The demand on land to produce food, for urban development such as housing, industry, shopping areas, infrastructure and also for recreation has increased during the history of mankind. This has resulted in such activities as the reclamation of swamps, flood plains, tidal areas and even lakes by impoldering. Model simulations can be use to gain insight into the system behaviour of different land use and soil composition under temperate humid and humid tropic conditions, The existing package OPOL, based on a non steady model, was further developed to the version OPOL5 for the simulation of hydrological conditions and optimization of the main components of water management systems in polder areas in the temperate humid and humid tropical zone. This model reveals a system's behaviour as well as the effects of variation in the main components of the systems to the overall costs. For example, the designs of pumped drainage systems in polder areas can be optimized by varying the main components until the annual equivalent costs are minimum. A GIS tool has been used to complement OPOL5 for the simulation of the real situation in an area with respect to land use, damage, topography, and soil type. The model package has been applied to two case studies: one in the temperate humid zone, namely the Netherlands and to Thailand in the humid tropical zone.