Local spatiotemporal dynamics of a simple aridity index in a region susceptible to desertification

Increased aridity is a robust proximate cause of desertification, both indirectly through greater rainfall variability and directly through prolonged droughts. The south of continental Portugal has large areas with high susceptibility to desertification. This study evaluates local dynamics in dryness in the south of Portugal based on the Aridity Intensity Index (AII), which is a numerical indicator of the degree of dryness of the climate and an indirect indicator of soil moisture availability. The AII was computed using daily precipitation data from stations with records within the 1940–1999 period in the south of Portugal. Annual scenarios of the AII were generated from 1940 to 1999 using direct sequential simulation (DSS). Those scenarios were then used to produce an additional set of maps that summarize their underlying space–time patterns. Two desertification indicators accounting for local dryness dynamics are proposed, namely the Dryness Susceptibility Indicator and the Dryness Trend Indicator. The results show that the southeast region is the most threatened by droughts and extreme dryness. Moreover, there is a tendency towards drier climatic conditions in coastal areas and in the centre of the study region. These findings are likely to have profound implications in agricultural planning and water supply management.