PREPARATION OF HYDROLOGICAL VARIABLES FOR MODELLING: Mean areal precipitation -- Baseflow and runoff -- Effective rainfall -- TREATMENT OF HYDROLOGICAL VARIABLES IN LINEAR SYSTEM MODELS: System-theoretical elements -- Concepts in linear systems -- Treatment of variables in the time domain -- Treatment of variables in the frequency domain -- Particular linear systems (conceptional models) -- Identification of linear systems -- Linear systems with stochastic inputs -- TREATMENT OF HYDROLOGICAL VARIABLES IN NON-LINEAR SYSTEMS: Volterra series model -- Composition of parallel submodels.