Identification of drought and wet periods has an important role in water resources management. In this paper, climatic and hydrologic droughts were evaluated using SPI and SWI indices in Chghalvandi Watershed. The Data used in this study included 18-year climatic and hydrometric data obtained from 8 climatic and 2 hydrometric stations and also groundwater level data. Drought area delineation was made by ArcGIS kriging method, while the linear trend of the phenomena was assessed using Minitab and Makesen softwares. The results showed that magnitude and duration of drought have been considerable in recent years. Comparison of the climatic and hydrologic droughts represented a lag time between these two types of droughts. Drought area delineation maps revealed more severity of the hydrologic and climatic droughts in the southern and central parts of the study area.

The frequency of floods is one of the characteristics of river flow statistics so thatanalyzing it has an important role to assess the hydrological and economical water resources projects. For determining flood frequency, the estimation of accurate skewness coefficient of annual peak discharges is required. Estimation of population skew for different regions will be improved when it is computed from the weighted average of the sample skew and an unbiased generalized skew estimate. There are different ways to develop a generalized skewness coefficient. The goal of this study is to analyze the methods for generating unbiased generalized skew coefficient and select the best method for creating the weighted generalized skewness coefficient. In the present study, to calculate weighted generalized skewness coefficient, initially the hurst index is calculated to analyze the adequacy of time series length. The case study of the present research (West Azerbaijan, Iran) has three basins containing different hydrologic regions. These three basins are: the Aras River, Urmia Lake and Zab River basins. Therefore, various hydrologic regions, with the help of provincial border and the borders between sub-basins, are combined to form three larger hydrologic regions.After the formation of three larger hydrologic regions, the homogeneity of skewness variance of annual peak discharge of hydrometric stations within each three hydrological groups are tested using theleuven statistical parameter. Also the Dunnett test is applied to identify areas whichare significantly differentiated with other hydrologic regions. To develop the generalized skewness coefficient of 67 hydrometric stations with different statistical periods (16 to 62 years), three methods containing statewide map of skewness in West Azerbaijan, skewness map with including three hydrologic regions, and weighted average of skewness for the three hydrologic regions were used. Finally, after calculating the errors of three methods of generalized skewness development using Mean Square Error (MSE) coefficient, a weighted technique is used to calculate the weighted generalized skewness using sample skewness and the best generalized skewness (the one which has the least error) and their corresponding errors. The results showed that most parts of the province have negative skewness values. The Hurst test results showed that the hurst coefficient is greater than 0.5 for all 67 hydrometric stations and lengthening of time series for the analysis is not required. Also, the results of the leuven statistical parameter showed that the homogeneous assumption is true for hydrological groups. Therefore, there is no reason for the variance heterogeneity. Moreover, the results of the Dunnett test stated that statistically, skewness means within the hydrological groups are not different. An error analysis showed that the Zab river basin had the least error amongthe studied basins. Among the methods studied for developing the skewness map, the division of the province into three hydrologic regions hada higher accuracy (MSE of Generalized skew coefficient = 0.55) than the other methods. However, this difference was very marginal. According to skewness maps, it can be seen that by considering hydrologic regions, the errors can be reduced in all three hydrologic regions. As the MSE in areas A and B is lower than the provincial level and in the region C, the error rate is close to zero. However, it should be noted that the number of hydrometric stations in region C, are much lower than other parts of the study area and this can be one of the reasons for error reduction in this area. Considering that the aim of this study was to evaluate the accuracy of the generalized skewness estimating methods in the calculation of weighted generalized skewness coefficients, it has been seen that a regional approach, in addition to reducing the error rate, the fracture lines on the skewness map of the annual peak discharges can be reduced. Unlike the regional approach, the averaging method has shown worse results in all three regions.We may conclude that the sample skewness coefficient alone can bring better results than the averaging approach. Also, by comparing errors in areas A, B, and C, it can be concluded that with increment in area of hydrologic regions and inadequate spatial distribution of hydrometric stations, the error rate increases.

