|  Request Password  |  Site Map  |  Print  | Español | 



GRAS (Climate) Information and Remote Sensing Systems Satellite´s images Vegetation Monthly Monitoring (NDVI)
Vegetation Monthly Monitoring (NDVI)

The normalized difference vegetation index (IVDN or NDVI) is a variable which permits estimating the development of a vegetation based on the measurement, with remote sensors, of the radiation intensity of certain bands of the electromagnetic spectrum that it emits or reflects.

Considering the situation of hydric deficiency of some zones of the country, the Vegetation Index that shows the situation at the end of October and starting of November is annexed.

In the following table are the images with the MAXIMUM values of NDVI observed for each month (monthly NDVI), as well as the deviations of those values from the “normal” value for each month (monthly deviations). These are the results of the interpretation of the images produced from the information caught by the NOAA-AVHRR satellite through the INTA Clima y Agua Castelar Reception Station in Buenos Aires, Argentina.

Images with the MAXIMUM values of NDVI 
 

Evolution of MAXIMUM NDVI for the last months. Click here.

Evolution of the NDVI per month since year 1997. In the following table click on the month you are interested:

 ENERO   FEBRERO   MARZO   ABRIL 
 MAYO   JUNIO   JULIO   AGOSTO  
 SETIEMBRE   OCTUBRE   NOVIEMBRE   DICIEMBRE

The NDVI values for a given month can be expressed in absolute values or in relative values compared to averages of hitoric series for that same month.

The NDVI uses channels 1 (VIS visible spectrum) and 2 (NIR near infrared) of the AVHRR sensor of the NOAA polar orbit satellites. Usually the live vegetation has low reflectance in the VIS spectrum and high reflectance in the NIR spectrum. When a vegetal cover in under stress, it trends absorve less solar radiation in the VIS increasing its reflectance and absorving more solar radiation in the NIR. In this way the difference between reflectance of channels 2 and 1: R2 – R1 (differential index) trends to decrease when the vegetal cover is affected by some stressing factor (water deficiency, diseases, pests, etc).

The normalized differential index is defined as : NDVI= (R2-R1)/(R2+R1) The values of NDVI range between -1 and 1. The index permits to identifying the presence of green vegetation on the surface and characterizing its spatial distribution as well as the evolution of its condition throughout time. This is determined mainly by the climatic conditions. The interpretation of the index should consider the fenological cycles and the annual developments to distinguish the natural oscilations of the vegetation from the changes in spatial distribution due to other factors.

·         Water has R1>R2 reflectance, so it has negative NDVI values.

·         Clouds have similar values for R1 and R2, so the NDVI is around 0.

·         Bare soil or with scarce vegetation has positive values but no too high.

·         Dense vegetation, humid and well developed has larger NDVI values.

The pixels of the AVHRR NOAA system have an approximate size of 1,1 x 1,1 km. However, for the calculation of the “normal NDVI” for each month a historical series of lower resolution (8 x 8 km approximately) is available. This is the reason why the NDVI maps have 1 x 1 resolution, while the monthly deviations maps are re-scaled at an 8 x 8 km resolution. The atmosphere, specially the clouds, influence on the R1 and R2 signals, tending to diminish the NDVI real value. This is why the monthly NDVI data correspond to the maximum value registered during the month for each pixel at the maximum value registered during the month for each pixel, in order to guarantee the lower incidence of clouds in the value of the index.