Volumes of rainwater harvested on-site are estimated by short-term storage capacity by two methods: graphical method and analytical method. The first method considers the difference between demand and supply over a specified period of time. The second uses random events to determine analytically, the storage capacity required to guarantee the draft. The comparison between the two methods indicates that there are minor differences. Based on the volume of water harvested after applying these methods, run-off coefficient for impervious surface in arid and semi-arid area was estimated. New analytical approach for long-term storage capacity is utilized to estimate the detention pond capacity off-site for the local natural streams. To apply this method, it is necessary to estimate the overall mean storage capacity in which the soil conservation service method is utilized. This technique is confirmed with graphical method.