Large volumes of potable water are imported into almost all urban areas of Iran at extremely high cost, while considerable volumes of urban storm water are disposed of out of the cities and wasted, mostly into bodies of saltwater. In this paper, urban storm water is examined as a potentially valuable and reclaimable resource. A model is introduced in which storm water runoff is captured and stored behind a small dam of height H with an overflow weir of length L to waste excess flows. At the same time, the storm water is diverted through a side weir of width W, to be conveyed to suitable recharge grounds for later Reclamation and use. A particular watershed, in the arid and rapidly urbanizing city of Bandar Abbas in southern Iran, with 24 years of rainfall records, was chosen and used in the model. The rainfall from nine other arid, semiarid and wet regions were used as input to the same watershed. The results show that the amount of reclaimable water as a percent of total runoff is almost the same for all the regions. This study provides a relationship, which defines the reclaimable water as a function of W,L and H. The relationship may be used for planning urban storm water Reclamation projects. The normal parameters defining arid, semiarid and wet climate are not of significance in this relationship. The relationship, however, may be further refined if one incorporates the number of intense storms as an extra parameter.