Pinet Sylvain, Martinez Jean-Michel, Ouillon Sylvain, Lartiges Bruno, Villar Raul Espinoza
Opt Express. 2017 Apr 17;25(8):A283-A310. doi: 10.1364/OE.25.00A283.
We investigated the relationships between inherent and apparent optical properties (IOP and AOP, respectively) and suspended sediment concentrations (SSC) in the main Amazonian river waters. In situ measurements of SSC, remote sensing reflectance (R), the diffuse light attenuation coefficient (K) and the total and non-algal particle (NAP) absorption coefficients (a and a, respectively) were conducted during three sampling trips along different streams of the Amazon River catchment (104 stations). The size distribution and chemical characteristics of the suspended sediment were also determined for 85 stations. We show that the particle size distribution (PSD) in the river water is best described by a segmented Junge power law distribution with a smaller slope value for the smallest particles (J = 2.4) and a larger slope value (J = 4.1) for the largest particles (> 10 µm). A strong relationship was found between AOPs and IOPs and SSC when the entire data set was considered. However, for the Madeira River, the primary Amazon River tributary in terms of suspended sediment discharge, a significant dispersion was detected for the R - SSC relationship but not for the K - SSC relationship. This dispersion has been shown by a previous study, using MODIS data, to display a seasonal pattern, which we investigated in this study using Mie modeling calibrated with suspended sediment characteristics. In the Madeira River, suspended sediment had a finer distribution size and a different mineralogy (e.g., a greater smectite content and a lower kaolinite content) during the rising water stage. Spectral variations of the imaginary part n(λ) of the refraction index also showed significant differences during the rising water stage. In contrast, other streams of the Amazon basin had very stable properties with respect to granulometry and mineralogy. Model simulations made possible to reproduce both field and satellite observations, showing that the R hysteresis observed in the Madeira River in the near infrared was mainly due to n(λ) seasonal variations, leading to a decrease of absorption during the rising water stage. K was shown to remain stable because of its strong dependency on scattering processes. The model was used to further understand how suspended sediment size distribution and refraction index drive the IOPs in large rivers: n(λ) variations were shown to control primarily the reflectance variability; R(850) presented limited variations as a function of PSD in the range typical of large rivers (J < 3) although it remained sensitive to particle mineralogical composition; R(670) showed the opposite behavior with a higher sensitivity to PSD variation for coarser PSD. Finally, we demonstrate that the use of the R ratio between the red and infrared channels allowed a reduction of the R sensitivity in all cases, by an average of 50% with respect to changes in the mineral composition or size distribution of suspended sediment. In particular, the R ratio varied by less than 5% for PSD representative of surface river waters.
我们研究了亚马逊河主要水域中固有光学特性（IOP）和表观光学特性（AOP）与悬浮泥沙浓度（SSC）之间的关系。在沿着亚马逊河流域不同支流进行的三次采样行程（104个站点）中，对SSC、遥感反射率（R）、漫射光衰减系数（K）以及总吸收系数和非藻类颗粒（NAP）吸收系数（分别为a和a）进行了现场测量。还对85个站点的悬浮泥沙的粒径分布和化学特征进行了测定。我们发现，河水中的粒径分布（PSD）最好用分段的容格幂律分布来描述，最小颗粒（J = 2.4）的斜率值较小，最大颗粒（> 10 µm）的斜率值较大（J = 4.1）。当考虑整个数据集时，发现AOPs、IOPs与SSC之间存在很强的关系。然而，对于马德拉河（就悬浮泥沙排放量而言是亚马逊河的主要支流），检测到R - SSC关系存在显著离散，但K - SSC关系不存在。先前一项使用MODIS数据的研究表明这种离散呈现出季节性模式，我们在本研究中使用根据悬浮泥沙特征校准的米氏模型对此进行了研究。在马德拉河，涨水期悬浮泥沙的粒径分布更细且矿物学特征不同（例如，蒙脱石含量更高，高岭土含量更低）。折射率虚部n(λ)的光谱变化在涨水期也显示出显著差异。相比之下，亚马逊河流域的其他支流在粒度和矿物学方面具有非常稳定的特性。模型模拟使得能够重现现场和卫星观测结果，表明在马德拉河近红外波段观测到的R滞后现象主要是由于n(λ)的季节性变化，导致涨水期吸收减少。由于K强烈依赖于散射过程，所以显示其保持稳定。该模型用于进一步理解悬浮泥沙粒径分布和折射率如何驱动大型河流中的IOPs：n(λ)变化主要控制反射率变化；在大型河流典型范围内（J < 3），R(850)作为PSD的函数变化有限，尽管它对颗粒矿物成分仍很敏感；对于较粗的PSD，R(670)表现出相反的行为，对PSD变化更敏感。最后，我们证明，在所有情况下，使用红波段和红外波段之间的R比值可降低R的敏感性，相对于悬浮泥沙矿物成分或粒径分布的变化平均降低50%。特别是，对于代表表层河水的PSD，R比值变化小于5%。
