Assessment of meteorological drought in Iran using standardized precipitation and evapotranspiration index (SPEI)

Drought is one of the main natural causes of damages to the agriculture, economy, and environment. After a long period without precipitation, drought usually occur. Determination of the start time, end, and extent of drought is very difficult. Quantitative determination of the severity, magnitude, and duration of drought is very difficult.  Several factors such as rainfall, temperature, evaporation, relative humidity affect the incidence, severity, and duration of droughts. The basic characteristic of drought, according to the available water resources, including ground water, surface water, snow pack, and water supply have been discussed by many scientists. In addition, some studies have examined the importance of temperature in determination of drought conditions systematically. Precipitation and temperature assessments in Palmer index show that this index has similar the same changes of precipitation and temperature parameters, and only small fluctuations of temperature can be controlled by precipitation. Thus, drought indices, which include temperature data in formulas (like Palmer Index), especially for applications of climate forecasts are appropriate. However, the necessity of several quantitative drought indices has not been considered in different hydrological systems and only different values of Palmer Index for drought types has been used. Thus, the drought is formulated according to three variables precipitation, temperature, and potentiality of evapotranspiration (PET) in a new index called the Standardized Precipitation and evapotranspiration (SPEI). SPEI merges the Palmer Index sensitivity with evapotranspiration (based on temperature fluctuation) using simple computation while considering multi-scale nature of the Standardized Precipitation Index (SPI). This index was first introduced in 2009 by Vicente Serrano et al. The present study is an attempt to investigate the use of SPEI in drought evaluation in Iran.
Total precipitation and average temperature data are considered for 104 synoptic stations across Iran. The meteorological data have been obtained from the Islamic Republic of Iran Meteorological Organization (IRIMO). The statistical periods are between 25 to 30 years (25 stations on a 25-year period and information about the rest of the stations is for a 30-year period). The interpolation and visualization of meteorological parameters and indices were performed using Arc Map 9.3 GIS software. In order to calculate SPEI, first, total precipitation is determined for the considered period (month, quarter, etc.) and year for each station. Then using the data and methodology the precipitation for each station was calculated using Thornthwaite method. Then potential evapotranspiration is deducted from the total precipitation for each station in considered time periods and years. With skewness calculation, the mean and standard deviations of the data set are determined. By assuming “n” is the number of precipitation data and” m” is the sequence number, the probability of the amounts of precipitation is calculated. Using the probability of precipitation and the inverse gamma function, the corresponding precipitation is determined. Next, using the probability of precipitation and the inverse normal function with mean and standard deviation, the corresponding precipitation is counted. Now, the reported station precipitation, gamma precipitation and normalized precipitation for each station are available. Thus, the probability density function of station precipitation and the corresponding cumulative function of the probability density function can be calculated and SPEI can be determined after normalization. Thus, drawing and analysis of the abnormal patterns of temperature, precipitation, and evapotranspiration for the long-term average and seasonal SPEI can be made. The results show that due to considerable decrease in temperature in winter, the effect of evapotranspiration may not be significant. During spring, summer and autumn the effect of evapotranspiration is influenced heavily on precipitation in most provinces, especially the southern provinces of Iran (including Hormozgan, Sistan- Baluchestan, Fars and Khuzestan) and drought has intensified (weaken) while precipitation abnormal has increased(decreased). Regarding the geographical situation of Iran (arid and semiarid), index of evapotranspiration, especially during the warm season in most parts of the country had an impact on the determination of droughts and hence, it will be better to consider in addition to precipitation, for assessing drought.