تحلیل فراوانی منطقه ای شدت، مدت و گستره خشکسالی هواشناسی استان لرستان

Introduction
Drought, which is one of the climatic variabilities, is repeated with different intervals in areas such as the western and southwestern regions of Iran (Lorestan province), which has a Mediterranean climate. Some of the droughts operate in wide areas and some in limited areas. Intensity, continuity, extent and frequency are among the features of drought which are determined by relying on drought indicators. In this research, in order to investigate the intensity, duration, frequency (return period) and the extent of drought in Lorestan province, 28 stations in the region that had the longest common statistical period (1990-2018) were used. For this purpose, SPI (standard precipitation) index was used in four-time scales of 1, 3, 6 and 12 months.

Methodology
After determine of the length of the statistical period, homogeneity and quality tests such as Run test, double mass curve and Mann-Whitney non-parametric test were performed on the data and corrections were made if necessary.
In order to analyze the temporal changes of drought, the SPI time series graph was drawn in different time scales (1, 3, 6, 9, 12 months) at different stations. According to these graphs, many drought events have occurred in almost all stations and the only difference between them is the beginning and end of the drought, the duration and severity of the drought.
In order to investigate the characteristics of droughts, all the drought events should be investigated in each station. In this study, the threshold level equal to -0.5 has been chosen. The results of the investigation of different drought events show that the studied stations have experienced drought events from at least 11 (Aligudarz station) to at most 19 (Lorestan station). The average severity of these droughts is -0.13 to -0.63 and its maximum severity is from -0.2 to -2.7.
Analysis the spatial distribution maps of drought in the region shows that the studied region has experienced drought in 1997, 1999, 2000, 2008, 2009 and 2011 to 2015, which includes moderate to severe conditions.
In this research, using the method of frequency analysis, the return period of droughts with a certain duration was calculated for each station and region and Severity-Duration-Frequency curves were obtained by region.
Then the Severity-Area-Frequency regional curves were calculated and drawn for the region. These curves show that droughts in the region are generally widespread and affect a wide area of the region.

Conclusion
In general, the investigations carried out in the region indicate that:
The analysis of SPI shows that many drought events have occurred in all of stations and the only difference between them is the time of beginning and end of drought, the duration and intensity of drought. The droughts of 1985, 1999, 2000, 2008 to 2009, 2011 to 2014 and 2018 are among the events that can be identified in most of the stations.
The study of SPI shows that most of droughts in all stations are droughts with 1 to 3 months' duration; duration of 4 to 7 months also are significant in study area. survey the characteristics of drought events shows that the studied stations have experienced from minimum 2 to maximum 19 drought events. The average intensity of these droughts is -0.13 to -1.41 and its maximum intensity is from -0.2 to -2.7.
SAF curves show that droughts with short return periods affect a small area of the region due to their high intensity, while droughts with longer return periods will cover a larger area of the region.
The spatial distribution of the probability of drought in the region (extracted from the SPI index) shows that the probability of drought in the region is between 12 and 17 percent. Most of the region has a probability of drought between 14 and 15.5%.
The results of the investigation of different drought events show that the studied stations have experienced drought events from at least 11 (Aligudarz station) to at most 19 (Lorestan station). The average intensity of these droughts is -0.13 to -0.63 and its maximum intensity is from -0.2 to -2.7.

Methodology
After determine of the length of the statistical period, homogeneity and quality tests such as Run test, double mass curve and Mann-Whitney non-parametric test were performed on the data and corrections were made if necessary.
In order to analyze the temporal changes of drought, the SPI time series graph was drawn in different time scales (1, 3, 6, 9, 12 months) at different stations. According to these graphs, many drought events have occurred in almost all stations and the only difference between them is the beginning and end of the drought, the duration and severity of the drought.
In order to investigate the characteristics of droughts, all the drought events should be investigated in each station. In this study, the threshold level equal to -0.5 has been chosen. The results of the investigation of different drought events show that the studied stations have experienced drought events from at least 11 (Aligudarz station) to at most 19 (Lorestan station). The average severity of these droughts is -0.13 to -0.63 and its maximum severity is from -0.2 to -2.7.
Analysis the spatial distribution maps of drought in the region shows that the studied region has experienced drought in 1997, 1999, 2000, 2008, 2009 and 2011 to 2015, which includes moderate to severe conditions.
In this research, using the method of frequency analysis, the return period of droughts with a certain duration was calculated for each station and region and Severity-Duration-Frequency curves were obtained by region.
Then the Severity-Area-Frequency regional curves were calculated and drawn for the region. These curves show that droughts in the region are generally widespread and affect a wide area of the region.

Conclusion
In general, the investigations carried out in the region indicate that:
The analysis of SPI shows that many drought events have occurred in all of stations and the only difference between them is the time of beginning and end of drought, the duration and intensity of drought. The droughts of 1985, 1999, 2000, 2008 to 2009, 2011 to 2014 and 2018 are among the events that can be identified in most of the stations.