Evaluation of Drought Temporal Changes using Mann-Kendall test and Sen’s Slope in Kerman Province for the Period 1990 to 2018

Introduction:
Due to locating most of Iran in arid and semi-arid climates and the consequences of drought, assessing the trend of drought change is very important. An overview of past researches indicates the occurrence of severe and long-term droughts in recent years, which highlights the effects of drought in Iran and other parts of the world. Climate change due to global warming, severe and prolonged droughts and consequently water scarcity is one of the major challenges, especially in arid and semi-arid regions. Therefore, it is necessary to study the trend of drought change as one of the consequences of climate change, which leads to severe limitation of water resources, so that in the presence of increasing drought changes, necessary measures will be taken to manage water resources. Therefore, in this study, the trend of meteorological drought changes in Kerman province is evaluated by SPEI index. Kerman province, like other arid and semi-arid regions of IRAN, is no exception to the phenomenon of drought and so far no study has been conducted to study the fluctuations of meteorological drought by existing indicators (SPI, SPEI, RDI, PDSI) in the whole province.
Materials and methods
The study area of this paper is Kerman province which located in the southeast of Iran. The climatology data, including mean monthly precipitation and mean monthly temperature, of 7 synoptic stations were used to conduct this study for the period of 1990-2018. These data were obtained from Meteorological Organisation of Kerman.
To conduct this study, first, Standardized Precipitation Evapotranspiration Index (SPEI) was calculated to extract drought periods in three time scales i.e., 3, 9 and 12 month. SPEI estimates dry and wet periods based on precipitation and temperature data. In order to evaluate the trend of time series changes, the first step is to examine the existence of autocorrelation between time series. For this purpose, the Pre-Whitening method (Von Storch, 1999) was used to determine the presence or absence of autocorrelation. After confirming the absence of autocorrelation between data, Mann-Kendall test was applied to SPEI values. In the next step, the Sen’s slope test for the time series is calculated and the significance of the slope at different levels of confidence is obtained. In other words, the assessment of upward and downward trend of SPEI series was evaluated using two both Mann-Kendall and Sen' Slope trend tests
Results and discussion
Considering the impact of climate change on precipitation and temperature in Iran and also the significant impact of precipitation and its fluctuations on agricultural production, it is important to assess the fluctuations of phenomena such as drought that are directly affected by precipitation and temperature. Therefore, in this study, drought changes were assessed by SPEI3, SPEI9 and SPEI12 indices in Kerman province as one of the regions with arid and semi-arid climate. The results of SPEI time series analysis showed that the highest number of drought events occurred in Shahrbabak station with a frequency of 80% during the statistical period 1990 to 2018. Also, the most severe drought occurred at this station in 2016, according to the annual SPEI average. The average annual SPEI values ​​showed that the study area experienced the most severe drought events between 1998 and 2010. The results of evaluating the monthly changes of drought by Mann-Kendall and Sen’s slope method showed that in almost all months of the year except May, an increasing and decreasing trend is observed, so that in January (increasing and decreasing trend) and October (increasing trend) there are the most significant drought changes. Seasonal changes of drought showed that drought changes in spring, autumn and winter have an upward slope that in some stations this increase is significant at the level of 99 and 95%, while the summer season during the study period was faced with increasing and decreasing changes. The increasing trend of drought in spring, summer and autumn has been reported by Malekinejad et al. (2012) in Tehran province. In general, the evaluation of annual changes in drought in the study stations showed that the whole area during the statistical period (1990-2018) had an upward trend. In other words, the severity of drought is increasing in Kerman province, which is one of the main reasons for climate change and consequently increase in temperature (Mirakbari and Ebrahimi, 2021; Mesbahzadeh et al., 2020). Also, the increasing trend of drought in Iran has been reported by several researchers, including Ghorbani et al. (2020), Mozaffari et al. (2021), Amani et al. (2021). The study of changes in drought periods showed that Kerman province has experienced three periods with different trend slopes during the statistical period under study. The first period (1996-1990) has a decreasing slope, the second period (1997-2010) and the third period (2018-2011) have an increasing slope, which is relatively consistent with changes in precipitation and average temperature throughout Kerman province. The results of this part of the research are consistent with the results of Ebrahimi et al. (2021), which reported the existence of three periods of drought changes with different trend slopes based on Domarten drought index in the whole country during the statistical period of 2011-2018. In general, the results of this study showed that drought in Kerman province is increasing, which requires more attention of experts and planners of water resources for proper management of water shortages caused by severe droughts.