Vegetation Optical Depth and Soil Moisture Retrieved from L-Band Radiometry over the Growth Cycle of a Winter Wheat
Vegetation Optical Depth and Soil Moisture Retrieved from L-Band Radiometry over the Growth Cycle of a Winter Wheat
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DOI：
10.3390/rs10101637
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发表时间：
2018-10
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通讯作者：
T. Meyer;L. Weihermüller;H. Vereecken;F. Jonard
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作者：
T. Meyer;L. Weihermüller;H. Vereecken;F. Jonard
在Selhausen遥感野外实验室（德国）对冬小麦林分的整个生长季节进行了l波段辐射计测量。在同一冬小麦林内的两个不同足迹上收集了l波段微波观测数据，以解开来自土壤和植被的排放。基于在由反射器覆盖的土壤表面（即阻挡来自土壤表面的辐射）组成的区域上进行的亮度温度（TB）测量，使用tau-omega （τ-ω）辐射传输模型检索植被光学深度（τ）信息。检索到的τ明显与极化相关，水平(H)极化值较低，垂直(V)极化值较高。此外，观察到V偏振的入射角对τ有很强的依赖性。此外，τ呈钟形时间演化，分蘖期和衰老期最低，开花期最高。后者对应的植被含水量（VWC）最高，叶面积指数（LAI）最大。为了证明时间、偏振和角度依赖性也高度依赖于所观察的植被种类，我们在一个短时间的实验中种植了白芥菜，并使用相同的实验装置进行了辐射计测量。这些结果表明，与小麦植被相比，芥菜植被具有更强的各向同性（即τ参数对入射角和偏振的依赖较小）。下一步，研究了τ与原位测量的植被特性（VWC、LAI、地上植被生物量总量和植被高度）之间的关系，发现整个生长季节的τ与VWC以及τ与LAI之间存在很强的相关性。最后，从没有地面反射器的第二个地块的TB观测数据中获取土壤湿度。使用时间、偏振和角度相关τ作为先验信息，与原位测量相比，检索结果有显著改善。这种改善可以通过更好地表示植被层对测量TB的影响来解释。
L-band radiometer measurements were performed at the Selhausen remote sensing field laboratory (Germany) over the entire growing season of a winter wheat stand. L-band microwave observations were collected over two different footprints within a homogenous winter wheat stand in order to disentangle the emissions originating from the soil and from the vegetation. Based on brightness temperature (TB) measurements performed over an area consisting of a soil surface covered by a reflector (i.e., to block the radiation from the soil surface), vegetation optical depth (τ) information was retrieved using the tau-omega (τ-ω) radiative transfer model. The retrieved τ appeared to be clearly polarization dependent, with lower values for horizontal (H) and higher values for vertical (V) polarization. Additionally, a strong dependency of τ on incidence angle for the V polarization was observed. Furthermore, τ indicated a bell-shaped temporal evolution, with lowest values during the tillering and senescence stages, and highest values during flowering of the wheat plants. The latter corresponded to the highest amounts of vegetation water content (VWC) and largest leaf area index (LAI). To show that the time, polarization, and angle dependence is also highly dependent on the observed vegetation species, white mustard was grown during a short experiment, and radiometer measurements were performed using the same experimental setup. These results showed that the mustard canopy is more isotropic compared to the wheat vegetation (i.e., the τ parameter is less dependent on incidence angle and polarization). In a next step, the relationship between τ and in situ measured vegetation properties (VWC, LAI, total of aboveground vegetation biomass, and vegetation height) was investigated, showing a strong correlation between τ over the entire growing season and the VWC as well as between τ and LAI. Finally, the soil moisture was retrieved from TB observations over a second plot without a reflector on the ground. The retrievals were significantly improved compared to in situ measurements by using the time, polarization, and angle dependent τ as a priori information. This improvement can be explained by the better representation of the vegetation layer effect on the measured TB.