NDVI - Vegetation Index
This article provides an overview of Normalized Differential Vegetation Index (NDVI) maps, an index used to estimate the quantity, quality and development of crops, measured through remote sensing from aerial or satellite systems, capturing the reflection of different wavebands emitted by the crops.
Remote sensing basic concepts
Prior to reviewing how NDVI works, the basic concepts of remote sensing should be reviewed, as detailed in this article: Remote Sensing Basics (extension.org)
The relationship between reflected, absorbed, and transmitted energy is used to determine spectral signatures of individual plants. Spectral signatures are unique to plant species. Spectral Signatures are the specific combination of reflected, absorbed, and transmitted energy at wavelengths which can uniquely identify a plant species; the following graph illustrates the different signatures for different crops
What is the NDVI?
This similar behaviour in plant reflectance is used to determine differences among crops, and also, to estimate indexes by combining values from wavelength ranges or wavebands, which can provide information on the crop evolution.
Normalized Difference Vegetation Index (NDVI) is calculated from the visible and near-infrared light reflected by vegetation. Healthy vegetation (left) absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation (right) reflects more visible light and less near-infrared light. The numbers on the figure below are representative of actual values, but real vegetation is much more varied. Measuring Vegetation - NASA Earth Observatory
The coefficient constitutes a good measurement of the physiological activity of the plants; also, in empirical studies, it shows a good level correlation with the crops yields.
The NDVI is an index calculated based on the radiation of remote sensing reflection bands:
NIR and RED are the reflectance of the near-infrared and red bands; in the particular case of Landsat images, it corresponds to bands 4 and 3.
These bands are selected because of the spectral response of the vegetation, which has a high reflectance to the near-infrared band, while the red band is used as a reference, since it always keeps a near constant value.
The NDVI varies between known parameters (-1, 1). This facilitates its clasification since values less than 0.2 the foliar activity is low, while values greater than 0.4 correspond to active levels of foliar activity.It has a direct relation with the quantity, quality and development of vegentation, and it correlates well with the yield of a given field in key dates.
NDVI and crop stages
NDVI values have a strong correlation with crop stages (Sample Crop Growth Stages). This relationship has been well documented in several research experiences, as illustrated in the figures below:
Effect of nitrogen levels on NDVI - showing NDVI related with DAS (Days After Seeding), from Spectral response of nitrogen fertilization in cotton (Gossypium species) (Ansari, Mahey, et al)
Average NDVI profiles and standard deviation for each cropping season and definition of four development phases boundaries, from Sugarcane yield estimates using time series analysis of spot vegetation images (Lobato Fernandes et al)
One aspect that should be considered, is that during the early stages of the crop, when the green leaf area is still small, it is highly sensitive to soil background effects induced by tillage, crop residues, or moisture. It also tends to saturate once the heavy crops have reached canopy closure.
Types of NDVI uses
- Scouting: NDVI can be used for scouting, allow agronomists and producers to identify problem areas, and to make timely decisions. Scouting Maps can be requested at key dates as a guidance for field visits. NDVI based scout maps show variations in the field, so the user knows where to look in the field to determine where corrective or preventative measures are needed. Users can plan their field visit locations, take it to their GPS or a printable pdf report, and accurately evaluate the reasons for in-field variability.
- Variable rate applications: Plant growth regulators, insecticides, herbicides, fungicides, mid-season fertilizers and harvest are valuable resources that can be effectively used to for better crop management while minimizing input costs. Since NDVI shows variation of biomass within a field and divides those differences into management zones, these maps help managers to make better scouting decisions and address those decisions with variable rate crop applications driven by variable rate prescriptions created with GeoAgro GIS.
- Crop monitoring: Another approach of using NDVI maps is for field monitoring; this involves processing all suitable images (with no cloud cover) during the crop season, and detect possible anomalies during the season. Usually there would be between 4 and 6 images available for analysis during the season. The following screenshots illustrate the NDVI images for different dates, for field monitoring purposes
- Yield estimation: A strong correlation has been demonstrated between yields and NDVI at certain crop growht stages, as described in this research:In-season prediction of corn grain yield potential using NDVI at various vegetative growth stages. In the most basic method, variables related with yields can be sampled in the field, correlate them with NDVI values, to extrapolate them to the entire field, and obtain an estimated yield map.
NDVI and spatial resolution
Different levels of Spatial resolution can determine possible uses of NDVI maps. For instance: at mid/low resolution (e.g.: 30mt), they can be used for the applications mentioned above, such as Scouting, supporting variable rate fertilizing, or estimating yields. At higher resolutions (e.g.: 1mt) there are more detailed uses such as evaluating Nutrient Deficiencies (tissue tests/soil samples), detecting equipment problems (compaction, depth control, etc) or locating tiles (tiles can be visible as the soil starts to drain for a brief period after a spring shower), Insects & Disease Pressure.
The following figures illustrate NDVI maps with three sample resolutions, at 30mt, 5mt, and 1mt.
|NDVI Map - 30 mt pixel||NDVI Map - 5 mt pixel||NDVI Map - 1 mt pixel|
- Remote Sensing Basics (extension.org)
- Precision Agriculture: Remote Sensing and Ground Truthing (University of Missoury Extension)