Showing 21 results for Trend
M. Sargolzaei, M. A. Edriss,
Volume 8, Issue 1 (4-2004)
Abstract
In this study, 14322 growth trait records of 2387 sheep bred and reared in the Rearing and Breeding Station of Bakhtiari Sheep during 1989-1997 were used to estimate the phenotypic, genetic, and environmental trends for some of the growth traits using Animal Model for lambs, rams, and ewes. The genetic trend of birth weight, weaning weight, six month's weight, daily gain from birth to weaning, daily gain from weaning to six month and daily gain from birth to six month were 12.2 (+2.9), 19.6 (+5.5), 28.7 (+8.7), .15(+0.04), 0.06 (+0.05) and 0.014 (+0.04) gr. for ewes as an indicator of genetic level of the herd respectively. Also, average of the genetic change rate per year for rams and lambs were calculated. Overall, the maximum progress rate was for six-month body weight (28.7 gr. per year) and the next was weaning weight (19.6 gr. per year). The results for lambs and rams also showed that these traits had the same rank for the genetic progress. The progress could be due to direct selection for these traits in the selection index criterion although there were negative phenotypic and environmental trends during the study years. Negative phenotypic and environmental trends could be due to bad environmental conditions, especially to nutrition of the sheep in an unsuitable (drought) climate during the study years. .
H. Shirgholami, B. Ghahraman,
Volume 9, Issue 1 (4-2005)
Abstract
A number of researches have indicated a gradual increase in mean temperature throughout the world. Yet, there are some reports on the reduction of annual mean temperature. In this research we investigated a long-term trend of annual mean temperature in 34 synoptic stations in Iran (2 stations in cold and humid climates 14 in humid and moderate climates, 11 in Steppe climate and 7 in desert climate - following Kopen climatic division) with a minimum statistical record of 30 years by applying the minimum square-error and Man-Kendall methods (Wald-Wolfowitz method had a different result).
The results confirmed a positive trend in 59% stations, while 41 % of the stations were negative for the whole time-horizon of data. Considering the significance level, 3 zones of positive, negative, and no trends for annual mean temperature were detected in Iran. However, it was hard to define a specific spatial theme for such a division. By taking another approach, we proceeded with a shared statistical time period of 1968-1998 for all stations. In this case, 68% of the stations showed a positive trend, while the remaining 32% was negative. There were some shifts in direction from one trend to another in some of the stations in the study, yet no well-defined spatial structure was reported. In contrast, at 5% level of significance, 44, 15 and 41 percentages of the stations demonstrated positive, negative, and no trend for annual mean temperature, respectively. In general the behavior was different for different climates and no specific pattern was found. So, despite the fact that some stations did not show significant trends, one may hypothesize that more regions in future will experience higher temperature values and their positive trends would be a clue for future warming.
M. Vatankhah, M. A. Talebi, M. A. Edris,
Volume 11, Issue 41 (10-2007)
Abstract
In this study 5025 records from the Lori-Bakhtiari sheep stud were used to predict phenotypic, genetic and environmental change in ewe traits from 1989 to 2004. Best linear unbiased prediction (BLUP) of breeding values were estimated by Drivative Free Restricted Maximum Likelihood (DFREML) procedure using single and multi-trait animal model. Phenotypic, genetic and environmental trends were calculated by regressing of the average phenotypic values, predicted breeding values and environmental values in the year of ewe birth respectively. The estimated phenotypic trends were –0.1223 kg for ewe body weight, -0.0415 kg for greasy fleece weight, 0.6639% for conception rate, 0.0003 for number of lambs born per ewe lambing, 0.0094 for number of lambs weaned per ewe lambing, 0.0380 kg for total birth weight per ewe exposed and 0.4227 kg for total weaning weight per ewe exposed. The estimated genetic trends were 0.0603 kg, -0.0004 kg, 0.0183%, -0.0012, -0.0007, 0.0030 kg and 0.0211 kg from single trait analysis and 0.0549 kg, -0.0006 kg, 0.0089%, -0.0008, -0.0008, 0.0030 kg and 0.0230 kg respectively from multi-trait analysis. The estimated phenotypic and environmental trends were significant but genetic trends were not significant (P<0.05) for often traits.
