An introduction to hydrology and hydrogeology: the basic equations describing fluid movement in groundwater systems will be derived and applied. The main techniques to investigate groundwater flow systems are highlighted. Water circulation within river catchments is discussed by means of the catchment water balance. The physical process represented by each component of the water balance will be covered as well as the current methods of quantifying these fluxes of water within the catchment . Principles of catchment modelling are outlined.