Testing the Relevance of Daily MODIS Data to Monitor Mediterranean Shrubland Canopy Water Content with Temporal Cross-Correlation Analyses

Testing the Relevance of Daily MODIS Data to Monitor Mediterranean Shrubland Canopy Water Content with Temporal Cross-Correlation Analyses

Carole Delenne (UMR HSM, Université de Montpellier 2, Montpellier, France), Jean-Stéphane Bailly (UMR TETIS et UMR LISAH, AgroParisTech, Montpellier, France) and Michel Deshayes (UMR TETIS, Institut National de Recherche en Sciences et Technologies Pour L’environnement et L’agriculture, Montpellier, France)
DOI: 10.4018/jaeis.2013010101
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Drought alert systems for forest fire prevention often rely on vegetation water content (VWC) monitoring which is a key parameter in forest fire hazard. In southern France, VWC is now monitored through regular field surveys. Thanks to the theoretical sensitivity of shortwave infrared reflectance to VWC, MODIS satellite data are potentially able to monitor VWC depending on plant species VWC magnitude. In this paper, a specific statistical approach based on temporal cross-correlations is developed in order to test the correlation between two MODIS water indices and VWC measurements coming from field surveys. This test assesses the ability of daily MODIS data to monitor Mediterranean shrubland canopy water content and detect any delay effect between MODIS and field survey temporal series. Statistical tests are carried out for 29 sites containing 18 dominant shrubland Mediterranean species. 67% and 54% of significant correlation were found using respectively the NDII and NDWI indices from MODIS data. Correlation were found low with a dominant negative delay effect, i.e., with a MODIS signal that reacts a few days after the field VWC. Test results show that, even if deeper pre-processing of MODIS data may be required, site soil, site vegetation cover, and heterogeneity at MODIS pixel scale, as well as species VWC sensitivity make correlation between field VWC and MODIS water indices non univoque and highly variable. Many obstacles are still to overcome, for an accurate monitoring of Mediterranean shrubland canopy water content using MODIS daily data.
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1. Introduction

Most of drought alert systems for forest fires prevention rely on vegetation water content (VWC) monitoring since this parameter plays a key role in the ignition and burning rate of woody or shrublands areas (Yi & Huili, 2010). In the Mediterranean area of southern France, the mean burnt area is about 20 000 ha per year (DGPM, 2012). Up to now, VWC monitoring is performed through a costly field survey network, requiring a periodic field work of a large number of agents followed immediately by a 24-hour laboratory data processing (Dauriac, 2004).

VWC is known to be sensitive to remote sensing infrared signal, especially shortwave infrared (SWIR) (Hunt & Rock, 1989; Ceccato et al., 2001; Chen et al., 2005; Ustin et al., 2008; Wang et al., 2008; Yilmaz et al., 2008). Yilmaz et al. (2008) recently succeed in the use of the Normalized Difference Infrared Index (NDII) from Landsat, ASTER and AWiFS images to remotely sense Equivalent Water Thickness (EWT) of corn leaves, which was found to be highly correlated to corn VWC.

With its high temporal resolution, the use of Terra-MODerate Resolution Imaging Spectroradiometer (Terra-MODIS) data has also been already explored for crop water content monitoring (Chen et al., 2005; Trombetti et al., 2008), for semi-arid ecosystems monitoring (Fensholt & Sandholt, 2003; Bajgiran et al., 2009) or for forest water content monitoring (Dasgupta, 2007; Liu et al., 2009; Qu & Ambrose, 2009). Walker et al. (2012) recently demonstrated the feasibility of a combined use of Terra-MODIS images (with high temporal resolution) and Landsat images (high spatial resolution) in order to monitor dryland forest phenology. In this study, forest dryland phenology was only measured through NDVI (Normalized Difference Vegetation Index) data and VWC was not considered. In addition, (Duveiller et al., 2011) pointed out the fact that modelling the instrument point spread as well as filtering MODIS data according to target and pixel overlaps, increase the accuracy in parameter estimations of crop phenomenology. However, the approach they proposed requires a precise spatial delineation of the objects to monitor, within MODIS pixels. This is hardly feasible in the case of Mediterranean shrubland VWC that corresponds to a continuous mixing of species. Most of these studies were developing using existing or newly defined near and mid-infrared bands ratio indices, among which can be cited: the Normalized Difference Water Index (NDWI) (Zarco-Tejada et al., 2003), the Simple Ratio Water Index (SRWI) (Zarco-Tejada et al., 2003), the Shortwave Infrared Water Stress Index (NDII) (Fensholt & Sandholt, 2003), the Vegetation Temperature Condition Index (VTCI) (Wan et al., 2004) and the Normalized Difference Infrared Index (NDII) (Hardisky et al., 1983).

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