Structure of the rainfall gradient in the Mediterranean-Sahara transition in Tunisia: geographical determinants and seasonality

A geostatistical analysis of interannual (1961-1990) mean annual rainfall and of its monthly components is made based on a network of 106 raingauges covering the Tunisian territory. The relevance of a power-type variogram, for the year and for most months confirms the existence of gradients. Analysis of directional variograms shows that the apparently north/south general gradient results from partial gradients which evolve through the seasons. The spatial characteristics of rainfall depend on geographical determinants, both local with a relatively short range which lead to micro-regionalisation, and regional with a longer range. The major role of topographic structures through the interception of rainfall-generating air masses is shown. The proposed analysis is a contribution to the setting-up of a synoptic hydro-climatic reference for Tunisia itself, and for the Mediterranean-Sahara transition zone of which it is an example.