The classification of a reservoir by volume is not as straight forward as it may seem. As the name implies, water is held in reserve by a reservoir so it can serve a purpose. For example in Thailand, reservoirs tend to store water from the wet season to prevent flooding and then release it during the dry season for farmers to grow rice. For this type of reservoir, almost the entire volume of the reservoir functions for the purpose it was built. Hydroelectric power generation, on the other hand, requires many dams to buildup a large volume before operation can begin. For this type of reservoir only a small portion of the water held behind the dam is useful. Therefore, knowing the purpose a reservoir has been constructed, and knowing how much water can be used for that purpose helps determine how much water is in possible reserve.