Biophysical Parameters

Access to an absolute quantification of the biomass and the nitrogen nutrition

In a world where population is growing fast while agricultural lands and resources are limited, many agricultural practices target to combine productivity with minimised environmental impact.

With this objective, it is now common usage to leverage satellite, aerial or UAV images to gather an overview of the parcel and help farmers apply the right level of fertilisers, pesticides and water at the right place within the field and at the relevant point in time. Nowadays, the most widespread methodology to estimate the vegetal development from these images is called NDVI (Normalised Difference Vegetation Index). It can be calculated easily, and provides a good overview of the plant development status differences within the field: the NDVI map will display where the quantity of live green vegetation is high and where it is low.

However, NDVI entails some limitations, because it presents a relative evaluation of the differences in vegetation development - NDVI being sensitive to the light conditions, the viewing angle, the sensor, and close to saturation when canopy closure is reached. As a result, access to absolute information with NDVI requires calibrating the model with ground measurements that should be synchronised with the image acquisitions. Otherwise, consistent monitoring over time is impossible with this index.

A second, more advanced method exists. It is called biophysical parameters extraction. Although it requires sophisticated, complex processing algorithms, it provides access to an absolute quantification of the biomass and the nitrogen nutrition, via calculation of different indicators such as green cover fraction (fCover), FAPAR (fraction of absorbed photosynthetically active radiation), leaf area index, chlorophyll content, brown cover fraction, etc.

Name	Meaning	Value	Interest
fCover (fractional cover of green vegetation)	% of surface of the ground occupied by the plant, seen from above	0 to 1, 1 corresponding to a situation where the ground is completely covered by vegetation	Mixed with other indicators, gives access to biomass assessment
FAPAR (Fraction of Absorbed Photosynthetically Active Radiation)	% of sunlight absorbed by the plant in the domain of photosynthesis	0 to 1, 1 corresponding to a photosynthesis fully active	Directly related to photosynthesis activity, mixed with other indicators, gives access to biomass assessment and production
LAI (Leaf Area Index)	Number of square meters of leaves present in one square meter of ground	Up to 5 to 7 for most crops (canopy closure is reached for 3-4)	Mixed with other indicators, gives access to biomass assessment
Chlorophyll content	Content of Chlorophyll A and B per unit area of leaves	Typical range 20 to 80 µg / cm2	Mixed with biomass, allows assessing nitrogen concentration and deriving nitrogen inputs recommendations
fNPV (fractional cover of non photosynthetic - brown - vegetation)	% of surface of the ground occupied by brown leaves, seen from above	It ranges from 0 to 1, 1 corresponding to a situation where the ground would be completely covered by brown vegetation	May indicate stress or a certain degree of senescence / maturity in the crop. Can be used to issue recommendations for harvest or irrigation

The biophysical parameters are robust and independent from the viewing angle, the sensor, and the light conditions. They can then be used in a straightforward way to follow evolution and compare situations at two different points in time with no bias, free of ground measurement. Used in combination with agronomical models, they can be efficiently derived into recommendations to accurately dose fertilisers and water, rightfully apply growth regulators and pesticides, finally saving on chemicals and lowering farming environmental footprint.

For over 15 years, Biophysical parameters have been used in our agricultural services, from intra-field precision agriculture up to nation-wide indexes maps for fodder insurance.