To calculate Aridity Risk, we use the variables [Copernicus Climate Data Store (CDS)]:
- Total wet day precipitation (TWD): annual sum of precipitation (in mm).
- Monthly mean of daily mean temperature
By calculating the annual mean temperature for each year, we can combine information on total annual precipitation P (input) and annual mean temperature T (output driver through evaporation and transpiration) to approximate the water balance, and thus estimate aridity risk.
The methodology described for calculating Aridity Risk is a form of climate classification based on indices that combine precipitation (P) and temperature (T). This approach is scientifically sound and based on a long tradition in climatology and hydrology.
- Meteorological Droughts in Europe: Events and Impacts – Past Trends and Future Projections This atlas formalized the use of the AI=P/PET index for mapping the planet's arid zones.
UNEP's Aridity Index (AI) is defined as AI=P/PET, where P is mean annual precipitation and PET is mean annual potential evapotranspiration. PET is a measure of how much water vapor could be emitted from a surface if water were not a limiting factor, and is calculated primarily from temperature.
Our method uses temperature (T) as a direct proxy for PET. This is a simplified but valid approach, as higher temperatures correspond to greater evaporative demand. The risk categories (from A to F) therefore correspond to UNEP-defined climate classes based on AI value.
- Kottek, M., et al. (2006). 'World Map of the Köppen-Geiger climate classification updated'
The Köppen-Geiger classification is one of the most famous and widely used climate classification systems in the world. It is based precisely on annual/monthly mean temperature and annual/monthly precipitation values to define climate zones.
Our methodology is a simplification focused on the 'humid-arid' axis of the Köppen system. In particular, the distinction between arid (BW - desert) and semi-arid (BS - steppe) climates is based, as in our case, on formulas combining P and T. Köppen established precipitation thresholds that vary based on annual mean temperature, as in our method.
- The De Martonne aridity index in Calabria (Southern Italy)
The De Martonne Index (I) is calculated with the formula I=P/(T+10), where P is annual precipitation in mm and T is annual mean temperature in °C.
This formula is the conceptual ancestor of our method. It establishes a direct and mathematical relationship between precipitation and temperature to define the degree of aridity of a location. The classes defined by De Martonne are similar to ours.
In conclusion, our aridity risk classification system is a well-founded empirical model, reflecting the consolidated scientific principles in aridity indices (UNEP, De Martonne) and climate classification systems (Köppen-Geiger). The cited sources provide the scientific validation of our methodological approach.