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101.
102.
Seasonal fishing closures are often used in fisheries management to conserve overfished stocks.As one of the unintended consequences,fishermen often contend for maximizing catches immediately after reopening fisheries.The resultant large catch landings in a short time period(i.e.,pulse fishing)may undermine the benefit of closure.We implemented an end-to-end model OSMOSE-JZB(Object-oriented Simulator of Marine ec OSystem Exploitation OSMOSE)modelling ecosystem in the Jiaozhou Bay located in China to evaluate the impact of pulse fishing on the effectiveness of seasonal closure at levels of fish community,population,and individual.Our study demonstrated that the three-month closure was successful in conserving fish stocks.There were small variations on ecological indicators(i.e.,total biomass of the community,mean trophic level of the community,mean trophic level of the catch,and Shannon-Wiener biodiversity index)when pulse fishing occurred.Pulse fishing seemed not to result in a great shift in community structure.Compared to other species,the biomass of two large predatory fishes were more susceptible to pulse fishing.Pulse fishing could change the pressure of predators to fish stocks via food webs,especially for young individuals.Our simulations indicate that we can improve the effectiveness of seasonal closure by managing pulse fishing.Although the results derived in this study may be specific to the target ecosystem,the general approach is applicable to other ecosystems when evaluating fishing impacts. 相似文献
103.
Albin Hammerle Alois Haslwanter Michael Schmitt Michael Bahn Ulrike Tappeiner Alexander Cernusca Georg Wohlfahrt 《Boundary-Layer Meteorology》2007,122(2):397-416
Carbon dioxide, latent and sensible heat fluxes were measured by means of the eddy covariance method above a mountain meadow
situated on a steep slope in the Stubai Valley in Austria, based on the hypothesis that, due to the low canopy height, measurements
can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order
to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were
subjected to a rigorous testing procedure using a series of quality control measures established for surface-layer flows.
The resulting high quality dataset comprised 36% of the original observations, the substantial reduction being mainly due
to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition
periods. The validity of the high quality dataset was further assessed by two independent tests: (i) a comparison with the
net ecosystem carbon dioxide exchange measured by means of ecosystem chambers, and (ii) the ability of the eddy covariance
measurements to close the energy balance. The net ecosystem CO2 exchange measured by the eddy covariance method agreed reasonably well with ecosystem chamber measurements. The assessment
of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on
the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated
by 28% and 29%, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements
made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain. 相似文献
104.
Freezing and thawing processes at the soil surface play an important role in determining the nature of Tibetan land and atmosphere interactions. In this study, land surface water and heat exchanges under different freezing and thawing conditions over the central Tibetan Plateau were investigated using observations from the Coordinated Enhanced Observing Period/Asia‐Australia Monsoon Project on the Tibetan Plateau, and the Simultaneous Heat and Water Model. During the freezing and thawing stages, significant diurnal variation of soil temperature resulted in a diurnal cycle of unfrozen water content at the surface. Radiation and energy components and evapotranspiration averaged over four freeze/thaw stages also changed diurnally. On average, the surface albedo (0·68) during the completely frozen stage was sharply higher than those during the freezing, thawing, and completely thawed stages due to the snow cover. The Bowen ratios were 3·1 and 2·5 in the freezing and thawing stages, respectively, but the ratio was only 0·5 in the completely thawed stage. Latent heat flux displayed distinctly better correlation with unfrozen soil water content during the freezing and thawing stages than during the completely frozen and thawed stages. This implies that the diurnal cycle of unfrozen soil water, resulting from diurnal freeze/thaw cycles at the surface, has a significant impact on latent heat flux. A surface energy imbalance problem was encountered, and the possible sources of error were analysed. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
105.
港口的检测与识别是合成孔径雷达(synthetic aperture radar,SAR)遥感海洋应用领域的重要内容。分析港口目标结构及其微波散射特性,提出了一种基于先验约束的SAR遥感图像港口检测与鉴别方法。首先利用港内水域半封闭性的特点,定义水域被陆地包围的封闭性测度提取港内水域,使用多向扫描法检测突堤,结合两者结果实现港口粗提取。然后依据港口的最小外接矩形,判定口门方向和突堤代表点,最后根据突堤代表点间海岸线的封闭性测度完成港口鉴别。实验结果表明,本文提出算法能够准确检测港口及口门方向,完整提取突堤结构。 相似文献
106.