F. Amiri, J. Khajeddin, K. Mokhtari ,
Volume 12, Issue 44 (7-2008)
Abstract
Ordination is the part of statistical ecology which has developed and integrated in recent years. Finding environmental factors which important in ecological structure determination of plant species is the final purpose of ordination. Ordination method was used for finding the effect of important variables on Bromus tomentellus species quantitative and qualitative changes in Esfahan's Fereidan region. For this purpose, fifteen sites were studied. Density, Cover percentage and Soil factors containing EC, pH, % caco3 , % gravel and stone, % clay, % silt, % sand, Na+, Ca2+, K+, Mg2+, Cl-, SAR, C/N and % OC. In A (0-30 cm) and B (30-60 cm) horizons over a line transaction in each site, were measured. The matrix of environmental and species characteristics were prepared. The relationship between environmental and species characteristics were determined using of PC-ORD and CANOCO software's and Redundancy Detrended Analysis (RDA) method. The results show that there are meaningful correlation between Density and Cover percentage with soil factors. The results implicate that C/N factor in A horizon and SAR in B horizon have the most effect on Bromus tomentellus density and cover percentage. EC, pH and …. Factors don't have much effect on Bromus tomentellus species characteristics.
R Sabohi, S Soltani,
Volume 12, Issue 46 (1-2009)
Abstract
Climate change has important effects on earth environment and human life. Therefor, investigation and study of climate change is very essential. This study investigated rainfall, temperature, relative humidity and wind variability by analyzing data for annual and monthly climatic factors collected at 13 synoptic stations (industrial cities of Iran) by using Mann-Kendall test. The results of monthly rainfall trends showed that most of synoptic stations have significant positive and negative trends in winter and spring months. About 23% and 1.7% of stations have significant negative and positive trends, respectively, in annual trend of this factor. The results of monthly number of rainy days showed the major number of significant trends occurs in spring. In autumn (September, October and November) like as summer most of the stations have no significant trends. Analyzing the annual number of rainy days trends also showed that 4 stations have significant positive trends and 2 stations negative trends. Trend of greatest daily precipitation is low throughout the year, so there is not any significant trend in winter. Annual investigations confirm the seasonal investigations. The major number of significant trends in monthly mean maximum temperature occurs in summer but there are not any significant trends in winter and March. The trend of mean minimum temperature is approximately high in all of the seasons and the major number of significant trends occurs in summer and autumn and then in spring and ultimately in winter. In annual investigation, most of the stations showed positive trends and only Oroomieh station has negative trends. Trend of mean temperature is high except for winter. Most of the stations showed positive trend, indicating increasing trends in this factor. Annual studies vertify the positive trends and about 63% of stations have significant positive trends.
S. Dodangeh, S. Soltani, A. Sarhadi,
Volume 15, Issue 58 (3-2012)
Abstract
This study performs trend analysis of hydroclimatic varibles and their possible effects on the water resources variability. Nonparametric Mann-Kendall and spearman tests were used to investigate trend analysis of mean annual and 24-hr maximum rainfall, flood and low flow parameters of 23 hydrometery and 18 synoptic stations in Sefid-Roud basin. The results showed that mean annual and 24-hr rainfall parameters are decreasing in few stations while most of stations representing negative trend for low flow and flood time series. Applying Sequential Mann-Kendall test revelad that this negative trend is started from 1965 to 1970 for rainfall parameters and from 1970 to1980 for flow (low flow and flood) parameters. Results show that climate change has probability affected variability of climatic variables, while changing of land use may have aslo affeteced extreme flow trends during recent decads. Therefor it can be noted that combination of climate chanege effects and human activities on water recources have affected the negative trend of hydroclimatics variables.
A. Khanamani, E. Dodangeh, F. Soleymani , H. Karimzadeh, S. Soltani,
Volume 18, Issue 67 (6-2014)
Abstract
Underground water resources considered as a major source of fresh water. Increasing need to water in Iran, causing to
immensely utilization and ground water balance disorder, so that state of ground water in many of area is negative.The
purpose of this study is to investigate the trend of changes in some of the characteristics of groundwater during the
period 1374 to 1387 is Segzi plain. For this purpose, data gathered from the Organization of regional water and
homogenous test with Tom test (Run-test) at 95% confidence level was performed on the data. The independence of
data evaluated by time series auto correlated functions (ACF), to do this, the amount of auto correlated data computed
in different time delays and finally Mann- Kendall test used to evaluate the trend of time series properties in
groundwater. The results of Run-Test showed that all of used series in this study were homogenous (P value< 0.05).
