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271.
In this paper, a generalized layered model for radiation transfer in canopy with high vertical resolution is developed. Differing from the two-stream approximate radiation transfer model commonly used in the land surface models, the generalized model takes into account the effect of complicated canopy morphology and inhomogeneous optical properties of leaves on radiation transfer within the canopy. In the model, the total leaf area index (LAI) of the canopy is divided into many layers. At a given layer, the influences of diffuse radiation angle distributions and leaf angle distributions on radiation transfer within the canopy are considered. The derivation of equations serving the model are described in detail, and these can deal with various diffuse radiation transfers in quite broad categories of canopy with quite inhomogeneons vertical structures and uneven leaves with substantially different optical properties of adaxial and abaxial faces of the leaves. The model is used to simulate the radiation transfer for canopies with horizontal leaves to validate the generalized model. Results from the model are compared with those from the two-stream scheme, and differences between these two models are discussed. 相似文献
272.
随着近10年来北京城市化步伐的加快,城市规模迅速扩大,北京三环以外的地区已由20世纪七、八十年代的城郊非均匀下垫面发展成现在粗糙复杂的城区下垫面,市区建筑物相应增多、增密和增高,导致城区地面动力学粗糙度明显增大.本文统计分析了北京325 m气象塔1994年和1997~2003年夏季平均场观测资料,结果表明:(1)在受下垫面影响最为剧烈的近地层,风向逐年趋于紊乱,现在气象塔周围近地面的流场已经具备了典型城市粗糙下垫面的流场特征;(2)近地面夏季平均风速呈现非常明显的逐年递减的趋势,而且距离地表越近,平均风速逐年递减的趋势也越为显著,这种风速逐年递减的趋势直到63 m左右才不明显,说明现在气象塔47 m以下的观测资料反映的是城市冠层的流场特征,城市冠层厚度约在47~63 m之间;(3)随着北京城市化的发展,城区近地面的平均风速存在逐年递减的趋势,但阵风并不存在相似的递减趋势,表明城市冠层的抬升对阵风的影响并不显著. 相似文献
273.
274.
Spatial variability of throughfall and stemflow in an exotic pine plantation of subtropical coastal Australia 总被引:3,自引:0,他引:3
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Junliang Fan Kasper T. Oestergaard Adrien Guyot David G. Jensen David A. Lockington 《水文研究》2015,29(5):793-804
Large‐scale exotic pine plantations have been developed for timber production in subtropical Australia. Few studies investigate the spatial variability of both throughfall and stemflow in such managed pine plantations despite their acknowledged effects on the heterogeneity of hydrological and biochemical processes of forested ecosystems. To examine the spatial variability of rainfall under a 12‐year‐old pine plantation in a subtropical coastal area of Australia, we observed gross rainfall, throughfall and stemflow over a 1‐year period. Our results show that the spatial variability of gross rainfall within a 50 m × 50 m plot is minimal. Throughfall is significantly different among three tree zones (midway between rows, west and east side of trunks), particularly for rainfall <50 mm, with the highest throughfall on the east side of the tree trunks (sum = 85% of gross rainfall) and the lowest in the midway between tree rows (sum = 68% of gross rainfall). These spatial patterns persist among 84% of recorded rainfall events. Spatial variability and time stability of throughfall are better explained by canopy interception of the inclined rainfall resulting from the prevailing easterly wind direction throughout the experiment. The annual stemflow is different among individual sample trees, which is mainly ascribed to the difference in tree size (e.g. projected canopy area and stem diameter). The outcomes of this study would help future investigators better design appropriate sampling strategies in these pine plantations under similar climate conditions. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
275.
Bao‐Lin Xue Zhanling Li Xin‐An Yin Tieliang Zhang Shin'ichi Iida Kyoichi Otsuki Takeshi Ohta Qinghua Guo 《水文研究》2015,29(6):1017-1026
A larch forest in eastern Siberia was characterized by the presence of two distinct storeys, the overstorey with a small leaf area index (LAI) and a dense understorey with a relatively large LAI. To understand the roles of the overstorey and understorey in forest–atmosphere water exchange, canopy conductance (Gc), a critical parameter used in determining the energy and mass exchange, was calculated on the basis of latent heat flux above the overstorey and understorey, measured separately. Results showed that Gc for the overstorey (Gco) and understorey (Gcu) experienced different seasonal fluctuations. Gco was smaller than Gcu during periods of leaf expansion and leaf fall and showed an increasing trend until 1 month after the onset of leaf expansion. In contrast, a sharp decrease in Gco was observed immediately before onset of leaf fall. Furthermore, Gco was slightly larger than Gcu during the fully foliated period. A simple model using solar radiation and vapour pressure deficit (D) as inputs successfully reproduced the Gc in fully foliated periods with acceptable accuracy. Furthermore, both the understorey and overstorey in this study have a large reference Gc (Gc at D = 1 KPa) than their counterparts of other boreal forests and would not be able to sustain a constant leaf–soil water potential difference as D increases. We speculated that this confers the forest with an advantage allowing it to be able to sustain carbon assimilation during large D days and thus provides for the survival of the ecosystem during the short growing season at this site. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
276.
