Iran, due to its geographical location, has low rainfall and is considered a dry land. As a result, different regions of the country grapple with DROUGHT. The presence of water management systems, such as aqueducts and reservoirs, in most parts of the country, along with a variety of methods for conserving water for irrigation, may contribute to this claim. Considering that DROUGHT is an inherent phenomenon in Iran's climate, people have invented and used numerous methods to combat it and store water. The aim of this study is to monitor and evaluate DROUGHT in Iran. In order to realize this goal, precipitation data from synoptic, rain gauge, and climatology stations were extracted over a 51-year period, from 1970 to 2020. The results obtained from examining DROUGHT occurrences in five ten-year periods reveal that, with the exception of the third decade (1991 to 2000), DROUGHT has prevailed in the majority of Iran's regions compared to other decades. On the other hand, in the recent decades leading to 2020, the intensity of DROUGHT occurrences, especially in the Middle Zagros, has intensified, which has consistently been among the regions with the highest rainfall in Iran after the Caspian region. This situation can cause concern in Iran, a country where its agricultural production hub is established along the Zagros mountain range. Moreover, the fluctuating behavior of Iran's DROUGHTs, with return periods of 2 to 5 years, has complicated the management strategies for these types of hazards. These conditions appear to have created numerous issues in many areas of Iran, particularly in the agricultural sector of the western provinces, due to the lack of conformity with these types of occurrences.

DROUGHT, as one of the most complex natural disasters, have adverse effects on human health, food security, economic activity, physical infrastructure, natural resources and the environment. So far, DROUGHT has not been taken into account as much as other natural disasters, as most of the natural disasters during the short period of time bring serious financial and life-damages to society, but DROUGHTs damages are heavier, long-lasting and gradual than other disasters. Iran as a wide country, due to its special location and topographical features, has different climates. Regarding the damages of DROUGHT in different sectors such as agriculture and economics of the country, its identification has special importance and requires more attention in political decisions. For quantification of DROUGHT, indices with different strengths and weaknesses were developed such as: Standardized Precipitation Index (SPI), Percentage of Normal Rainfall (PNPI), Anomaly Index Rainfall (RAI), and Palmer DROUGHT SEVERITY Index (PDSI). Coefficients and equations of some indices are related to conditions of origin area and their use in other areas may be accompanied by high errors. To obtain reliable results in other areas, these indices must be calibrated. Palmer index initially developed to measure the anomaly of soil moisture storage in the United States and is based on the equation of soil water balance. In general, the computational process of this index can be examined in three steps: Step 1, Like many indices, the humidity departure of the desired climate (d) is obtained in each period; Step 2, the moisture departure at different times and locations, that is determined by the coefficient of weight of the climatic characteristic (Ki), is compared, in other words in this step, the rate of departure of the moisture content (Zi) is determined; Step 3, to analyze the SEVERITY of the event and determine its beginning and end of it, the amount of moisture departure is converted into a graded form. The aim of this study is calibration of the Palmer DROUGHT SEVERITY Index coefficients in Iran. In this study for calibration of the Palmer Index, the monthly temperature and precipitation data related to the time period (1990-2002), area, latitude, available water capacity (TWC) and total soil moisture at the beginning of the period (September 1990); for 506 hydrological units were used and their weighted average were obtained for the 17 basins of country (as studied basins). Then the Palmer index was calibrated separately for wet and dry conditions. Calibrated coefficients of Palmer index were obtained (0. 892, 0. 002) and (0. 946, 0. 002) for wet and dry conditions in Iran, respectively, whereas coefficients of original Palmer index were (0. 897, 0. 333) only for dry conditions. The overall results of this study showed in most of the studied basins of the country, 2000 and 2001 were the driest and 1992 and 1993, were the wettest years (2002-1990). These results were consistent with the results of the analysis of precipitation conditions of these basins in those years. Also, the result shows that the calibration of the Palmer Index for each region is practicable and reliable. The results of the comparison of the dry and wetness SEVERITY, in both calibrated index and the primary index, showed that the primary index in most of the basins and years has estimated intensely much more than the real state in dry and wetness spells. Since this index can be used in the programs of water resources management, it is better to be calibrated and calculated based on the current status of the region, same as index calibration in this research which has resulted in good outputs. The results provide the basis for calibration and evaluation of Palmer DROUGHT Index in different climatic zones of Iran. By the results of this study, the effects of climate change on water resources and DROUGHT are possible using the calibrated Palmer index in Iran. It is suggested that instead of using of 17 hydrologic sub-basins for extraction of climatic characteristics and DROUGHT equations, 30 or more sub-basins be considered to analyze DROUGHT and wet and dry events with better accuracy. It is also suggested that, in addition to available water capacity, other outputs of hydrological models such as runoff, soil moisture content at all time intervals, and potential evapotranspiration be used and their outputs be compared with the water balance model used in the Palmer method, also is suggested the best model be selected to associate with the Palmer index.