The result of trend analysis test for region’s wells showed a significant increase in chlorine in underground water
resources (P value< 0.05). Calcium has an increasing Trend too about 3 units. Results also showed that all used series in
this study are random and Mann- Kendall trend analysis test can be an appropriate for trend evaluating in data series. As
regard to irregular utilization of underground water resources by increasing depth of water level, amount of different
salts such as chlorine and sodium increased, that causing to surface source degradation like soil and plant cover in
agricultural area.
M. Khoshravesh, J. Abedi-Koupai, E. Nikzad-Tehrani,
Volume 19, Issue 74 (1-2016)
Abstract
During the past few decades, the southern part of the Caspian Sea has more frequently experienced extreme climatic events such as drought and flood. Trend analysis of hydro-climatic variables was conducted using non-parametric Mann-Kendall test and regression test for Neka basin in the north of Iran. Trends of precipitation and stream flow characteristics including maximum flow, mean flow and low flow indices were analyzed at the annual, seasonal and monthly time scales from 1358 to 1391 (34 years). Results showed a general decrease in annual and winter precipitation and decrease in daily maximum precipitation, with an increased trend in daily maximum precipitation of spring season. A decreasing trend was observed in 7-day low flow in summer for all sub-basins. Annual and monthly mean flows specifically in winter in all sub-basins decreased, but annual maximum flow increased from upstream to downstream. Land use changes showed that deforestation and urbanization increased during 34 years in the mid and downstream sub-basins. The analysis showed that low flow indices and mean flows are strictly sensitive to climate change. Overall, from hydrological perspective, these results indicate that the study region is getting dryer and facing more severe drought events. The results of this study can predict future droughts to make better decisions for irrigation planning and management of water resources.
S. H. Sadeghi, A. Allbuali, R. Ghazavi,
Volume 20, Issue 76 (8-2016)
Abstract
Nowadays, the increasing population and water demand in various sectors of agriculture, industry, drinking and sanitation has brought about tremendous pressure on groundwater resources. Changes in groundwater quality and salinity of the water resources are currently major threats to development, especially in the dry and too dry lands. The aim of this study is evaluation of the trend of changes in groundwater quality, both temporally and spatially, in Kashan plain over a period of 12 years (2002-2013) using geostatistical methods and classification methods namely Shouler and Wilcox. Thereby, Export Choice has been used and each parameter has been weighted according to its effect on water quality changes. Then, the weighted average of water quality parameters was used for zoning the drinking and agriculture water. The results showed that among the geostatistical methods, circular Kriging based on the correlation coefficient has more acceptable performance. Moreover, the results of spatial and temporal changes in water quality based on Shouler and Wilcox indicate a decrease of drink and agriculture water quality in the study area. Besides, 1.75 km2 of high quality drinkable water was annually decreased between 2002 and 2013 and replaced with moderate or poor quality water. Also, the same but more remarkable decline happened in agriculture water so that 11.06 km2 of high quality agriculture water annually diminished from 2002 to 2009 and plunged zero by 2009.
N. Zohrabi, A. Massah Bavani, E. Goudarzi, M. Heidarnejad,
Volume 20, Issue 77 (11-2016)
Abstract
Since climate change is regarded as a serious threat to different parts of life cycle, separation of factors intensifying this phenomenon seems necessary. This research has investigated the temperature and precipitation trend using the multiple trend test in the upstream Karkheh basin located in west of Iran. For this purpose, two-dimensional graphs of temperature and precipitation anomalies of the CGCM3 Model (1000-year data) were drown for the study area. Then, the attribution of changes in climate variables due to climate internal fluctuations or greenhouse gases affected by human factors were investigated. Based on the findings of this study, in different parts of the study area, the range of natural climate variables for temperature and precipitation changes (95% probability) in the west of the study area are
± 1.4ºC and ±76%, respectively.
The results showed increase and decrease in temperature and precipitation in most of the studied stations, respectively. The variables of temperature and precipitation are affected by climate change and as we approach latest years, especially in the western and central parts of the study area, the impact of greenhouse gases in increasing temperature and reducing precipitation becomes more evident. According to the current results it can be concluded that changes in land use in Iran caused by human interventions can be introduced as a significant factor for the ascending trend of temperature. However, it can be noted that the most important factors of the increased greenhouse gases in recent years are human activities such as land use changes. These changes certainly have affected water resources in the study area.
