a risk limit, was calculated for the current and changing climatic conditions in order to study the frequency of damaging snowfalls. Compared to the baseline period 1961–1990, the risk of snow-induced forest damage and the amount of damaging snowfalls were predicted to decrease from the first 30-year period (1991–2020) onwards. Over the whole country, the mean annual number of risk days decreased by 11%, 23% and 56% in the first, second and third 30-year period, respectively, compared to the baseline period. In the most hazardous areas in north-western and north-eastern Finland, the number of risk days decreased from the baseline period of over 30 days to about 8 days per year at the end of the century. Correspondingly, the shares of the forest area at risk were 1.9%, 2.0% and 1.0% in the first, second and third 30-year period, respectively. The highest mean annual carbon stocks of young stands at risk were found in central, north-eastern and north-western Finland in the first and second 30-year period, varying between 0.6 and 1.2 Mg C ha???1 year???1, meaning at highest 3% of the mean carbon stock (Mg C stem wood ha???1) of those areas. This study showed that although the risk of snow-induced forest damage was mainly affected by changes in critical weather events, the development of growing stock under the changing climatic conditions also had an effect on the risk assessment. However, timely management of forest stands in the areas with a high risk of snow-induced damage contributes to the trees’ increased resistance to the damage. 相似文献
9.
T. Nuutinen J. Matala H. Hirvelä K. Härkönen H. Peltola H. Väisänen S. Kellomäki 《Climatic change》2006,79(3-4):315-333
The purpose of this study was to optimize forest management for a forest region (the total area of forest and scrub land 1.54
mill. ha) under changing climate by using the large-scale forestry scenario model MELA and sample plot data from the geo-referenced
National Forest Inventory (NFI). The MELA model is based on integrated simulation and optimisation; in the simulation it utilises
empirical tree-level models into which the impacts of climate change were introduced by transfer variables derived by using
the physiological model FinnFor. Six scenarios with differences in climate and forest management were defined. In simulations,
the accelerating tree growth caused by climate change resulted in an increase in maximum sustainable removal of trees at regional
level. Changes in regionally optimized forest management were also detected during the analysis period of 30 years; the proportion
of thinnings increased because the stands fulfilled the thinning requirements earlier than in the current climate. This study
was the first attempt to solve endogenously maximum sustainable timber production and corresponding forest management at the
regional level under different climate scenarios. When implemented in the MELA system, which is widely used in Finnish forestry,
the transfer variables offer means of disseminating the results from physiological studies to planning of adjustment and mitigation
measures under changing climate. 相似文献
10.
The more humid, warmer weather pattern predicted for the future is expected to increase the windthrow risk of trees through reduced tree anchorage due to a decrease in soil freezing between late autumn and early spring, i.e during the most windy months of the year. In this context, the present study aimed at calculating how a potential increase of up to 4°C in mean annual temperature might modify the duration of soil frost and the depth of frozen soil in forests and consequently increase the risk of windthrow. The risk was evaluated by combining the simulated critical windspeeds needed to uproot Scots pines (Pinus sylvestris L.) under unfrozen soil conditions with the possible change in the frequency of these winds during the unfrozen period. The evaluation of the impacts of elevated temperature on the frequency of these winds at times of unfrozen and frozen soil conditions was based on monthly wind speed statistics for the years 1961–1990 (Meteorological Yearbooks of Finland, 1961–1990). Frost simulations in a Scots pine stand growing on a moraine sandy soil (height 20 m, stand density 800 stems ha–1) showed that the duration of soil frost will decrease from 4–5 months to 2–3 months per year in southern Finland and from 5–6 months to 4–5 months in northern Finland given a temperature elevation of 4°C. In addition, it could decrease substantially more in the deeper soil layers (40–60 cm) than near the surface (0–20 cm), particularly in southern Finland. Consequently, tree anchorage may lose much of the additional support gained at present from the frozen soil in winter, making Scots pines more liable to windthrow during winter and spring storms. Critical wind-speed simulations showed mean winds of 11–15 m s–1 to be enough to uproot Scots pines under unfrozen soil conditions, i.e. especially slender trees with a high height to breast height diameter ratio (taper of 1:120 and 1:100). In the future, as many as 80% of these mean winds of 11–15 m s–1 would occur during months when the soil is unfrozen in southern Finland, whereas the corresponding proportion at present is about 55%. In northern Finland, the percentage is 40% today and is expected to be 50% in the future. Thus, as the strongest winds usually occur between late autumn and early spring, climate change could increase the loss of standing timber through windthrow, especially in southern Finland. 相似文献