Drought is a natural phenomenon that causes water resources shortage in various sectors such as drinking water, industry and agriculture. Drought is one of the climatic phenomena that can be observed not only in arid and semi-arid regions, but also alternately in wet areas. It should be noted that aridity and drought are different, aridity is a climatic trait specific to arid and semi-arid regions and is a permanent feature of the climate of the dry region (Eslamian et al, 2012). Liu et al (2011) with the study of meteorological and hydrological droughts in Oklahoma State discovered the two-day drought incidence of 2 months and the correlation coefficient between SPI and SDI profiles in an optimal state was 0.63. Eskandari-Damaneh (2015) investigated temporal and spatial relationships between meteorological and hydrological droughts in Tehran province. The results showed a correlation of 99% between meteorological and hydrological droughts and the increasing trend of drought in Tehran province. Mahmoudi et al. (2015) assessed the drought of meteorology in Sistan and Baluchestan Province using a standard score index and its impact on water quality in the province. Delpla et al (2009) examined the impact of climate change on the quality of surface waters in relation to drinking water production and concluded that climate change has reduced the quality of drinking water that has potential health impacts. Also, Mosley (2015) indicated that drought increased salinity, algae and soluble ions, which greatly reduced the quality of freshwater. According to drought studies, the main objective of this study is to study the drought of meteorology and hydrology, as well as to determine the probable delay between the occurrence of these two types of drought and their impact on river water quality, for water resources management in Godarkhosh basin which is one of the most important basins in Ilam province.

Rainfall/ precipitation, as one of the most important inputs to hydrological systems, is one of the most significant parameters in many hydrological models. In the recent decades, different types of forecasting methods are employed for forecasting and analyzing monthly precipitation rates. Linear regression is one of the methods are being used for this purpose. Recently, the use of singular spectrum analysis in water resources studies for removing random components of hydrological series has extensively increased. The main objective of this study is to investigate the use of linear regression coupled with singular spectrum analysis for monthly precipitation forecasting. The monthly data of Ponel raingauge station which span the period from 1991 to 2010 (i.e. 20 years) were used to develop the proposed model. The proposed model was compared with regular linear regression and the results indicated the superiority of combined linear regression and singular spectrum analysis models.

Although there are many studies on meteorological drought in Isfahan province, there is no specific study on hydrologic droughts. In this study, trend analysis of low flows as the most important hydrologic drought indicators for Isfahan province watersheds is carried out for the first time. Mean daily stream flow as a hydrologic indicator and 1-, 7-, 15-, 30- and 90-day low flows as hydrologic drought indicators for 22 gauging stations with at least 20 years of data record were calculated and their trends were estimated over the province. Results of trend analysis using Mann-Kendall and Spearman correlation indicated that mean daily stream flow and low flows have negative trend at many gauging stations. Apart from Ghaleh Shahrokh station, Low flow indices in Zayandehrud dam basin have negative trend so that Eskandari station show highest degree of negative trend. Daily stream flow and low flows reduction especially in recent years is a warning for water resources management in Zayandehrud basin.

Landslide is one of significant mass movement which causes normous annual human and sesmic region. Therefore, finding hazardous areas for planning and management of them is necessary. So, in this reasearch, occurred landslide in watershed region of bidvaz dam which has area of 161 square kilometers and 125 landsliding points are surveyed in different hydrological conditions by index model of slope stability. For specified slopes, graiding test was done and geotechnical parameters were attained. Then, by using Arc view and Arc GIS packages and calibrating parameters in SINMAP and incorporating the geotechnical and hydrological parameters with digital elevation model, SINMAP was conducted. After that, validation was done in two common and high hydrological conditions including rainfalls with a return period of 2-10 years and 20-50 years Respectively. Results showed that the performance of model in simulation of landslides is not appropriate at common hydrological cases. However, increase in hudrological humidity condition, performance of model improves. In calibration and validation, prediction of model increase 21 % and 26 %, respectively. Therefore, utilizing SINMAP model in high hydrological humidity conditions can be useful for choosing dangerous ways according to the landslide map.

