The SPI was designed to be a relatively simple, year round index applicable to all
water supply conditions. Simple in comparison with other indices, the SPI is based on
precipitation alone. Its fundamental strength is that it can be calculated for avariety of time
scales from one month out to several years. Any time period can be selected, often
dependent on the element of the hydrological system of greatest interest. This versatility
allows the SPI to monitor short-term water supplies, such as soil moisture important for
agricultural production, and longer-term water resources such as groundwater supplies,
stream flow, and lake and reservoir levels.