Aims Drought has become a main concern in Iran. The drought is posing a serious threat to life in Sistan and Baluchestan Province. This study aimed to evaluate the severity and extent of drought in Sistan and Baluchestan Province, Southeastern Iran. Materials & Methods Standardized precipitation index (SPI) and reconnaissance drought index (RDI) were used to determine the severity, duration, and extent of drought in Sistan and Baluchestan Province. Seven synoptic stations located across the province with data period of 2001– 2012 were used in monthly timescale. DrinC software was used to calculate SPI and RDI indices. Drought zoning maps were generated using ArcGIS software. Findings Iranshahr and Saravan stations showed the most severe drought in 2003– 2004 water year and Nikshahr station had the lowest drought severity in the same water year. The drought zoning maps indicated that the lowland areas and plains, especially in the southwestern part of the province and Iranshahr and Nikshahr stations, were vulnerable at the time of drought occurrence. Conclusion Both drought indices behaved similarly, but the RDI index was more sensitive in a specific condition due to using potential evapotranspiration of reference crop. it can be recommended that the RDI index can be used as an appropriate indicator for drought monitoring in Sistan and Baluchestan Province.

Introduction: Drought is a creeping natural phenomenon, which can occur in any region. Such phenomenonnot only affects the region subjected to drought, but its adverse effects can also be extended to other adjacentregions. This phenomenon mainly starts with water deficiency (say less than long- term mean of variable understudy such as rainfall, streamflow, groundwater level or soil moisture) and progress in time. This period can beended by increasing the rainfall and reaching the mean level. Even after the ending of a drought period, itsadverse effects can be continued for several months. Although, it is not possible (at least at this time) to preventthe occurrence of drought in a given region, it is not impossible to alleviate the drought consequences byscientific water management. Such a management should be employed before drought initiation as well asduring it and continue on even after the end of the drought period. The frequency of the main drought characteristics is a major concern of this study. The Northwest of Iran recently encountered severe and prolongeddroughts, such that a major portion of the Urmia Lake surface disappeared during the last drought in recentyears. In order to study drought characteristics, we used the Reconnaissance Drought Index (RDI). This index isbased on annual rainfall and potential reference crop evapotranspiration (abbreviated by PET here). This studyemployed the Monte Carlo simulation technique for synthetic data generation for analysis.Materials and Methods: The information from the 17 synoptic weather stations located in the North-west ofIran was used for drought analysis. Data was gathered from the Islamic Republic of Iran’s MeteorologicalOrganization (IRIMO). In the first stage of research, the ratio of long term mean annual precipitation toevapotranspiration was calculated for each of the stations. For this purpose, the Penman-Montheis (FAO 56) method was selected for PET estimation. In the second stage, the 64 candidate statistical distributions were fittedfor the mentioned RDI’s of each station. The best statistical distribution was selected among the 64 candidatedistributions. The best fitted distribution was identified by the chi-square criterion. The parameters of thedistribution were estimated by the Maximum Likelihood Estimation (MLE) scheme. Then 500 synthetic timeseries (each of them have the same number of observed data) were generated employing the parent populationparameters. The three main drought characteristics (namely duration, severity and magnitude) were obtained foreach of the mentioned artificial time series. The maximum values for each of the mentioned droughtcharacteristic were selected for each year. Then, a new time series having the 500 elements were obtained bycollecting the chosen values for each station. Once again the best distribution was selected for each series. Drought characteristics for different return periods (2, 10, 25, 50, 100 and 200 years) were estimated for eachstation.Results and Discussion: Preliminary results indicated that a negative trend existed in annual rainfall timeseries for almost all of the stations. On the other hand, the pattern of monthly PET histograms were more or lesssimilar for all of the selected stations. The peak of the PET was mainly observed in the hottest month of year, whereas the lowest value of the monthly PET belonged to the coldest month of year. The results showed that theamount of annual rainfall time series decreases sharply, after the year 1991. However, PET values significantlyincrease for all of the selected stations. After calculation of RDI values, the histogram of annual RDI’s wasplotted against the year. This is repeated for all of the selected stations. Figure.6 shows the mentioned diagramfor Tabriz station as an example. In the mentioned Figure, negative values of RDI (shown by red bars) indicatedthe drought years. A critical prolonged drought with a sixteen years duration period (neglecting the 2001 inwhich RDI value was a small positive value) was experienced in Tabriz. The maximum drought severity inTabriz was estimated to be about -7 in RDI units. Urmia station experienced the longest drought period, startingfrom 1995 and ending in 2005. It can be concluded that although few sparse wet years were observed in some of the selected stations in the studied period, they cannot compensate the water deficiency accumulated duringseveral consecutive years. The results showed that the lowest value of the ratio of drought severity in a 100 yearreturn period to the corresponding value for 2 year return period was about 2.13 (belonged to the Tabriz station), whereas the highest value was 3.17 (belonged to the Tekab station). On the other hand, the lowest value for theratio of drought duration in 100 year return period to its corresponding value for 2 year return period was 1.95 (experienced in the Makoo station). The highest mentioned ratio was 9.18 (observed in the Sardasht station). Thelowest and highest value of the ratio of drought magnitude in 100 year return period to its corresponding valuefor 2 year return period were 1.17 and 2.74, respectively. The mentioned drought magnitude ratios wereobserved in the Urmia and the Khalkhal stations, respectively. The isoplethes of the three main drought characteristics (severity, magnitude, duration) for a 10 year return period was illustrated for the study area (Northwest of Iran).Conclusion: In the present study RDI values were used to analyze drought characteristics of Northwest ofIran. The Penman-Montheis method was used to estimate PET (needed for RDI) values of the stations. The mainthree drought characteristics were calculated for each of the 500 synthetic time series. The results showed thatnearly all of the areas under study experienced severe and prolonged droughts. It can be concluded that a sharpdecrease in annual precipitation as well as the increase in PET (due to greenhouse effects of consuming fossilfuels as the main source of energy in the region) from 1995 to 2005 was observed in the study area. Scientificmanagement of available water in the study area is extremely vital to alleviate the adverse consequences ofdrought. Several economic and social problems were anticipated in these arid and semi-arid regions of Iran.