The Palmer DROUGHT SEVERITY Index (PDSI), which uses hydrometeorological variables to solve a simple water balance equation in the soil and considers the DROUGHT or wet conditions as dynamic phenomena, is used for the assessment of DROUGHT conditions in many parts of the world. The main goal of this study was to assess the PDSI based on its original assumptions, its regionalized status, using the outputs of already calibrated and validated SWAT model in central regions of Iran. The PDSI was assessed through five methods: 1) original Palmer Index without calibration in which the climate coefficients and the SEVERITY equation were derived for Kansas and central Iowa; 2) original Palmer Index in which the coefficients of SEVERITY equations were adjusted; 3) the Palmer Index with the calibration of equations in central areas of Iran; 4) the Palmer Index using the soil moisture and potential evapotranspiration from SWAT model; and 5) the Palmer Index using the soil moisture, potential evapotranspiration and runoff from SWAT model. The evaluation was conducted for 17 major basins covering the entire country with a monthly time step for the period 1990-2002. Then, using all five methods, the SEVERITY of the DROUGHT for 160 sub basins located in central Iran was calculated and evaluated. The results of this study indicated that method 4 provides more acceptable results. Also, the results of this research showed these methods clearly demonstrated (1992) as the wettest year and (2001) as the driest year. The approach used in this study is applicable to regional calibration of Palmer Index and the outputs of other hydrological models.

