Drought Frequency Analysis and Zoning of Iran Using the Standard Precipitation Index and the Geographical Information System

The standard precipitation index (SPI) was used to analyze 32 years (1974-2006) ofprecipitation data collected at 119 rain gauging stations throughout the I.R. of Iran todetermine drought frequencies in different ecological zones of that country. The resultsrevealed that the droughts at the shorter time intervals were more frequent than those with the longer time intervals. The study also showed that drought occurrence is a climatic property happening at certain time intervals. The assessments indicated that drought has been more frequent, yet of less intensity, during the recent years. Furthermore, the frequency of these droughts was higher in the central, southern, southeastern, and eastern parts of the country.Drought zoning was achieved using the kriging method with spherical, exponential, Gaussian, and power variograms. The results showed that the spherical and power variograms were the best predictors of 12- and 3-month frequencies, respectively. The values of root mean square error (RMSE), mean absolute error (MAE), and medium error (MDE) were 0.857, 0.672 and 0.31 mm, respectively, for a 12-month period; they were 0.941, 0.714 and -0.002 mm for a 3- month period. The zoning analysis revealed that the central, southern and some regions of eastern Iran were more susceptible to droughts than the other region of the country. It further indicated that on an annual basis, most regions experience droughts of low intensity. Severe and very severe droughts were not detected in our analysis. Overall, 61% of the area of Iran suffered from droughts of different severities.