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Extreme climatic events are associated with climate changes. Understanding the diversifications in these events is important in planning to reduce their effects. In this research, using daily temperature data of 8 synoptic stations in the north and west of the country, 18 extreme temperature indices as defined by the Expert Team on Climate Change Detection Indices (ETCCDI) were evaluated. The results showed that the temperature intensity indices at night had a higher trend than the temperature intensity indices during the day. The frequency of the hot night index and hot day index in most of the studied stations had a significant increasing trend. Moreover, the cool night frequency index had a negative and significant trend in the north and west of the country and showed a higher trend in the north of the country than in the west. The cool day frequency index was negative for both study areas. Cold extreme indices including the number of cold days, number of frost days, and cold spell duration index were obtained in both regions of the country with a negative trend. While warm indices showed the number of hot days, warm nights, and warm spell duration index with a positive and significant trend.

The meteorological drought phenomenon and its relationship to the hydrological response of the catchments are of particular importance in the development of programs related to the comprehensive water management of the basin. In this study, a common period (1982-2017) in five corresponding hydrometric and rainfall stations in the Kashkan watershed was considered. The regression relationship between rainfall and discharge was investigated. Then, the standardized precipitation index (SPI) and the standardized discharge index (SDI) were calculated in time scales of 3, 6, 9, 12, 18, and 24 months. The interrelationships of SPI and SDI in sub- catchments were analyzed using the correlation method. The results showed that in more than 50% of the study period, the meteorological drought was close to normal. hydrological drought investigation of the sub-catchments showed that in the catchments with fewer karst formations, the frequency of years with severe drought was higher. The maximum drought with a severe situation in the Afrine catchment was 20% and the minimum was 6% in the Cham-Anjir catchment. The trend of changes in correlation coefficients between SPI and SDI in Sarab- Seyed Ali, Pol-e Dokhtar, and Cham-Anjir were similar and the maximum was at 18- and 24-month time scale with a coefficient determination of 0.67.

Flow duration curve (FDC) is one of the most appropriate criteria for recognizing the hydrological response of catchments. Understanding the interaction of drought phenomenon along with FDC indices is a suitable tool for optimal water resources management of catchment. The aim of this study was to investigate the temporal relationship between drought and FDC indices in the Kaka-Reza, karstic catchment. In this study, the common period (1982-2017) was considered for the corresponding hydrometric and rain gauge stations. The standardized precipitation index (SPI) and standardized discharge index (SDI) were then calculated at time scales of 3, 6, 9, and 12 months. Precipitation duration curve (PDC) and FDC were plotted and curve indices were calculated and standardized. The relationships between drought index and FDC indices were investigated and analyzed. The results showed the general trend of changes in SPI and SDI in the respective time scales follow each other. However, the most consistent trends are related to the time step of three months. The rate of relative changes in the section of low flow indices (Q90/Q50) were very small and around 0.6% and in the high flow (Q25/Q50) was around 4.41%. The correlation between FDC and PDC showed coefficient of determination of 0.904. The rate of slope change was from 0.5 to 5% and trend of its changes was a slight upward slope during the research period. The small amount changes of FDC indices and slope of FDC showed the role of karst phenomenon in determining the shape of FDC as well as controlling flow fluctuations and providing stable flow during the drought period.

In recent years, over using of water, climate and land use changing in different river basins, have had different impacts on the river flow, particularly low-flow. With respect to the extraction of the groundwater resources and its effect on event time and the length of this time, it is essential to study the low-flow in different stations. The purpose of this study was to investigate and determine the trend of spatio-temporal low-flow in selected hydrometric stations in Alborz south mountains basins. For this purpose, daily discharge data were collected for different hydrometeric stations within the study area and 39 stations were selected. Daily discharge of these stations were reconstructed with correlation method and outliers were eliminated. Then, the time series of 5, 7, 10, 15, 30, 60 days of stations were calculated. Basins were then clustered into two classes including plain and no plain. The trend of low flows was calculated using Mann Kendall, Teil-sen, and spearman tests. For each cluster, a regression relationship was established between the low flow trend and basin upstream area. Results showed that about 10 stations had no significant trend and other stations found have significant negative trend at 1, 5, 10% levels. Plain basins experienced lower negative low flow trends as the basin area increased, which could be attributed to the over extraction of groundwater resources, but in no plain basins direct correlation was obtained as area increased.