Drought is a natural disaster and repeatable that influencing all climates, but the frequency of its occurrence is more in arid and semi-arid regions such as Iran. Due to water scarcity and the non-structural damages caused by drought, a system of continuous surveillance on the phenomenon, drought monitoring, is inevitable. Drought indices are used for monitoring and decreasing the effects of drought. Among the indicators can be mention to Reconnaissance Drought Index (RDI). Therefore, due to dry weather conditions in South Khorasan province and its undeniable role in the production of saffron, the relationship between saffron yield and RDI was studied in four prone sites for production and cultivation of saffron. In this regard, RDI of three months in the period 1989-2014 (25 years) calculated in synoptic stations of Birjand, Qaen, Ferdows and Nehbandan and then comparing with the yield, regression equation between drought index of each month and yield was determined. Regression analysis and the getting results, in addition to confirming the existence of the relationship between the RDI of saffron yield confirmed this point that the drought index of February, March and April months in Birjand and Qaen; February in Ferdows and months of January, February, March in Nehbandan were effective on saffron yield. The extreme drought occurred in Birjand and Qaen in years 2000-2001, 25 years under study, and in Ferdows and Nehbandan in the years 1999-2000, so that moderate drought (normal) has the most continuity in all the studied stations and its occurrence is Imminent each three years once.

Several indicators have been proposed by researchers for the study of drought, each of which having certain strengths and drawbacks depending on the regional conditions and parameters affecting drought. In recent years, RDI has received greater attention as it accounts for precipitation and evapotranspiration, and is sometimes used as the basis for comparison of drought severity and the assessment of its potential damage at the national level. The present research examines RDI index along with benefits, drawbacks and problems of zoning and generalization of findings at the national level to provinces. For this purpose, the statistics derived from synoptic stations of Yazd province was utilized and after calculating RDI for each station, a comparison was drawn with the results presented in the national RDI map for the same time period. The results of this study provide technical points and considerations for calculating and zoning RDI at the national scale and generalizing findings to provincial levels, with special emphasis on Yazd province. It also offers relevant executive suggestions for improving the precision of drought zonation studies and methods across the country, which have been discussed in details.

Iran is located in arid and semi-arid regions in the world and therefore, drought monitoring has an important role in there. Drought assessment is conventionally based on drought indices such as Reconnaissance Drought Index (RDI). On the basis of definition, Reconnaissance Drought Index is estimated by fitting Log-normal distribution on the ratio of precipitation to potential evapotranspiration (calculated by Thornthwaite method). In this research, the effect of the change of distribution function and potential evapotranspiration calculation method was investigated in estimating Reconnaissance Drought Index in Iran. For this purpose, meteorological data of 30 synoptic stations were used in the period of 1960-2014. The potential evapotranspiration was calculated using the FAO Penman-Monteith as standard method. Beside standard method, 20 different methods were used to calculate evapotranspiration value. The superior evapotranspiration calculating method was obtained using Root Mean Square Error statistic. Then, the Reconnaissance Drought Index values were calculated using different evapotranspiration values and by fitting several probability distribution functions. On the basis of obtained results, the change of probability distribution function and evapotranspiration calculation method can affect the determining Reconnaissance Drought Index. Also, the comparison results of the calculated Reconnaissance Drought Index based on the fitting the superior probability distribution function on the ratio of precipitation to potential evapotranspiration which was calculated using superior and standard methods showed that the FAO Penman-Monteith method should be used in 90% of the studied stations to calculate Reconnaissance Drought Index in cases of data available.

