Gaussen Index

The Gaussen Index (or Bagnouls-Gaussen Index) or xerothermic index is a method of calculating and comparing aridity.

According to Henri Gaussen (French botanist and biogeographer), a given period is said to be arid, when: ${\displaystyle P<{2\times T}}$.^[1][2]

(P: total precipitation in millimeters over the given period, T: average temperature in °C over the given period)

The resulting index number indicates the number of biologically dry days in a year for a given location (it therefore ranges between 0 and 365). The data includes not only precipitation stricto sensu but also fog, dew and humidity of the air.

In general, it is accepted that an environment is non-arid when the index is less than 100, semi-arid between 100 and 290, arid between 290 and 350, and hyperarid between 350 and 365.

This index is very useful for the use of an ombrothermic diagram [fr], the latter always constructed on the scale model: 1 °C = 2 mm precipitation.

Other indices such as the Louis Emberger rainfall quotient (which is not unique) have been defined. However, the Gaussen index which is simple and precise is still preferable.^[3] Indeed Henri Gaussen defines precisely the 4 nuances of Mediterranean climate just against this index,^[4] while Emberger defines the level of humidity in a region of Mediterranean climate but does not support precisely this Mediterranean climate.^[3]

The calculation does not reflect reality because it is based on averages. For example, according to the calculation, we find a total of 0 biologically dry days in Lyon, for 60 biologically dry days in Marseille.^[5]