Despite the need to predict the flow regime in rivers, especially in hydrological drought conditions, little progress has been made in this area, and practically time series of discharge or non-dimentional indices are used instead. In this study, the prediction of hydrological conditions in rivers based on the basin condition in terms of rainfall is done using Markov chain concepts for the case study of Anzali Wetland catchment with 9 meteorological stations and 20 hydrometric stations over the years. The data from 1985 to 2015 have been used. Two-dimensional Markov probability matrix was used to predict the hydrological status of the rivers by observing the meteorological condition of the basin. The two-dimensional transition probability matrix is different from what has been customary so far, using two different phases of moisture flow in hydrometeorological processes in a matrix so that its columns represent the meteorological state and its rows represent the hydrological conditions. Evaluation of Anzali catchment conditions showed that meteorological and hydrological conditions are the same in each month by the probability of higher than 50%. Based on the results of the 2D matrix of the probability of transfer, the probability of being in normal, wet and dry hydrologic conditions, if these conditions occur in a step ahead in the meteorological phase, is 80, 50 and 40%, respectively. Based on the achieved results, with the expected value of the pertinent discharges in the forecasted hydrological condition concerning the former step climatological state, the probable discharge in the river is predicted.

In order to assessment and prediction of hydrological processes with inherent complexity, processes changes effects on the nonlinear behavior of watershed system in the past and future and specification of water and soil conservation operation, using indirectly methods such as simulation of watershed processes therewith directly methods is necessary. The objective of this research is evaluation of runoff generation in space and time using distributed hydrological WetSpa model in Taleghan watershed of the Sefidrood basin. Thus flow hydrographs and peak discharges in any space of the stream network were predicted also spatially distribution of hydrological processes such as runoff was simulated using incorporating of both daily meteorological data in an 11 year period and three base maps information. Simulated results reveal that there is good agreement between observations and simulations. The Nash- Sutcliffe criteria, 83.3% and accuracy of the simulation show the high performance of the model in this watershed. Regarding to the outputs and spatially distributed hydrological factors in daily time step the model is capable to analyze topography, soil type, and land use effects on the hydrological behavior of the watershed.

Today, watersheds are severely affected by natural and man-made stresses, and their ability to recover and adapt to altered conditions depends on the resilience of watersheds. Therefore, due to the importance of this issue and the necessity to explain  management models in  order to promote resilience in the country's watersheds, the present paper aims to analyze the concept, application, and methods of hydrological resilience assessment as one of the resilience dimensions in comprehensive watershed management. Studies in this field are very limited worldwide but have an upward trend. Accordingly, the methods used to evaluate hydrological resilience have so far been limited to the use of simple compilation methods such as determining the arithmetic mean of some important hydrological indicators, using the Budyko curve and the Convex model. Budyko's curve analysis was mostly based on rainfall, evapotranspiration, and runoff production. But the convex model is used by considering the failure thresholds of hydrological indicators and establishing a relation between the process of long-term changes and the failure thresholds of indicators. The indicators used are multitude and have different applications depending on the hydrological conditions of each watershed. Among the most important of them can be the ratio of drought index to runoff, temporal trend and frequency of low and high water flow, change in annual water yield, groundwater level, surface runoff intensity, river enrichment from nutrients, and heavy metals, forest degradation percentage, soil erosion, sediment yield, and saline water levels are all the result of the interaction of other influential environmental factors, such as climatic, ecological, economic, biophysical and social.

In recent years, flood phenomenon has been one of the hydroclimatic events and one of the most serious natural hazards that has threatened human societies. Indiscriminate exploitation of forests and pastures and land use change and their transformation into unsuitable agricultural lands, along with the indiscriminate construction of residential areas, have caused an increase in floods. Therefore, the flood phenomenon is very important. Therefore, the present study was conducted with the aim of analyzing the spatial correlation of changes in vegetation cover with the height of runoff in the Gorgan River watershed in a 21-year period. For this purpose, from the data of slope, soil type, land unit and land use to prepare the hydrological response unit and Landsat images of different years (2000-2021) to investigate changes in vegetation density using the NDVI index in each of Hydrological response units are used. In this regard, 72 hydrological response units were prepared as basic units in the study area. Also, the height of the runoff was calculated by the SCS method in each of the hydrological response units using the criteria of rainfall height, soil hydrological group, vegetation, land use type and soil texture, so that the cities of Agh Qola, Simin Shahr and Gomish Tepe and the Gorganrood River in the height of the runoff There are many. The results of the analysis of 78% of the spatial correlation between the height of runoff and the density of vegetation showed that the agricultural and garden lands in the plains have increased significantly, while the density of forest and pastures has decreased in the 21-year period. This factor is caused by human activities such as the destruction of forests and pastures and the cultivation of these lands and the development of cities, the height of the runoff has increased. While in the highlands, due to more infiltration, despite the decrease in vegetation density, the runoff height is also low.