David Dunkerley 《水文研究》2000,14(4):669-678
The interception storage capacity has been measured for a range of dryland plants. Interception losses over time, however, arise in rain events that deliver either less or more than the canopy capacity. The fate of water in these cases depends on the efficiency with which the intercepted water is returned to the atmosphere by evaporation from the plant canopies. Two primary methods to estimate interception losses are (i) calibrated process‐based models of interception and evaporative loss and (ii) direct measurement. Models have been applied only rarely to dryland plant communities, and direct measurement techniques are in need of additional testing and refinement. Most published estimates of interception loss in dryland plant communities therefore appear to be based upon inadequate data and methods. Research needs in this area are highlighted. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
277.
Yanqiu Xing Alfred de Gier Junjie Zhang Lihai Wang 《International Journal of Applied Earth Observation and Geoinformation》2010
Light Detection And Ranging (LiDAR) has a unique capability for estimating forest canopy height, which has a direct relationship with, and can provide better understanding of the aboveground forest carbon storage. The full waveform data of the large-footprint LiDAR Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat), combined with field measurements of forest canopy height, were employed to achieve improved estimates of forest canopy height over sloping terrain in the Changbai mountains region, China. With analyzing ground-truth experiments, the study proposed an improved model over Lefsky's model to predict maximum canopy height using the logarithmic transformation of waveform extent and elevation change as independent variables. While Lefsky's model explained 8–89% of maximum canopy height variation in the study area, the improved model explained 56–92% of variation within the 0–30° terrain slope category. The results reveal that the improved model can reduce the mixed effects caused by both sloping terrain and rough land surface, and make a significant improvement for accurately estimating maximum canopy height over sloping terrain. 相似文献
278.
基于多次散射的植被-土壤二向反射模型 总被引:6,自引:0,他引:6
运算过于复杂是目前多数植被二向反射模型存在的一个突出问题。对冠层的各次散射、透射和地面土壤反射分别进行分析和考虑 ,建立各次反射光谱分量与叶面积指数的函数关系。在Verstraete (1 986 )的物理模型的基础上加上多次散射和土壤反射的分量 ,建立了基于多次散射的植被 土壤二向反射模型 ,并证明仅考虑前三次散射即可满足一般精度要求。该模型克服了原模型的不足 ,并可根据叶面积指数等参量直接计算二向反射率 ,避免了运算量巨大的叠代运算 ,故便于应用。实验证明 ,该模型具有较高的精度 相似文献
279.
280.
Tawanda W. Gara Andrew K. Skidmore Roshanak Darvishzadeh Tiejun Wang 《地理信息系统科学与遥感》2019,56(4):554-575
In remote sensing applications, leaf traits are often upscaled to canopy level using sunlit leaf samples collected from the upper canopy. The implicit assumption is that the top of canopy foliage material dominates canopy reflectance and the variability in leaf traits across the canopy is very small. However, the effect of different approaches of upscaling leaf traits to canopy level on model performance and estimation accuracy remains poorly understood. This is especially important in short or sparse canopies where foliage material from the lower canopy potentially contributes to the canopy reflectance. The principal aim of this study is to examine the effect of different approaches when upscaling leaf traits to canopy level on model performance and estimation accuracy using spectral measurements (in-situ canopy hyperspectral and simulated Sentinel-2 data) in short woody vegetation. To achieve this, we measured foliar nitrogen (N), leaf mass per area (LMA), foliar chlorophyll and carbon together with leaf area index (LAI) at three vertical canopy layers (lower, middle and upper) along the plant stem in a controlled laboratory environment. We then upscaled the leaf traits to canopy level by multiplying leaf traits by LAI based on different combinations of the three canopy layers. Concurrently, in-situ canopy reflectance was measured using an ASD FieldSpec-3 Pro FR spectrometer, and the canopy traits were related to in-situ spectral measurements using partial least square regression (PLSR). The PLSR models were cross-validated based on repeated k-fold, and the normalized root mean square errors (nRMSEcv) obtained from each upscaling approach were compared using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test. Results of the study showed that leaf-to-canopy upscaling approaches that consider the contribution of leaf traits from the exposed upper canopy layer together with the shaded middle canopy layer yield significantly (p < 0.05) lower error (nRMSEcv < 0.2 for canopy N, LMA and carbon) as well as high explained variance (R2 > 0.71) for both in-situ hyperspectral and simulated Sentinel-2 data. The widely-used upscaling approach that considers only leaf traits from the upper illuminated canopy layer yielded a relatively high error (nRMSEcv>0.2) and lower explained variance (R2 < 0.71) for canopy N, LMA and carbon. In contrast, canopy chlorophyll upscaled based on leaf samples collected from the upper canopy and total canopy LAI exhibited a more accurate relationship with spectral measurements compared with other upscaling approaches. Results of this study demonstrate that leaf to canopy upscaling approaches have a profound effect on canopy traits estimation for both in-situ hyperspectral measurements and simulated Sentinel-2 data in short woody vegetation. These findings have implications for field sampling protocols of leaf traits measurement as well as upscaling leaf traits to canopy level especially in short and less foliated vegetation where leaves from the lower canopy contribute to the canopy reflectance. 相似文献