IntroductionThe weather has an ongoing impact on human living and working environments. DROUGHT is a natural disaster that ranks first in frequency of occurrence, financial losses, and even human casualties among natural disasters that endanger humans and their environment. Due to its complexity and imperceptibility, this phenomenon—one of the primary and recurring features of various climates—has significantly impacted the human environment more than any other hazard. Its effects can also accumulate gradually over time and last for years afterward.DROUGHT cannot be avoided, but if its nature and characteristics are researched and understood, we may be able to forecast when it will occur, lessen its adverse effects through planning and preparation, and perhaps even control it. MethodologyThe political region of Iran is the subject of this study (Figure 1). Soil moisture and monthly precipitation are among the data used. The necessary data, which included 516 precipitation files and 516 soil moisture files with a spatial resolution of 0.5*0.66 in NC format, as well as DROUGHT and its characteristics calculated for all points, were acquired from the MERRA website for 43 years (1980–2022) to conduct this research. Iran's DROUGHTs were estimated over 3, 6, 9, 12, and 24 months using the MSDI. The characteristics of DROUGHTs, such as frequency, duration, SEVERITY, and magnitude, were computed, analyzed, and presented as a map in addition to examining the actual DROUGHTs.Results and discussionAnalyzing the DROUGHT characteristics for 1546 points across Iran revealed that the frequency and number of DROUGHT periods decrease as the time scale increases. In contrast, the values of other traits—such as continuity, intensity, and magnitude—also rise as the time scale increases. The increase in DROUGHT characteristics with increasing time scale, in terms of magnitude, duration, and SEVERITY, has been highlighted in studies by Nouri and Homai (2020) and Torabinejad et al. (2023). Geographically, the southeast and eastern parts of Iran have seen the highest frequency of DROUGHTs throughout the 43 years; in the southeastern part of the country, which is centered on the provinces of Sistan and Baluchestan, Kerman, and Hormozgan, there have been 28 to 34 periods of DROUGHT in 3 months. Conversely, the least amount of DROUGHT occurred in the northern coasts during the short-term periods of three and six months, with six to twelve periods, and in the coasts of the north and the southwestern region of the country during the long-term periods of twelve and twenty-four months, with two to six periods. The provinces of Sistan Baluchistan, Kerman, and Hormozgan in the southeast, as well as the far east of Khorasan Razavi and South provinces in the east of the country, experienced the worst DROUGHTs in terms of SEVERITY. Severe DROUGHTs lasting 12 or 24 months are concentrated in the southwest, in Bakhtiari and Chaharmahal, Kohgiluyeh and Boyer Ahmad, and part of Khuzestan.The findings from the computation and analysis of the duration and magnitude of DROUGHTs in Iran over the period under discussion indicated that, for two features, the country's northern regions, centered on the north coasts, mainly Mazandaran province, had more prolonged and severe DROUGHTs than other regions within the time scales of three and six months. By looking at the country's southwest from the perspective of location over periods of 12 and 24 months, the provinces of Fars, Bushehr, south of Kohgiluyeh and Boyer Ahmad, and Khuzestan province make up the central core of the most prolonged and most extensive DROUGHTs. ConclusionsBased on soil moisture and precipitation data, the results of a DROUGHT calculation in Iran demonstrated that DROUGHTs occur throughout the country with varying degrees of SEVERITY and weakness. Their occurrence has become a permanent feature of the country's climate, particularly in recent decades. The analysis of Iran's DROUGHT chronology revealed that, according to the MSDI index's range, most of the country's DROUGHTs are weak and moderate. It was also discovered that as time scales increase, the frequency of DROUGHTs decreases while the continuity, magnitude, and SEVERITY increase. Geographically, the country's southeast and east saw the highest frequency of DROUGHTs, while its northern coast—mainly the province of Mazandaran—and southwest—where the provinces of Chaharmahal & Bakhtiari and Kohgiluyeh & Boyer Ahmad—saw the lowest frequency. The maximum SEVERITY of DROUGHT in the country is -1.93 on the 3-month scale in the southeast and -2.2 on the 24-month scale in the southwest. Two of the study's notable findings are the continuity and high magnitude of the DROUGHTs that struck the country's north and southwest. Thus, the most extensive DROUGHTs, with values between -16 and -31 on short-term scales and between -35 and -68 on 12- and 24-month scales, have happened in the country's north and southwest, respectively. These results suggest an overall decrease in soil moisture and precipitation in these regions.

DROUGHT is a meteorological event the adverse effects of which can be observed not only in arid and Semi arid, but in humid regions as well. To reduce these effects, DROUGHT should be studied in a scientific manner, and in correct frameworks. As DROUGHT can also be considered as a hydrologic event, we can apply hydrologic methods to analyze it. Markov chain is one of the most important and classic stochastic models having many applications in hydrology as well as in meteorology. To prepare a map of DROUGHT probability occurrence, the probabilities of different conditions derived from Gibbs and Maher procedure for 22 stations in Isfahan province were considered as Markov chain and the return period for each condition was calculated for each station. Applying ordinary Krigging, the map of DROUGHT return period was drawn for Isfahan province.

In this study, we used monthly Palmer SEVERITY DROUGHT Index (PDSI) data of National center of oceanic atmospheric administration (NOAA) during 1/1950 to 12/2005 (56 year). The time and space resolution of data is monthly and 2.5*2.5 degree respectively. Therefore 26 pixels located in Iran political boundary. A matrix 26*672 carried out that pixels located on rows and month located on column. The cluster analysis by Ward method showed that Iran classified to four regions. The most DROUGHT SEVERITY occurred during year 2000 in northwestern of Iran. It seems that location of regions related to the atmospheric passing route on Iran and their effective radius. There is a significant correlation between teleconnection patterns and DROUGHT in Iran. The effect of teleconnection patterns is different for regions. In total, in autumn there is a highest correlation between teleconnection patterns and DROUGHT.

