The permanence of rainfall shortage compared with the long-term average of meteorological droughts, reduces river flow, groundwater level and hydrological droughts. To quantify the drought, several Indexes have been defined. In this study, two drought Indexes, including Standardized precipitation Index (SPI) and Standardized Streamflow Index ((SSI)), were used for meteorological and hydrological drought monitoring. The aim of this study was the monitoring the drought in 5 stations in Tehran province during the period of (1985-2016), determining the most appropriate cumulative distribution function for calculating the (SSI) drought Index and also comparative study of meteorological and hydrological drought indicators in this province. The results show that the most suitable cumulative distribution function at the Darake station is Gen-Loglogestic, at the Firuzkuh and Abali station is Gamma, at the Namrud station is Fatigue life and at the Sharifabad station is Beta. Among the stations with proper functions, Derek with the highest P-value (0. 996) and the lowest RMSE (0. 916) had the best fit with the Gen-Loglogestic distribution function. Also, the similarity in the process of drought resulting from two different indicators, indicates that the meteorological and hydrological indicators are completely same, in other words, surface water resources in Tehran province are completely influenced by rainfall.

Drought is one of the most destructive and important climate phenomena, whose effect is usually more important on a regional scale. The importance of this phenomenon is more evident in the Karkheh basin due to its size and important role in providing the country's water resources. We aim to monitor hydrologic drought using the accurate calculation of standardizes Streamflow Index ((SSI)) in one month time scale based on fitting frequency distribution to monthly data and goodness of fit test for each station in Karkheh basin for 30 years (1986-2016). The findings of this research showed that the generalized Pareto distribution was selected as the most appropriate distribution in most months, unlike the previous research that fitted and used only the Gama distribution on the data. The time series of the standard flow Index indicated the occurrence of super-drought in 2008 to 2015 years. Also, the significant impact of the construction of hydraulic structures upStream of the basin on the average flow rate was observed in some stations. The results of direct and annual monitoring of the drought situation showed that the Karkheh basin has experienced hydrological drought in recent years, and the drought trend is increasing.

Awareness of drought interrelationships and hydrological response of watersheds is a prerequisite for optimal management of surface and groundwater resources. The aim of the present study was to compare the impact of drought on surface and subsurface waters in the Kaka-Reza basin. In this study, after controlling the data, a common period (1982-2017) for hydrometric and rainfall stations was considered. Then the Standardized precipitation Index (SPI) and the Standardized discharge Index (SDI) were calculated for the 1-, 3-, 6-, 9-, 18-, 12-and 24-month time steps. The base flow and the corresponding Index were calculated by B-flow Lyne and Hollick recursive digital filter. Then, the Standardized base flow Index (SBFI) was calculated by adapting the SPI method for the corresponding time steps. Finally, the relationships between SPI, SDI and SBFI were investigated and analyzed using correlation method. The results showed that the greatest effect of drought on groundwater for the 9-and 12-month time step with coefficient of determination 0. 87, While the effect of drought on surface water for the corresponding time step has a coefficient of determination of about 0. 4.

Introduction: According to some researchers, predeting and preventing water shortages is not po(SSI)ble. However, by fitting the distribution of the probability to the data of a river, it is po(SSI)ble to determine the incident corresponding to the probability of the occurrence or a certain period of return, and with the management and implementation of the drought management programs to reduce its effects and consequences. Hydrological droughts affect vast areas and are detected by reducing lake water storage, lowering groundwater levels, and decreasing river flow flux. This decline in river flow fluctuates from two environmental and management perspectives on water resources. On the other hand, hydrological drought affects the agricultural sector and causes irreparable damages to this sector. Therefore, the importance of studying hydrological droughts is shown in relation to other droughts. Droodzan dam basin is one of the mountainous basins of central plateau of Iran located in the northwest of Fars Province. It provides approximately 760 million cubic meters of water per year, including drinking water, and agricultural and industrial facilities in and around the area. Recently, due to drought, it has faced severe water losses. Therefore, the preservation and maintenance of the reservoir of this dam and the continuity in exploitation of water resources requires attention to its watershed and the potential of water production in the river basin. Therefore, studying the hydrological status of surface waters of the basin can have a special place in the management of drought in the country. Methodology: In this study, 5 stations that had appropriate data during a long statistical period were selected. Reconfiguration of the statistical defects was done using the correlation between stations and using the SPSS software. After reconstructing the statistical defects and completing the data, their homogeneity was analyzed using run test and SPSS software. Then, the river flow Index was calculated at 3, 6, 9, and 12-month intervals during the statistical period of 28 for each station. In this study, for the frequency analysis, the data from series of limit values were used and fitted with the probabilistic theoretical fit. Chi-Square and Kolmogorov-Smirnov tests were used to determine the appropriate distributions for each time scale using Chi-square fit test. Then, the return period for each of the drought conditions was obtained on the basis of the river flow Index at each time period and was extracted from the frequency analysis curve of each station. Results: The results of the data homogeneity were studied. The data of all stations were at a level of confidence of at least 99%. In general, the results of the river flow Index were very similar at different time scales and indicated the severity of the drought in these years (64-65, 80-78, and 92-87). This increase was observed at 9 and 12-month intervals. Also, the number of years in which mild drought occurred at a time scale of 9 and 12 months, was compared to the 3 and 6-month time scales. In the abundant analysis, the normal distribution of the 3-month river flow Index, the distribution of generalized limit values for the river flow Index of 9 and 6 months, and the exponential distribution of the 12-month river flow indices were recognized as the best distributions. Discu(SSI)on and conclusion: The results obtained from the calculation of the studied indices showed an increase in the severity of the hydrological drought, especially in the recent decades. River flow Index is one of the indicators of the hydrological drought assessment, which can be a good measure for asse(SSI)ng the drought phenomenon in the region. This indicator is also very efficient and has a high sensitivity to other drought indicators. In addition, the zoning maps showed that in the northern and eastern regions of the mild drought, less than the other regions, except for the 6-month period, they were less likely to return to the rest of the period. In the case of the moderate and severe droughts, this was true in the northern parts of the region. In general, with the increase in the return period, the extent and severity of the drought in the area increased. Particularly the northern and eastern areas of the studied watershed were more prone to these natural disasters. Therefore, this area was exposed to economic and social damages and hydrological droughts, so water resources studies require more attention. Thus, the drought risk poses these areas to severe economic, social, environmental, agricultural, and ecological degradation. These areas can serve as short-term and medium-term goals of integrated strategic management and operational plans.

