Statistical analysis, time tendencies of real evapotranspiration and diagnostic of a functional relationship between insolation and evapotranspiration. Case of ndjamena-chad. Period from 1986 to 2010

This work particularly concerns statistical distribution and time tendencies of the evapotranspiration in Ndjamena and surroundings for the period from 1951 to 2010 included. As solar radiation is the main cause of the evapotranspiration and keeping in mind that Chad and particularly Ndjamena are closer to the desert of Sahara, the authors at first have paid their attention to the insolation, e.i. the duration of the solar radiation on the earth surface which, measured using the heliograph Campbell-Stokes. Concerning the evapotranspiration, it is usually estimated by empirical formulas elaborated for some specific zones. Moreover, these formulas need information on some atmospheric parameters which are not currently measured in many meteorological stations, particularly in developing countries. Thus, in this paper was analyzed real evapotranspiration measured with the bac of evapotranspiration installed according to the World Meteorological Organization prescriptions. The authors have diagnosed a relationship between insolation and evapotranspiration. It would enable users to faster estimate the second parameter knowing the first as it is measured in almost all the meteorological stations. To do the study, 1951-2010 was divided into five years sub periods. In each one, monthly means and standard deviation were calculated, corresponding values in other sub periods were compared between themselves. This permitted us to analyze their statistical distribution and to deduce their time trends. According to the analysis, the main results were that highest values of both parameters were registered during the dry season, particularly in March-April, and lowest values – in the rainy season, mostly in August. The values of insolation were dispatched closer their mathematical means than what was observed for the evapotranspiration. For the moment, establishing a relationship between the two parameters seems to be difficult as all requested information is not yet available. Effort should be done to complete the bank off data with missing one.