Drought is the most complex of all natural hazards, that is so difficultto define, detect, and measure, and resaerchers have been striving to develop indices to accomplish these tasks. In 1993, researchers at Colorado State University developed a new drought index, the Standardized Prediction Index (SPI), which is based on precipitation alone. The SPI was designed to be a relatively simple, year-round index applicaple to all water supply conditions. Its fundamental strength is that it can be calculated for a variety of time scales from one month out to several years. This versatility allows the SPI to monitor short-term water supplies, such as soil moisture important for agricultuteral production, and longer-term water resources such as grounwater supplies.
Drought impacts are usually first apparent in agriculture, but gradually move to other water-dependent sectors. Recovery time for water stored in surface and subsurface systems can be quite long under severe drought conditions. Risk of drought is still a major conceren in parts of I. R. of Iran, where precipitation amounts are low extremly variable. The SPI is a relatively new index, and it has not been widely applied or tested. In this research, we tested the SPI for different climatic regions and investigated its potential use as a tool for monitoring drought in I. R. of Iran. SPI values have been computed for 100 synoptic stations of IRIMO network for 3-,6-, 12-, and 24-month scales, covering 1960-2002.The results of the research shows that at the 3-month scale drought frequency increases but its duration decreases. On the other hand, as the time scale increases, the index responds more slowley. |
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