估算您的Irr视野中的顶篷分数
一个常见的问题在许多人的脸canopy temperature measurements with an infrared radiometer (IRR) involves estimating the fraction of the field of view that is occupied by canopy (leaves and stems) and soil. This is important in many settings, because plants do not completely cover the ground and the resulting IRR temperature measurement will be a weighted average of canopy components and soil in the IRR field of view. Estimation of field of view occupied by canopy is particularly important early in the growing season when crops are small and a large fraction of soil is viewed. However, knowledge of field of view composition is important in any application where a uniform surface is not being measured due to different surface components. In this case, canopy and soil will typically have different temperatures. The equation to approximate the fraction of IRR field of view occupied by the canopy (fc) is dependent on plant leaf area and is based on the equation to calculate the canopy extinction coefficient for direct beam radiation [1] and is given as:
其中Lai是叶面积指数，K是消光系数。k依赖于两个附加变量：叶角分布参数（x）和irr的视角（相对于Nadir View = 0°）由用户选择。这些变量将在下面简要描述。有关更多帮助计划您的IRR的视角，点击这里。
叶面积指数（莱）：每单位地面叶子区域[m2m-2]，可以表示为总叶面积或单面叶面积。单面叶面积（投影区域）是总叶面积的一半，通常被称为半表面积。除非冠层主要由针叶物种组成，否则通常报告为每单位地面区域的半表面区域。在未直接测量LAI的情况下，它通常从冠层高度近似。来自科学文献的一些例子在下表中给出。
| 庄稼 | 赖等式（其中H在米中） |
| 剪裁草[2] | = 24 * h |
| alfalfa [2] | = 5.5 + 1.5 * ln（h） |
| 玉米[3] | = exp（1.64 * ln（.141 * h）） |
| 大豆[3] | = exp（2.38 * ln（4.17 * h）） |
消光系数（k）：阴影区域由遮篷除以叶片中的叶片区域，或换句话说，叶片上的平均投射到水平表面上。以下等式可用于近似k：
叶角分布参数（x）：垂直表面上的水平表面上的冠层元素的平均投影区域与投影区域的比率。X定义从0（水平）到90度（垂直）的植物冠层中叶倾角角的分布。对于具有更水平导向的叶片的植物檐篷，X大于1，对于具有更垂直导向的叶片的植物檐篷，X小于1。X = 1是许多植物檐篷的良好近似，因为叶面积在所有叶片倾斜角度之间相当均匀地分布。下图显示了三个檐篷的倾斜角度密度[4]。x，叶片越多。这些分布的等式由Campbell [5]给出。
一个dditionally, X values for various crops have been estimated as shown in the table below [4].
| 庄稼 | X |
| 黑麦草 | 0.67-2.47 |
| 玉米 | 0.76-2.52 |
| Rye | 0.8-1.27 |
| 小麦 | 0.96 |
| 大麦 | 1.20 |
| 蒂莫西 | 1。13 |
| 高粱 | 1.43 |
| 卢塞恩 | 1.54 |
| 杂交瑞典人 | 1.29-1.81 |
| 甜菜 | 1.46-1.88 |
| 强奸 | 1.92-2.13 |
| 黄瓜 | 2.17 |
| 烟草 | 1。29-2.22 |
| 土豆 | 1。70-2.47 |
| 马豆 | 1。81-2.17 |
| Sunglower | 1.81-4.1. |
| 白三叶草 | 2.47-3.26. |
| 草莓 | 3.03 |
| 大豆 | 0.81 |
| 玉米 | 1.37 |
| J.朝鲜蓟 | 2.16 |
一个简单的小部件在我们网站上的IRR产品部分中，可以帮助实现必要的计算来估计IRR所观看的冠层的一部分。
[1]坎贝尔，G.S.（1986）使用椭圆倾角分布计算的植物檐篷中的辐射的消光系数。农业和森林气象36：317-321。
[2] Allen, R.G., et al. (2001) The ASCE Standardized Reference Evapotranspiration Equation. Pg B-6. http://www.kimberly.uidaho.edu/water/asceewri/appendix.pdf.
[3] M.C. Anderson, et al. (2004) Upscaling ground observations of vegetation water content, canopy height, and leaf area index during SMEX02 using aircraft and Landsat imagery. Remote Sensing of Environment 92:447-464.
[4]坎贝尔，G.S.和Norman，J.M.（1998）第2届环境生物物理学介绍。PG 250-253。
[5] Campbell，G.S.（1990）衍生具有椭圆形叶角分布的檐篷的角度密度函数。农业和森林气象49：173-176。