The application of a weighted infrared-red vegetation index for estimating leaf area index by correcting for soil moisture.

The simplified reflectance model described earlier (Clevers, 1988b) for estimating leaf area index (LAI) is further simplified. In this model the nearinfrared reflectance was corrected for soil background (in particular differences in soil moisture content) and subsequently used for estimating LAI by applying the inverse of a special case of the Mitscherlich function. In the specific situation that the ratio of the reflectances of bare soil in the red and near-infrared is constant for a given soil background, the corrected near-infrared reflectance is now ascertained as a weighted difference of total measured near-infrared and red reflectances (socalled WDVI = weighted difference vegetation index). The validity of this approach is confirmed by simulations with the SAIL model. The above concept was tested at the experimental farm of the Agricultural University Wageningen, by using reflectance factors measured in field trials by means of multispectral aerial photography. The soil type at the experimental farm yielded constant ratios between green, red, and near-infrared reflectances independent of soil moisture content (that is, as a function of time). The difference between measured near-infrared and red reflectances provided a satisfactory approximation of the corrected near-infrared reflectance. The estimation of LAI by this corrected near-infrared reflectance for real field data yielded good results in this study, resulting in the ascertainment of treatment effects with larger precision than by means of the LAI measured in the field by conventional field sampling methods