Abstract: Originally, water balance models were introduced to evaluate the importance of different hydrologic parameters under a variety of hydrologic conditions but its present applications are the most common studies at water resources management. In spite of the simple concept of water balance equation, specific considerations are need to proper application. With numerous affecting factors on hydrologic processes, the parsimony trait of water balance equation can cause huge errors or complexities throughout study processes. It is beyond a general computation to create an appropriate portrait of water circumstances with a parsimonious equation that should be considered as an art. Practically, water balance computations are used in five separate categories at least: watersheds, groundwater aquifers, farms, urban water distribution networks and particular areas such as glaciers and landfills; totally, they are directed along three main lines: watershed hydrology reconstruction, evaluation of water supply and water demand systems and assessment of climatic changes impacts. This study is to concentrate on some specific hints which their ignorance leads us to less reliability on water balance results and misunderstandings of actual situations. Finally, the methods used in Iran are investigated in five separate watersheds in the north east of the country and their results are compared with two other published results.