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Does measuring azimuthal variations in sap flux lead to more reliable stand transpiration estimates?
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Hikaru Komatsu Yoshinori Shinohara Tomonori Kume Kenji Tsuruta Kyoichi Otsuki 《水文研究》2016,30(13):2129-2137
Stand transpiration (E) estimated using the sap‐flux method includes uncertainty induced by variations in sap flux (F) within a tree (i.e. radial and azimuthal variations) and those between trees. Unlike radial variations, azimuthal variations are not particularly systematic (i.e. higher/lower F is not always recorded for a specific direction). Here, we present a theoretical framework to address the question on how to allocate a limited number of sensors to minimize uncertainty in E estimates. Specifically, we compare uncertainty in E estimates for two cases: (1) measuring F for two or more directions to cover azimuthal variations in F and (2) measuring F for one direction to cover between‐tree variations in F. The framework formulates the variation in the probability density function for E (σE) based on F recorded in m different azimuthal directions (e.g. north, east, south and west). This formula allows us to determine the m value that minimizes σE. This study applied the framework to F data recorded for a 55‐year‐old Cryptomeria japonica stand. σE for m = 1 was found to be less than the values for m = 2, 3 and 4. Our results suggest that measuring F for one azimuthal direction provides more reliable E estimates than measuring F for two or more azimuthal directions for this stand, given a limited number of sensors. Application of this framework to other datasets helps us decide how to allocate sensors most effectively. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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如何在阅读教学中培养学生语篇分析能力 总被引:9,自引:0,他引:9
郭晓玲 《成都信息工程学院学报》2001,16(2):153-157
语篇分析是系统功能语言学有别于其它语言学流派的重要特征,通过探讨语篇分析在英语阅读教学中起到的作用,以求有效地提高学生的阅读理解能力。 相似文献
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Lopez Ivan Oyarzun Roberto Marquez Alvaro Doblas-Reyes Francisco Laurrieta Alejandro 《Earth, Moon, and Planets》1998,81(3):187-192
In absence of other mechanisms, the main input of CO2into the Venusian atmosphere is via volcanic out gassing. Since Venus can be regarded as a planet-wide large igneous province,
we can expect large quantities of CO2 being transferred into its atmosphere via volcanic out gassing. We have quantified the maximum possible amount of CO2 that can be out gassed via a single massive episode of resurfacing of the planet. This figure (5.6 × 10up>19up> kg of CO2) is about 8 times smaller than the total CO2 present in the Venusian atmosphere (4.55 × 10up>20up> kg CO2). The lack of planet-wide, efficient mechanisms for the recycling of CO2 on Venus indicates that CO2 has progressively accumulated in the atmosphere. Based on these considerations we suggest that the “equivalent” to eight
global resurfacing episodes would be required to account for the present values of CO2 atmosphere.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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A regime shift is considered to be a sudden shift in structure and functioning of a marine ecosystem, affecting several living components and resulting in an alternate state. According to this definition, regime shifts differ from species replacement or alternation of species at similar trophic levels, whereby the ecosystem is not necessarily significantly altered in terms of its structure and function; only its species composition changes. This paper provides an overview of regime shifts, species replacements and alternations that have been observed in the northern and southern Benguela ecosystems over the past few decades. Bottom-up control, initiating and sustaining regime shifts or species replacements via environmental forcing, is documented for both the southern and the northern Benguela ecosystems. Fishing (a case of top-down control) appears to have played an important role in regime shift processes in the Namibian ecosystem. Very low biomass levels of exploited fish stocks associated with less efficient energy transfer in the northern Benguela are indicative of a regime shift. Very high biomass levels have been reached in the southern Benguela in the 2000s. However the alternation between sardine and anchovy that has been observed in the southern Benguela over the last two decades appears not to have had major effects on the overall functioning of the ecosystem. The consequences of regime shifts for exploitation are highlighted, suggesting that fisheries managers should move towards a more effective ecosystem approach to fisheries. 相似文献
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孤东油田地处黄河入海口,是黄河泥沙填海尚示成陆地围海筑堤开有析一个年产原油500成t的大油田。由于地貌特殊,围堤潜在问题较多。本文叙述了孤东油田围堤建设前后的地域情况,通过计算分析认为黄河洪不、风暴潮对围堤构成的威胁破坏最大,同时指出内涝、软基、坝体质量、植被生态中存在的问题;提出了采取引黄淤临、机械吹填洼地、导沙入海、锥探灌浆等措施,可解决孤东油田围问题。 相似文献
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In boreal forested wetlands, the observed increase in the water table level after clearcutting (watering‐up) is often a threat to sustained ecosystem productivity. Hydrologic recovery refers to the processes by which a water table progressively drops back to its initial level after the cut. In eastern Canada, drainage is used operationally after clearcutting wet sites in order to lower the water table level and accelerate hydrologic recovery. The objective of this study was to evaluate the duration of the watering‐up caused by timber harvesting and the extent to which drainage affected the water table recovery on five peatlands and three hydromorphic mineral sites located in the St. Lawrence Lowlands of Québec (Canada). The mixed wood stands studied are dominated by balsam fir (Abies balsamea (L.) Mill.), eastern white cedar (Thuja occidentalis L.), and red maple (Acer rubrum L). Results indicate that, 10 years after clearcutting, water table levels in undrained plots are still 5 to 7 cm higher than the pre‐cut levels. The slight recovery in water table level plateaued after the third year. Rainfall interception by vegetation was also monitored, and after 10 years had reached nearly 50% of the pre‐cut rate. The immediate water table drawdown following drainage mitigated watering‐up within 40 m of a ditch. The persistent watering‐up observed in this study should encourage using sylvicultural systems adapted to boreal forested wetlands in order to prevent productivity loss and stand conversion. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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