Abstract Examples of numerical modelling of surface water hydrodynamics and water quality are presented. Their meaning for the development of the EIA 3D model system is analyzed from the almost 70 tracer studies (mainly 1971–1974) until the 3-dimensional (3D) solutions which became dominant since 1982. Up to summer 2008, the number of 3D applications has increased to almost 230 while the number of all applications exceeds 300.

The specific applications considered are from: Porttipahta reservoir in Northern Finland (I), Porttipahta and Lokka reservoirs (II), combination of reservoirs, lake, river sections, Kemi estuary and the sea (III), Kemi estuary and other coastal applications in the Gulf of Bothnia (IV), Lake Haukivesi in Eastern Finland (V), Lake Näsiselkä in South-West Central Finland (VI), and Kymi River and Kotka estuary in Southern Finland (VII).

A detailed description of the 3D model system is given in the application of Näsiselkä (VI). It is completed with drastically changing water levels, drying and wetting of immersed areas, characteristics of pulsing system, and internal loading in the application of Porttipahta (I). The application of the Kymi River and Kotka estuary (VII) shows the sensitivity of tracers as transport indicators and the validity of the transport model. In Lokka and Porttipahta (II) the methods of validity tests are extended to include field tests, laboratory experiments, and comparisons with expert evaluations and analytical solutions.

A strong indication of model validity is obtained in Näsiselkä (VI). When the loading after the model work was changed according to a plan included in the computation, the observed changes of water quality corresponded closely with those predicted by the model. Another severe indication of the model validity is from Porttipahta and Lokka (I, II). With the parameter values based on data from 1967–1986, a recent application indicated a close agreement with the observed data from 2000–2006. In Haukivesi (V) and Näsiselkä (VI) the agreement between the model and observed results is extended to the biological indicators of the algal biomass.

The integrated application to the network of a planned and two existing reservoirs, a strongly regulated lake, river sections, Kemi estuary and the sea (III) shows the usability of the model system to all types of water bodies. As a practical result it highlights the decay of the effects of a new impoundment with time and distance, including fast dilution in the estuary and the sea.