Spatial structures of reference evapotranspiration and correlated climatic variables over Tunisia
Résumé
The study investigates the spatial organization of inter-annual (1986-2003) reference evapotranspiration over 10-day periods, and of the climatic variables used for its assessment (air temperature, air relative humidity, wind speed and duration of daily sunshine), over Tunisia, which displays a transition from sub-humid Mediterranean to arid Saharan climates. The study is based on variogram modelling with assessment of the parameters (nugget, slope or sill and range). We also consider indicators of the variogram-origin behaviour and of the spatial structure variation for the studied variables. It appears that models of variograms change according to both the variables and the periods within the year. Nevertheless they are mainly of linear type. The variogram of the reference evapotranspiration has no nugget for any period of the year. Concerning the other variables, especially maximum air temperature, relative humidity and duration of daily sunshine, the nugget is higher during the hot period of the year. The variogram of the wind speed is spherical with a large range for the whole year, while the sunshine duration structure displays a pure nugget effect during summer and is linear for the rest of the year. Reference evapotranspiration displays a structural gradient similar to that of relative humidity over the whole year, but different from those of all other variables.