Reconnaissance Drought Index (RDI) is based on fitting a Log-normal distribution to the ratio of precipitation to evapotranspiration (ETo) values in selected periods. In this index value of ETo was calculated based on mean temperature by Thorenth-Waite (Th) method. Th method may underestimated ETo values comparing to the actual in arid and semi- arid regions. The log-normal distribution maynot befitted to the ratio of precipitation to ETo values of some regions. In order to investigate the effects of these two limitations on drought situations' changes, meteorological parameters have been used during 50 years period at 8 Synoptic Stations of Iran. In the first step, the values of RDI (Th) for any stations during the mentioned time were calculated. Then, ETo values were calculated from best fitted empirical equation in any situation of lack of parameters. Subsequently RDI (select) index were established. The Kolmogorov–Smirnov (KS) test is used to assess the goodness of fitting most appropriate distribution function to the ratio of precipitation to ETo values. Then, according to equi-probability transformation the values of RDI (Th) were modified to *RDI (Th). The occurrence of different classes of drought according to RDI (select) and/or *RDI (Th) comparing to RDI (Th) showed the elimination of any mentioned limitations may leads to changing the amount of occurrence of any drought classes in RDI (Th). Hence, the RDI (Th) modified to *RDI (select) by estimating ETo values from selected method and applying appropriate distribution function to the ratio of precipitation to ETo values.

Drought is one of the most common natural events that has a great negative impact on agriculture and water resources. Using the indices is necessary in order to monitor, evaluate and statistically analyze drought in each area. Recently, a powerful drought index, the Reconnaissance Drought Index (RDI), is gaining wide acceptance mainly in the arid and semiarid climatic regions. Since RDI is based both on precipitation and potential evapotranspiration (PET), it is interesting to assess the effect of the PET calculation methods on the drought severity characterization obtained by the RDI. This paper compares the results of the RDI for various reference periods using some popular empirical PET methods with minimum data requirements. The selected methods are: Hargreaves-Samani, Thornthwaite, Blaney-Criddle, Jensen-Haise and FAO Penman-Monteith. The FAO Penman-Monteith method is used as reference method. The data used are from meteorological stations in thr Tehran Province representing different climatic conditions. No significant influence on RDI was detected by using the selected PET methods. However, the Blaney-Criddle method performed relatively better. This supports the conclusion that the RDI is a robust drought index not dependent upon the PET calculation methods.

Soil moisture is one of the most important factors that are affected by drought. Soil Moisture Deficit Index (SMDI) is one of the most important indicators that monitors drought based on soil moisture. But due to lack of direct measurement of soil moisture in weather stations and lack of information, it has been used to a limited extent. In this research, the meteorological data of Zarghan synoptic station was used during the statistical period of 30 years (1992 to 2022) and with the help of ET0 and AquaCrop models, the amount of soil moisture was estimated at the depths of 5, 15, 45 and 95 cm. Then the percentage of soil moisture deficiency and SMDI index were calculated to monitor drought. Also, the results of this index were compared and analyzed with two common indices SPI and RDI which are based on precipitation and evapotranspiration data.The highest value of this index occurred in 1996 after a period of heavy rain in 1994 and 1995, and the lowest value occurred in 2011 after the dry years of 2008 to 2010. Considering that the highest and lowest amount of rainfall is in 2004 and 2021, SPI and RDI drought indices have shown the most severe drought in the mentioned years. Severe droughts occurred in 2010, 2011, 2016 and 2021, and all three indicators had similar results. In examining the relationship between meteorological parameters and indices, the results showed that the SMDI index has the lowest coefficient of explanation with evapotranspiration (0.25), RDI index (0.4) and then SPI and precipitation index (0.45). Also, the results showed that there is a strong correlation between SPI and RDI. Regarding moderate and very severe droughts, where the amount of precipitation is greatly reduced and evapotranspiration increases, the RDI index is more accurate and shows the severity of the drought with higher accuracy, and its sensitivity to weather conditions is higher than the SPI index. Therefore, in arid and semi-arid regions where the severity of drought reaches the severe and very severe threshold, it is recommended to use indicators that take into account other factors in addition to precipitation.