Relationships among the leaf area index, moisture availability, and spectral reflectance in an upland rice field

The leaf area index (LAI) and ratio of actual evapotranspiration to potential evaporation (ma) were estimated using spectral reflectance. Two indices, a vegetation index for LAI (VILAI) and chlorophyll concentration (VICC), were calculated using wavelengths of 550, 680, 800, and 980 nm. These indices were validated using data collected by upland rice field measurements. Indices, such as NIR/Red, NIR/Green, normalized differential vegetation index (NDVI), green normalized differential vegetation index (GNDVI), modified chlorophyll absorption in the reflectance index (MCARI), transformed chlorophyll absorption in the reflectance index (TCARI), soil-adjusted vegetation index (SAVI), optimized SAVI (OSAVI), modified SAVI (MSAVI), and VILAI, developed in this study, were statistically effective in estimating LAI. The correlation between ma and VICC was higher than that of other indices accounting for coefficients of determination (R2) = 0.97. This indicates that an index with four visible and near-infrared wavelengths expresses ma better than the traditional two or three wavelength indices. VICC can be considered to relate to ma through the chlorophyll concentration becoming low, reducing the photosynthetic ability and decreasing evapotranspiration.