Abstract

ODI-LARA, Magali; PAZ-PELLAT, Fernando; LOPEZ-URREA, Ramón  and  GONZALEZ-PIQUERAS, José. Limitations in the estimation of biophysical variables in crops using spectral vegetation indexes: foliage density effect. Agrociencia [online]. 2010, vol.44, n.7, pp.807-819. ISSN 2521-9766.

High density foliage crops produce the rapid saturation of the red band and reduction of variations in curve slopes of the same vegetation (iso-LAI), on which many vegetation indexes (VI) are based. For this reason, it is important to develop alternative indexes that do not become saturated and that are easily parameterized with information from the field. To this end, temporal patterns of spectral indexes (expo-linear model as reference) were evaluated based on the slopes of the iso-LAI lines, as well as the biophysical variables aerial biomass (Bm), aerial cover (fv) and height (h) of two crops with different foliage density: high (sorghum) and medium (cotton). When an expo-linear model is applied, it is expected that it would have a bi-linear pattern, as long as the variables coincide in their initial and end phases. In general, the Bm exponential and linear phases do not coincide with those of fv and h; this shows that there are differences among the structural characteristics, those of spatial vegetation distribution (fv and h), and photosynthetic function (foliage and biomass). The medium density crop exhibited well-characterized bilinear patterns. In contrast, the high density crop saturated rapidly in the exponential phase of Bm. To reduce the problem of saturation, the use of the additive constant a₀ was explored as an alternative to the indexes based on slope. The relationship between the additive constant and Bm exhibited patterns of the double expo-linear type with problems in the definition of its transitions; the relationship with the variable fv were bi-linear, both for the medium density crop and for the high density crop, thus facilitating parameterization the vegetation indexes (use of fv) in the field; with height, the relationship was linear-exponential.

Keywords : vegetation indexes; foliage density; iso-vegetation lines; sorghum.

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