The multipurpose water resources system Danube-Tisza-Danube (WRS DTD) (flood control. drainage. irrigation, water supply, navigation, water quality control, etc.) is hydraulically very complex. WRS is controlled through a number of weirs which send the water in various directions, depending on the actual hydrologic or water resources conditions, To improve the operation of the system a control model has been developed. The complex development task is realized by a chain of gradually developed MM. First, WRS was decomposed: (a) spatially (divided into subsystems); (b) temporally (by time intervals — or the periods of quasy-steady and unsteady flow); according to the hydraulic function (the weirs operation MM, network flow MM, inflow estimation MM. etc.); and (d) according to numerical aspects. The non-standard problems of unsteady flow with varying discharge throughout the WRS (tributaries, pumping stations for irrigation and drainage) are solved by using the latest advancements in the computational hydraulics theory (Preismann scheme, Cholesky scheme, el). A very operable unsteady-flow MM has been developed and its introduction to practice will improve the operation efficiency of the WRS.