Abstract Live fuel moisture content (LFMC) is an important factor in fire risk management in the Mediterranean region. Drawing upon a large network of stations (the Reseau Hydrique) measuring LFMC for operational fire danger assessment in the south-eastern region of France, this study assesses the ability of several long-term passive microwave remote sensing indices to capture the LFMC temporal dynamic of various Mediterranean shrub species. Microwave remote sensing has a high potential for monitoring LFMC independently of several constraints ( e . g ., atmospheric and cloud contamination effects) associated with optical-infrared and thermal remote sensing observations. The following four microwave-derived indices are considered: (1) the Essential Climate Variable near-surface soil moisture (ECV_SM); (2) the root-zone soil moisture (ECV_RZSM) derived from ECV_SM; (3) the microwave polarization difference index (MPDI) computed from five microwave frequencies (C, X, Ku, K and Ka-band corresponding to 6.9, 10.7, 18.7, 23.8 and 36.5 GHz respectively); and (4) the vegetation optical depth (VOD) at C- and X-band (from the Advanced Microwave Scanning Radiometer for the Earth observing system, AMSR-E). Firstly, an evaluation of the root-zone soil moisture ECV_RZSM against a network of soil moisture measurements (SMOSMANIA in southern France) gave satisfactory results. For most of the Reseau Hydrique sites, the present study found good agreement between LFMC and individual microwave indices, including root-zone soil moisture, VOD at X-band, and MPDI at X and Ku-bands, all averaged over the 15 days preceding the in - situ LFMC measurements. VOD at X-band showed the best agreement with the in situ LFMC data (median of correlation coefficients over all in situ sites = 0.43). Further comparisons between LFMC data and several optical indices computed from the Moderate Resolution Imaging Spectrometer (MODIS) data including normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), visible atmospheric resistant index (VARI), normalized difference water index (NDWI), normalized difference infrared index 6 (NDII6), normalized difference infrared index 7 (NDII7) and global vegetation moisture index (GVMI) were made. The comparisons showed that VARI and SAVI, as optical greenness indices, outperform the microwave indices and other optical indices with median of correlation coefficients of 0.66 and 0.65, respectively. Overall, this study shows that passive microwave indices, particularly VOD, are efficient proxies for LFMC of Mediterranean shrub species and could be used along with optical indices to evaluate fire risks in the Mediterranean region.

中文翻译：

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用于监测地中海地区活燃料水分含量的微波遥感评估

摘要 活燃料水分含量 (LFMC) 是地中海地区火灾风险管理的重要因素。本研究利用测量 LFMC 的大型站网络（Reseau Hydrique）在法国东南部地区进行运营火灾危险评估，评估了几个长期无源微波遥感指数捕捉 LFMC 时间动态的能力。各种地中海灌木物种。微波遥感具有独立于与光学红外和热遥感观测相关的若干限制（例如，大气和云污染影响）监测 LFMC 的巨大潜力。考虑了以下四个微波衍生指数： (1) 基本气候变量近地表土壤水分 (ECV_SM)；(2)由ECV_SM导出的根区土壤水分(ECV_RZSM)；(3) 由五个微波频率（分别对应于 6.9、10.7、18.7、23.8 和 36.5 GHz 的 C、X、Ku、K 和 Ka 波段）计算的微波极化差异指数（MPDI）；(4) C 波段和 X 波段的植被光学深度 (VOD)（来自地球观测系统的高级微波扫描辐射计，AMSR-E）。首先，根据土壤水分测量网络（法国南部的 SMOSMANIA）对根区土壤水分 ECV_RZSM 的评估给出了令人满意的结果。对于大多数 Reseau Hydrique 站点，本研究发现 LFMC 和单个微波指数之间具有良好的一致性，包括根区土壤水分、X 波段的 VOD 以及 X 和 Ku 波段的 MPDI，所有这些指数都是前 15 天的平均值原位 LFMC 测量。X 波段的 VOD 显示与原位 LFMC 数据的最佳一致性（所有原位站点的相关系数中位数 = 0.43）。LFMC 数据与从中分辨率成像光谱仪 (MODIS) 数据计算的几个光学指数之间的进一步比较，包括归一化差异植被指数 (NDVI)、土壤调整植被指数 (SAVI)、可见大气阻力指数 (VARI)、归一化差异水指数 ( NDWI)、归一化差红外指数6(NDII6)、归一化差红外指数7(NDII7)和全球植被水分指数(GVMI)。比较表明，VARI 和 SAVI 作为光学绿度指数优于微波指数和其他光学指数，相关系数的中位数分别为 0.66 和 0.65。全面的，