It was found that the systems analysis technique is a useful tool for hydrologic systems and is not only applicable to artificial hydrologic systems but also to natural catchments. The general equation describing the relationship between surface runoff and rainfall excess of a hydrologic system is a second order nonlinear equation. The damping coefficient for hydrologic systems is approximately unity and the other parameters in the transfer function (Eq. 5-4) such as the time constant and the nonlinear parameter depend on basin characteristics and input intensity. / Ph. D.