This book is intended to be a simple, clear and elementary introduction to conceptual rainfall-runoff hydrological models. Modeling the runoff becomes more challenging as runoff generation process is highly complex, nonlinear, dynamic in nature, and affected by many interrelated physical factors. Further, the temporal and spatial variability of these factors cause more uncertainty in the parameterization of the model. Parameter estimation for the existing rainfall-runoff models has been a subject of investigation since last few decades. With increasing capacity of computational effort, several complex optimization algorithms have emerged in the recent past, but none of the algorithms provide a unique parameter. This book describes how to estimate parameters of the NASH model by analyzing geomorphologic and hydrologic characteristics of watershed.