M. Nouri, M. Homaee, M. Bannayan,
Volume 22, Issue 1 (6-2018)
Abstract
In this study, the trends of changes of the standardized precipitation index in a 12-month timescale (SPI-12) and seasonal and annual precipitation were investigated in 21 humid and semi-arid stations of Iran during the 1976-2014 time period. After removing the serial correlation of some series, the trend of precipitation and SPI-12 was detected using the Mann-Kendall nonparametric trend test. The results revealed that the trends of annual precipitation had been declining in all stations over the past 39 years. The seasonal precipitation trend in winter, spring, autumn and summer was downward in approximately 90, 95, 47 and 37% of the studied stations, respectively. In addition, the descending trend of wintertime precipitation was significant in Sanandaj, Khoy, Urmia, Hamedan, Mashhad, Torbat-e-heydarieh, Nozheh and Qazvin. Also, the temporal trend of SPI-12 was decreasing in all surveyed stations except Shahrekord. Furthermore, SPI-12 showed a significant downward trend only in Sanandaj and Fasa. Moreover, the most severe meteorological drought occurred in the period 1999-2000, in Ramsar, Urmia and Hamedan, and in the period 2008-2009, in Tabriz, Sanandaj, Shiraz, Fasa, Qazvin, Mashhad, Torbat-e-heydarieh, Shahrekord, Gorgan and Kermanshah stations. Overall, the results of this study indicated that the trend of precipitation in most studied sites, particularly in semi-arid parts of the northeast and southwest of Iran, has changed due to the severe and long metrological drought that has occurred in the recent decade (2005-2015).
F. Banan Ferdosi, Y. Dinpashoh,
Volume 22, Issue 3 (11-2018)
Abstract
In this study, in order to analyze the trends of annual precipitation, the information from 21 synoptic meteorological stations located in the Urmia Lake basin in a 30-year time period (1986-2015) was used. For this purpose, the Sequential Mann-Kendall test was used. The date of sudden change (if exist) in the precipitation time series of each station was identified. Significance of the trend in each of the time series and its direction (decrease or increase) in each of the stations were tested at 0.05 level. The results showed that 10 out of the 21 stations had a significant decreasing trend. Three stations (Sarab, Bostanabad and Sardasht) had significant increasing trends. Precipitation trends of eight stations were insignificant. Also, the study of sudden breaking points in the annual rainfall time series of the selected stations revealed that about 57.143 percent of the stations (12 stations) showed a significant sudden change in their annual rainfall series. In other words, more than half of the selected stations exhibited a sudden change in their time series. The date of the sudden change of precipitation in eight stations (namely, Bonab, Sarab, Urmia, Oshnavieh, Kahrizi, Miyandoab, Bokan and Saghez) belonged to the middle part of the time series (i.e. 1996-2005). The sudden change date of t hree stations (namely, Sardasht, Nagade and Tekab) belonged to the first decade of time series (i.e. 1986-1995) and only the sudden change date of one station (namely, Maragheh) belonged to the last decade of time series (i.e. 2006-2015).
F. Negahban Khajeh, Y. Dinpashoh,
Volume 23, Issue 2 (9-2019)
Abstract
Studying the trends of water table in any region especially in arid and semi-arid regions is an important issue. This study focuses on assessing groundwater table changes in Tabriz plain. For this purpose, non-parametric Mann-Kendall test is used. In studing groundwater level the information of 14 pizometric wells in the period of 1991-2013 was used. Significant levels of 1, 5 and 10% were used for the trend test. Slope of trend lines is estimated using the sen's estimator method. The homogeneity of trends were tested using the Van Belle and Hughes method. The results showed that groundwater level in the most of pizometric wells have decreasing trend, That was significante in 1% sifnificance level. According to the research, trend of groundwater level was negative in all of the stations in April and maximum negative trend was belong to Dizaj Leily Khany station (Z= -6/47) that was significante in 1% sifnificance level. Also the minimum negative trend was belong to Ana Khaton station (Z= -0/322). The minimum groundwater level was -1.45 in Said-Abad station.