The present DROUGHT is a phenomenon that can occur in any climate, hence, due to its creeping and mysterious nature, economic losses, social effects as well as crises in agricultural, natural resources and ecosystems, its study is of great importance. Therefore, in this study, by using 9 DROUGHT indices including SPEI, SIAP, DI, SPI, PN, MCZI, CZI, RDI and ZSI, the DROUGHT was analyzed using 40 meteorological and synoptic stations in Fars Province, Iran during the last half century.  In order to select the best DROUGHT index, three methods including minimum amount of precipitation, normal distribution, and correlation were used. Also, the SEVERITY, duration and frequency of DROUGHTs and their return period were determined using Run Theory (RT) method and SDF curves.Finally, after determining the best index, the DROUGHT events of the region were interpolated using ArcGIS techniques along with the simple and conventional kriging methods with spherical, exponential, and Gaussian models as well as the inverse weighted distance (IDW) method. In order to determine the most appropriate interpolation method, Cross-Validation method and MAE and MBE indices were used. The results showed that the SPI index performed as the best indicator to describe the DROUGHT. The results of RT method and SDF curves showed that by increasing time scale and return period, DROUGHT continuity and magnitude increase and as DROUGHT persisted, the SEVERITY of DROUGHT not increase at a constant rate. According to the results, the most severe and widespread DROUGHTs in the province occurred in 1970, 1993, 1999, 2007, 2014 and 2016. Also, Gaussian conventional Kriging method was the best method of DROUGHT interpolation in the study area due to its lower error rate. Therefore, by spatial monitoring and distribution of DROUGHTs, necessary measures can be taken to better deal with and manage water and natural resources.

DROUGHT is one of the most important natural hazards that affects many countries all over the world and causes socio-economic, political and cultural problems. Awareness of and ability to forecast DROUGHT may help to reduce its consequences. In this research some geostatistical methods including simple kriging (SK), ordinary kriging, inverse distance weighing (IDW) of 1 to 5 degrees for DROUGHT SEVERITY mapping in the Province of Fars in 2007 using the percentage of normal index. Comparison of results based on RMSE and MAE showed that Using the SK method resulted in the lowest values (RMSE=11.44 and MAE=6.94), the refores it was selected as the most appropriate method for DROUGHT SEVERITY mapping in the mentioned year. Based on the prepared map, the province of Fars suffered showed two classes of DROUGHT, severe and very severe, DROUGHT which covered 21.58 and 78.42, respectively, of the Province area.

Groundwater is important water resource supply, especially in arid and semi-arid regions. Increased utilization of the ground water aquifer leads to significant reduction in the storage of reservoirs. This study evaluates the hydrogeological DROUGHT in Garmsar plain using Groundwater Resource Index (GRI). First, we used 17 piezometric wells data over 2001-2011 statistical period to calculate GRI in the beginning, middle and end of the period. So, we used different interpolation method including geostatiscal method ordinary kriging (OK), simple kriging (SK) and deterministic methods including inverse distance weighting (IDW) to prepare the maps over three periods. The mean absolute error (MAE) and root mean square error (RMSE) indices were used to evaluate the accuracy of simple kriging, ordinary kriging and IDW classification based on the DROUGHT maps. The results showed that the values of MAE and RMSE criteria for simple Kriging is better than the other methods and indicates the suitability of this method for zoning GRI. According to the results, the most severe hydrogeological DROUGHT in Garmsar plain was at the end of 2011, that 91. 16 % of the study area was suffered from severe DROUGHT. SPI was used for considering the effects of meteorological DROUGHT in the time scale of 3, 6, 9, 12, 24 and 48 months on groundwater. The correlation between SPI and GRI showed long-term timescale of 48 monthly has the greatest correlation with groundwater level.