Abstract
The monitoring of herbaceous fuel moisture content is a crucial activity in order to assess savanna fire risks. Faced with the difficulty of managing wide areas of vegetated surfaces, remote sensing appears an attractive alternative for terrestrial measurements because of its advantages related to temporal resolution and spatial coverage. Earth observation (EO)-based vegetation indices (VIs) and the ratio between Normalized Difference Vegetation Index (NDVI) and surface temperature (ST) were used for assessment of herbaceous fuel moisture content estimates and validated against herbaceous data collected in 2010 at three open savanna sites located in Senegal, West Africa. EO-based estimates of water content were more consistent with the use of VI as compared to the ratio NDVI/ST. Different VIs based on near-infrared (NIR) and shortwave infrared (SWIR) reflectance were tested and a consistent relationship was found between field measurements of leaf equivalent water thickness (EWT) from all test sites and Normalized Difference Infrared Index (NDII), Global Vegetation Moisture Index (GVMI) and Moisture Stress Index (MSI). Also, strong relationships were found between fuel moisture content (FMC) and VIs for the sites separately; however, they were weaker for the pooled data. The correlations between EWT/FMC and VIs were found to decrease progressively as the woody cover increased. Although these results suggest that NIR and SWIR reflectance can be used for the estimation of herbaceous water content, additional validation from an increased number of study sites is necessary to study the robustness of such indices for a larger variety of savanna vegetation types.
Highlights
Savanna fires play a major role in ecosystem dynamics in dry tropical areas [1,2]
During the 32 days of sampling collection of biomass for herbaceous water content estimates, the equivalent water thickness (EWT) results showed that the herbaceous water content increased from the north (Sahelian savannas) to the south (Sudanian savannas) following the rainfall gradient (Figure 4)
The temporal evolution of the water content was characterized by a continued downward trend for all sites. 40 days after the last rain, the herbaceous vegetation in the northern site had almost lost all its water content (0.003 g·cm−2 being close to the dry weight), whereas it was
Summary
Due to an increased number of fires ignited by humans, fire frequency on savanna ecosystems has increased during the last decades to a level where it has become a threat to biodiversity and ecosystem stability [3]. The vegetation fuel moisture is a critical parameter in fire ignition because flammability is closely limited by leaf water content [5]. Ground measurements of herbaceous water content are labor intensive and expensive if performed over larger areas and difficult to apply at the regional scale. The phenology adds another level of complexity which is related to the short time window during which fuel moisture drop down rapidly (a few weeks after the end of the rainy season)
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