Investigation of temporal and spatial variations of meteorological drought in Lorestan province

Drought is a deficiency in precipitation over an extended period. It is a part of normal climate variability in many climate zones. The duration of droughts varies widely. Drought can develop quickly and last only for a matter of weeks, exacerbated by extreme heat and/or wind, but more commonly drought can persist for months or years. Decreased rainfall and severe fluctuations in rainfall are factors that have exacerbated the water crisis in recent years, as well as numerous droughts. Drought is one of the most important natural disasters that is called lack of rainfall in a long period Drought is a natural phenomenon that has a significant impact on Human life exists and its temporal and spatial variations can be effective in dealing with the effects of this phenomenon. The purpose of this study was to investigate the drought process using SPI and Mann-Kendal test as well as meteorological drought spatial variations in Lorestan province using GIS.

Drought is often grouped into four basic types: 1) meteorological or climatological, 2) agricultural, 3) hydrological, and 4) socioeconomic. Meteorological and climatological drought is defined in terms of the magnitude of a precipitation shortfall and the duration of this shortfall event. Lorestan province with an area of 28064 km in western Iran is located between 46 degrees and 51 minutes to 50 degrees and 3 minutes’ east longitude of the Greenwich meridian and 32 degree and 37 minutes and 34 degrees and 22 minutes north latitude of the equator.The Standardized Precipitation Index (SPI) was developed to determine drought and wetness at each station, and the values obtained (SPI) indicate different intensities of drought in an area. The Standard Precipitation Index (SPI) is a relatively new drought index based only on precipitation. It's an index based on the probability of precipitation for any time scale. Some processes are rapidly affected by atmospheric behavior, such as dry land agriculture, and the relevant time scale is a month or two.

Using SPI index, rainfall data of 9 synoptic stations and 23 rain gauging stations in the statistical period (1998-2018) were analyzed at the provincial level. The Mann-Kendal test was used to examine the trend of annual changes and the appropriate interpolation method was used to prepare zoning maps using the information of all 32 stations. The Mann-Kendal test is used to check the time trend for each set of data. This test is based on non-parametric linear regression logic. In this study, for interpolation, the methods of inverse distance weighting, Global polynomials, local polynomials, radial basis functions and kriging were investigated and the most accurate method was selected.Mann-Kendal test results showed that the trend of drought index changes of Alashtar at the 95% probability level was significant and increasing and the rest of cities lacked Are meaningful. The results indicate that according to the value classification scale (SPI), Aligudarz city was very dry in 1999 and 2008, with moderate humidity in 2017 and moderate drought in 2000 and in the other years has been in normal condition. The most suitable method was Kriging method for zoning drought in the province. For this purpose, the years 2012, 2016 and 2018 were designated as representative of the dry, wet and normal years, respectively. According to the zoning maps mentioned in three years, in the year 2012 districts west and south have been subjected to severe drought, in the northern part of the province in the wet period in the year 2016, and in the eastern part of the province in the year 2018 have been most exposed to drought. Also, considering the number of droughts in different years for the selected stations, the driest station, Kohdasht station and the humidest station, Nourabad station was introduced.

Based on result of study, the driest station can be introduced as Koohdasht and the wettest station can be introduced as Noorabad. According to the results of this study and the occurrence of various droughts in different parts of the province, to deal with the effects of this phenomenon, proper planning of water resources and insurance of agricultural products, especially in the city of Koohdasht, which more than other parts of the province with Drought has been suggested.