E. Soheili, H. Malekinezhad, M. R. Ekhtesasi,
Volume 23, Issue 4 (12-2019)
Abstract
The Kor River in Fars province supplies an important part of water requirement in the Doroodzan dam basin and its surrounding area. In this study, the meteorological and hydrological droughts of this area were investigated in the last four decades. For this purpose, the temporal and spatial trend variability of the stream flow was investigated in monthly, seasonal, and annual time scales in the 6 selected stations. The trends of Standardized Precipitation Index SPI, as the drought index, in the 5 selected stations were also studied by the modified Mann-Kendall method. The results indicated that the trend in the stream flow was decreasing in all time scales. Significant downward trends were observed at 95% confidence level on monthly, annual and monthly time scales, especially in the warm months from May to September. These significant downward trends were located spatially in the stations located near the agriculture area, in the middle part of the basin. The significant upward trend existed only at the Doroodzan dam station, at the outlet in the area and in the warm months of the year. In the case of the SPI index, trends were decreasing in all time scales and were significant only at 2 stations in the long-term periods, 9, 12, and 18 months, at 95% confidence level. The results, therefore, indicated the occurrence of severe droughts (SPI<-2) during 1982-83 and 2007-8 periods.
S. Jahanbakhsh Asl, B. Sari Saraf, T. Raziei, A. Parandeh Khouzani,
Volume 23, Issue 4 (12-2019)
Abstract
In this study, the temporal and spatial variation of snow depth over the mountainous region of Zagros, in the western Iran, for the period 1979–2010 was investigated for the cold season when the probability of snow occurrences was high. For this purpose, daily gridded snow depth data relative to Era-Interim/land were retrieved from the European Centre for Medium-Range Weather Forecasts (ECMWF) and used for spatiotemporal analysis of snow in the region. Furthermore, monthly maximum, minimum and mean air temperature relative to the weather stations distributed over the region were also used to investigate the relationship between snow depth and air temperature variability in the region. In each grid point, the rate of temporal changes in the snow depth was estimated using the Sen’s slope estimator, while the modified Mann-Kendall Test was applied to assess if the change identified was statistically significant. The results showed that in almost all of the studied months, especially February and March, the snow depth was significantly reduced in the region, which was statistically significant at 5% significant level. Unlike the observed statistically significant decreasing trend in the depth snow in the region, a significant increase in the maximum, minimum and average temperature was observed for all the studied months and the stations. The result suggested that the observed decrease in the snow depth in the region was related to the increasing trend in the temperature during the study period, which could be attributed to the global warming and climate change.
F. Soroush, F. Fathian,
Volume 25, Issue 1 (5-2021)
Abstract
In the present study, the spatial and temporal changes of climate variables such as pan evaporation (Ep), temperature (T), relative humidity (RH), sunshine duration (SD), wind speed (W) and precipitation (P), as well as their relationship with altitude, were investigated. For this purpose, 68 meteorological stations with 30 years of data (1987-2016) throughout Iran on both seasonal and annual time scales were selected. Trend analysis of climate variables showed that over the past 30 years, most areas of Iran have become warmer and drier although all trends have not been significant. Investigation of the relationship between the trend slope of climate variables and altitude illustrated that there was no significant relationship between them during the study period on the annual time scale (p>0.1). However, in winter, the rate of increase in T (minimum, maximum and mean temperatures) and SD (p<0.1), as well as the rate of decrease in P (p<0.01), was significantly enhanced by increasing the altitude. The increase in mean and maximum T (p<0.1) and SD rates (p<0.001) in summer were significantly lower in the highlands than in the lowlands. In autumn, the trend slopes of minimum and mean T (p<0.05) were negatively correlated with altitude; in addition, the rates of increase in P and RH (p<0.05) in the highlands demonstrated a sharper increase. It seems, therefore, that most changes in climate variables have occurred in both autumn and winter. The results also showed that in winter, the highest rates of increase in Ts were related to the altitude of 1500-2000 m; however, the highest decrease in P belonged to the altitude of 2000-2500 m. In autumn, the highest rates of decrease in minimum and mean Ts had occurred in the altitude of 2000-2500 m; as well, he highest rate of increase in P was observed in the altitudes of both 0-500 m and 2000-2500 m.
M. Motavallizadeh Naeini, R. Modarres,
Volume 25, Issue 4 (3-2022)
Abstract
Dust storms in arid and semi-arid regions have harmful impacts on the environment, the economy, and the health of local and global communities. In this study, the frequency of annual dust events in twenty-five stations and five climatic variables including rainfall, maximum annual wind speed, average annual wind speed, maximum annual temperature, and average annual temperature in arid regions of Iran up to 2014 were used to show the effects of climatic change on dust storms. Annual correlation coefficient time series between climatic variables and dust storms were first calculated based on monthly observations. Then, the trend in climatic variables, dust storm frequency, and their correlation were assessed using the Mann-Kendal method. Results indicated that the correlation coefficients had fluctuations in time and are both significant and insignificant in different years that reach from 0.6 to 0.9 for wind speed and temperature and -0.2 to -0.6 for precipitation. This trend in correlation has the same direction with climatic variables and shows co-movement between climatic change and dust storm fluctuations in central Iran. Results also showed that wind speed and temperature have a high impact on dust storm fluctuations and rainfall reduction has an increasing effect on dust storms.
