Methodologies based on the use of naturally occurring isotopes are, at present, an
integral part of studies being undertaken for water resources assessment and management.
Applications of isotope methods aim at providing an improved understanding of the overall
hydrological system as well as estimating physical parameters of the system related to flow
dynamics. Quantitative evaluations based on the temporal and/or spatial distribution of
different isotopic species in hydrological systems require conceptual mathematical
formulations. Different types of model can be employed depending on the nature of the
hydrological system under investigation, the amount and type of data available, and the
required accuracy of the parameter to be estimated.