Citation: Acknowledgements: The author is very grateful to Professor univ. Ph.D. Elena Gavrilescu (from University of Craiova), researcher II Ph.D Ion Marinica (from Meteorological Regional Center Craiova) and mathematician Florin Jianu (from Center of calculus of Jiu Water Directorate, Craiova) for helpful discussions about this subject which helped the author to improve this paper. Also, the author thanks Alexandra Pasparuga for translating the article into English.