The Pumping test is one of the most useful means for determining the hydraulic properties of aquifer and confining beds. Results can be obtained that, in general, are representative of an area larger than those obtained by means of point observations (Kruseman and de Ridder, 1990). Theis (1935) was the first to develop a formula for unsteady-state flow that introduces the time factor and the storavity. The unsteady-state equation, wich was derived from analogy between the flow of groundwater and the conduction of heat introduced the expotential integral (Chiccoli et al., 1990) (which is written symbolically W(u)). Classical methods use approximations to ‘well function’, where you must fulfill conditions (constraints) on the physical variables for minimal error (Theis, 1935; Chow, 1952; Cooper and Jacob, 1946). This work proposes a method to estimate the transmisivity (T) and the storativity (S) for unsteady-state flow. Our proposed method generates more robust and accurate estimation leading to a more reliable characterization of the hydrogeological specified area.