Management practices for irrigation schemes can often be improved by modelling the behaviour of a scheme and by evaluating its efficiency statistics. However, looking at an irrigation scheme without regard for other water uses within the river basin may not be effective. Efficiencies computed in this way are local and, since water may be used many times within the same basin, may not be realistic. Changes in water uses (e.g. irrigation, industrial, power production, urban water supply, navigation, environmental, recreational) will often have effects on other uses; and changes in irrigation schemes (management,structural, crop pattern) may affect other users within the basin. In addition, all water users will be affected by external changes such as changes in land cover or in climate. As demand for water increases, these links become more important until, at some stage, there is insufficient water for all users and hard choices must be made. Hydrological modelling is a tool that can be used to relate irrigation schemes to the other water uses within a river basin and can help in assessing real productivities and in evaluating alternative patterns of water usage. This paper discusses the techniques available to simulate irrigation schemes within overall basin water resources using, as an example, an intensively utilized basin in western Turkey.