The need for reliable tools, which can assess the hydrological consequences of climate change, as well as of land use change is evident with the increasing environmental concerns. In this framework, with the aim of improving hydrological modelling approaches, different models have been employed, which reproduce internal catchment processes. These have been assessed and validated with field observations, measurements and experimental catchments. This process has identified model weaknesses, of primary importance for improving modelling conditions, and for obtaining a better understanding of the behaviour of catchments. The various hydrological models which have been employed and improved include a physically-based model (SHETRAN), a parsimonious model (TOPMODEL), submodels concerned with forest interception and snow melting, as well as a methodology for the hydrological use of ground probing radar (GPR). Environmental tracing has also been used, for the development of a new software package for the uncertainty analysis of hydrograph decomposition. Models have been tested in a range of environmental conditions across Europe, as well as land uses and changes, e.g. erosion processes, snow melting, flow generation and partitioning, for validation with experimental data and detailed observations of catchment internal hydrological processes. The procedure is now available for demonstration. Other studies include digital terrain modelling (DTM), the use of global positioning systems (GPS), a new TOPKAPI model, and comparisons with the two widely used water management models SACRAMENTO and BROOK. The advantage of the series of models developed and improved is that it has significantly progressed hydrological modelling, necessary for evaluating different climate change scenarios, as well as the corresponding land use changes. On a practical level, such information is also useful for assessing different watershed management strategies for preserving water resources.