10.1117/2.1200904.1623 Multiband drought index enhances soil and vegetation moisture monitoring Lingli Wang and John Qu Satellite multispectral measurements provide accurate images of the intensity of forest fires over time. Drought, the most complex and least understood of all natural hazards, also affects more people than any other. 1 Traditional drought monitoring is based on observations at specific loca- tions, and thus lacks continuous spatial coverage. Satellite re- mote sensing offers an effective means to continuously monitor drought across wide areas. Most drought monitoring from space observes vegetation moisture using spectral indices that reflect variations in the spectral signatures caused by water stress. Very few studies focus on equivalent soil-moisture monitoring, how- ever. Simple spectral indices allow interpretation of changes in vegetation water content using a normalized ratio of a sensi- tive and an insensitive band. Uncertainties arise because these indices cannot completely remove background soil effects. The reflectance of a canopy is strongly influenced by the background soil reflectance, which is primarily determined by the moistness of the soil surface at a given location. Therefore, soil moisture must be taken into account in drought analysis. We performed sensitivity studies of three short-wave IR (SWIR) bands of the Moderate Resolution Imaging Spectrora- diometer (MODIS) and observed that the reflectance of each SWIR band responds differently to variations in soil and vegeta- tion moisture. 2 We therefore defined the Normalized Multiband Drought Index (NMDI), NMDI = R 0.86μm - (R 1.64μm - R 2.13μm ) R 0.86μm +(R 1.64μm - R 2.13μm ) , where R represents the reflectance at the wavelengths denoted by the subscripts. 3 Unlike traditional vegetation indices, which use a single SWIR channel, the NMDI uses the difference between two liquid-water absorption bands (at 1.64 and 2.13μm) Figure 1. (left) Model simulated soil spectra at various soil moisture levels. (right) Canopy spectra at different leaf water levels (C w ). Both graphs use Moderate Resolution Imaging Spectroradiometer (MODIS) bands B1-2 and B4-7. as a soil and vegetation water-sensitive band. The NMDI is based on the characteristic ‘slope variation’ in response to different kinds of moisture changes. As shown in Figure 1, the slope between the 1.64 and 2.13μm channels becomes steeper as soil moisture increases, but flatter as leaf water content increases. To show that the NMDI can be used to monitor both soil and vegetation moisture from space, we investigated its sensitivity to bare soil or weak vegetation as well as to heavy vegetation. For bare soil, higher values of the NMDI indicate increasingly severe soil drought. For heavily vegetated areas, on the other hand, lower NMDI values indicate increasingly severe vegeta- tion drought. The usefulness of NMDI for remote sensing of soil and vegetation moisture has been validated and reinforced us- ing soil spectra and satellite measurements, respectively. 3 Soil moisture and vegetation water content can influence the occurrence and behavior of wildland fires. 4 Since the NMDI can simultaneously monitor vegetation and soil water content, it should provide valuable information about wildland fire con- ditions. We used the 2007 wildfires in southern Georgia (USA) Continued on next page