H. Ahmadzadeh, A. Fakheri Fard, Mohammad Ali Ghorbani, M. Tajrishy,
Volume 26, Issue 3 (12-2022)
Abstract
Determining the actual evapotranspiration value and analyzing its temporal trend is essential for optimal water resources management in a basin. In the present paper, the actual evapotranspiration time series is simulated and its trend is analyzed according to the trend of climatic variables and land use in the Ajichi basin during the period of 2015-1987. The comprehensive SWAT model was set up, calibrated, and validated for the Ajichi basin. Also, the average of simulated actual evapotranspiration of crops (in wet years) was compared with similar values in the National Water Document. The results of the Mann-Kendall trend test showed that the annual rainfall in most meteorological stations had a decreasing trend and the rainfall trend in the ten stations decreased significantly. While the annual maximum temperature at all stations and the annual minimum temperature in most of them have significantly increased. Investigation of land use maps illustrated that the irrigated land area of the basin has increased by a 39% during the study period. According the study's results, the potential evapotranspiration of the basin has had a significant increasing trend with a rate of 2.54 mm per year. The results indicated that despite the increasing trend of potential evapotranspiration and irrigated land area, the actual evapotranspiration of the basin had a significant decreasing trend with a rate of 2.2 mm per year due to the decrease in rainfall.
F. Fathian, M. Ghadami, Z. Dehghan,
Volume 26, Issue 4 (3-2023)
Abstract
In this research, the trend of spatial changes in extreme indices of temperature related to the health and agriculture sectors such as the number of frost days, number of summer days, number of icing days, number of tropical nights, growing season length, diurnal temperature range, cold spell duration index, and warm spell duration index were investigated for 54 synoptic stations throughout Iran for observational (1976-2005) and future (2025-2054) periods. Daily maximum and minimum temperature data of three regional climate models namely, CCSM4, MPI-ESM-MR, and NORESM1-ME from the CORDEX project under RCP4.5 and RCP8.5 scenarios were downscaled for each station using a developed multiscale bias correction method. Then, trends and changes of extreme temperature indices were investigated using Mann-Kendall and Sen’s trend line slope methods. The results indicated that the warm indices such as the number of summer days and tropical nights indices have had a positive trend at most stations in both observational and future periods. In contrast, cold indices like the number of frost days have had a decreasing trend in most stations. The results of cold and warm spell duration indices showed that most stations have had no trend for both periods. The growing season length has increased in more than 60% of stations (45% having a significant trend) mainly located in the northern, northwestern, and western regions of the country. Based on the results, it can be concluded that without considering thoughtful climate adaptation measures, some parts of the country may face health risks and limited habitability and agriculture in the future.
H. Nazaripour, M. Hamidianpour, M. Khosravi, M. Vazirimehr,
Volume 26, Issue 4 (3-2023)
Abstract
In this study, the decade variability of frequency and severity of drought in Iran has been investigated. The one-month scale data from the standardized precipitation-evapotranspiration index (SPEI 01) in the period 1956 - 2015 have been used. Based on the common numerical thresholds, the characteristics of the frequency and severity of drought for each pixel have been calculated and they are the basis for the analysis of the drought situation. Then, the frequency of drought severity classes was calculated and its trend was investigated using the non-parametric Mann-Kendall test. The findings indicated the spatio-temporal variability of drought frequency and intensity patterns in Iran. The frequency of mild droughts has decreased from south to north and from east to west; while the frequency of more severe droughts has increased from north to south and from west to east. The frequency of mild droughts in the southeast, northwest, and northeast has increased by 5 to 40 percent. While the frequency of more severe droughts in most parts of Iran has increased between 10 and 20 percent. Variability in the frequency of more severe droughts is more pronounced in the Central Plateau catchment area as well as in the Persian Gulf-Oman Sea. The trend of drought intensity is decreasing (drought intensification) at the same time as the prevailing rainfall regime in Iran. A significant increase in drought intensity (wet season intensification) is observed only in southeastern Iran at the same time as the monsoon regime. However, extra-arid and arid regions of southeastern Iran are affected by the frequency and severity of drought and have a high degree of vulnerability.