Understanding the interaction of drought phenomenon and hydrological response of catchments can lead to obtaining some information for optimal water resource management. The aim of this study was to investigate the temporal relationship between drought and groundwater contribution to Stream flow, in Kaka–Reza Sub-catchment. In this study, the common period of 1382-2017 was considered for the corresponding hydrometric and rain gauge stations. The Standardized Precipitation Index (SPI) was then calculated at time scales of three, six, nine, 12, 18 and 24 months. Base flow and related Index were calculated by B-flow-Line and Halick digital filter method in monthly, annual and total time bases. Then the Standardized Base flow Index (SBFI), the Base flow Duration Curve (BFDC) and the base flow duration curve shape Index (SBFDC) was calculated. Changes in BFDC and the corresponding shape Index and the relationships between SPI and SBFI were investigated and analyzed using the correlation method. The results showed that the average Base flow Index during the research period was equal to 0.52 and its minimum and maximum were equal to 0.46 and 0.57, respectively. Also, the trend of bas flow changes is a decreasing trend with a low slope. The trend of changes SBFDC decreases over study period with a low slope. The rate of slope change is also from one to three percent, and close to the straight line, which indicates the tendency for base flow to be stable in the long time. The greatest impact of the drought phenomenon on the river base flow is in the time step of nine and twelve months. The correlation between SPI and SBFI with a coefficient of determination 0.87 confirms the increase in the groundwater contribution to base flow, with a delay of nine to twelve months.

In analyzing the frequency of drought as a multifaceted phenomenon, summarizing the event in one variable reduces the reliability of the results. Bivariate and multivariate drought frequency analysis using copula functions makes it po(SSI)ble to estimate the return period of a drought event with specific intensity, duration and peak. In the present study, the Stream drought Index (SRI-12) was used to investigate the characteristics of drought using the monthly Stream of Barnetin Station of the Esteghlal Minab Dam watershed. Spearman's correlation coefficient was used to analyze the dependence between hydrological drought variables and six marginal distribution functions for their modeling. Then, by using Archimedean and elliptical family copula, the best joint was selected and conditional multivariate analysis of hydrological drought was done. The results showed that gamma, lognormal, and Weibull distributions are the best marginal distribution functions for drought intensity, duration, and peak, respectively. The results of the copula fit show that the best copula function in analyzing the dependence between variables is Frank's copula. The three-variable conditional probability of drought severity in 20, 50, 80, and 100-year continuities and peaks of 1 and 1.5 were investigated. These thresholds were chosen considering that they were the most frequent in the study area. The results showed that the probability of drought intensity decreases with the drought duration remaining constant and the peak increase from 1 to 1.5, but with the drought duration and intensity increasing, the drought peak also increases.

River flow behavior is one of the major problems considered in the design, operation and studies related to water resources. Recently, applying new techniques such as chaos theory in hydrology and water resources due to its innovation and ability has been considered. An application of chaos theory is characterization and quantitative analysis of river flow time series. The aim of this study is determining properties and prediction of Ahar-chai river daily flow during 19 years using the concepts of chaos theory. Chaos time series state space reconstruction is based on selecting two suitable parameters namely delay time and embedding dimension. In this study, average mutual information and false nearest neighbors method are implemented for estimating these two parameters. Correlation dimension method is used for determining the chaos ability and estimating the fractal dimension of daily river flow. The results of computations show that fractal dimension of 4 (chaotic low), with delay time of 65 days and embedding dimension of 13 for reconstruction the river flow dynamic state space, can be used. Local prediction algorithm for prediction of time series and correlation coefficient, and also Nash-Sutcliffe Index and root mean square error are used for asse(SSI)ng the performance. Results illustrate good and acceptable accuracy of chaos theory in prediction of river flow.

1. Introduction: Most investigations on the SPI (Standard Precipitation Index) have focused on drought analysis yet. A few researchers including Seiler et al. (2002) and Du et al. (2012) using monthly data showed SPI2, SPI12, and SPI24 have the potential to forecast floods occurrence. The present research aimed at using SPI for flood occurrence forecasting. The results of the study are important from two points of view: first, the applied model uses just one parameter that imposes less uncertainty through decreasing errors in preparing and introducing data to the model, and second, the model uses the most commonly available data with acceptable length everywhere; i. e. precipitation. While in drought analysis monthly scale is common, in this research the SPI was used in daily scale to fit the purpose of the study. For the case study, two different watersheds were selected; one in a humid area (Kasilian watershed in the north of Iran) and the other in a semi-dry region with a river of rainfall-snowy hydrologic regime (Kailak Darahsi watershed in the north-west of Iran)...