在三角高程对向观测法理论的基础上,根据对向观测法的闭合差条件,提出其偶然误差特性的检验,并结合数例进行包括闭合差正负号检验、正负误差分配顺序检验、误差数值和的检验和个别误差值的检验.最后得出三角高程对向观测法闭合差具有偶然误差特性,并具有一定的正确性和现实指导意义. 相似文献
107.
Water scarcity is one of the main factors limiting agricultural development in arid or semi-arid areas. Accurate Evapotranspiration (ET) observations and estimations are crucial in water cycle studies to estimate water losses from the terrestrial surfaces to the atmosphere to close the regional water budget. The eddy covariance (EC) method is an important technique measure ET and other land surface energy fluxes. However, the underestimation of energy fluxes and the problem of EC energy balance non-closure are far from solved. In this study, a new method is proposed to account for advection in order to correct EC data under advective environments. This advection based method was applied to data from Bushland, TX, which is subject to dry air and strong winds. Observations from two identical EC systems as well as two precision monolithic weighing lysimeters were used in this analysis. Both EC sites showed significant underestimates of evapotranspiration (ET) compared with lysimeter measurements. The daily energy balance closure for NE01 and SE02 sites were 0.78 and 0.74 respectively. The advection correction method provided improved performance in daytime, and it is more suitable for ET estimate than forcing closure under the advective environment. For nighttime, two methods (NCM1 and NCM2) were proposed to correct EC underestimates. Finally, all the corrected ET values were compared with the lysimeter measurements. For NE01 site, the MAD (mean absolute deviation) and the RMSD (root mean square deviation) were 47.72 W/m2 and 67.66 W/m2, respectively; and the r2 (coefficient of determination) was 0.85. For SE02 site, the MAD and RMSD were 30.59 W/m2 and 44.43 W/m2; and the r2 was 0.93. The statistical measures illustrated that the proposed methods are functional and appropriate under an advective environment. The accurate estimate of actual evapotranspiration will benefit both the strategic planning of optimal water uses and the improved understanding the environmental and hydrological processes. 相似文献
108.
Adrie F. G. Jacobs Bert G. Heusinkveld Albert A. M. Holtslag 《Boundary-Layer Meteorology》2008,126(1):125-136
Observations for May and August, 2005, from a long-term grassland meteorological station situated in central Netherlands were
used to evaluate the closure of the surface energy budget. We compute all possible enthalpy changes, such as the grass cover
heat storage, dew water heat storage, air mass heat storage and the photosynthesis energy flux, over an averaging time interval.
In addition, the soil heat flux was estimated using a harmonic analysis technique to obtain a more accurate assessment of
the surface soil heat flux. By doing so, a closure of 96% was obtained. The harmonic analysis technique appears to improve
closure by 9%, the photosynthesis for 3% and the rest of the storage terms for a 3% improvement of the energy budget closure.
For calm nights (friction velocity u
* < 0.1 m s−1) when the eddy covariance technique is unreliable for measurement of the vertical turbulent fluxes, the inclusion of a scheme
that calculates dew fluxes improves the energy budget closure significantly. 相似文献
109.
A computational scheme for an improved Mellor–Yamada(M–Y) Level-3 model with condensation physics is proposedand its performance is examined against large-eddy-simulationdata on radiation fog. The improved M–Y model greatlycorrects several shortcomings of the original M–Y model:the underestimations of the mixed-layer depth and themagnitude of turbulent kinetic energy, and the discrepanciesin the formation and dissipation times of the fog. Inaddition the improved M–Y model can reproduce theoccurrence of Kelvin–Helmholtz instability and periodicoscillations due to its energy cycle. It is shown that theoptimization of both the closure constants and the masterlength scale is required for this improvement.The improved M–Y model has an improvement also in theLevel-2.5 version. Although the performance of theLevel-2.5 version is not so good as that of the Level-3version, the former has the advantage of relatively lowcomputational cost and is popularly used in operationalweather forecasts. Our computational scheme for theimproved M–Y model allows us to switch its hierarchylevels easily according to the purpose. 相似文献
110.