مجله علوم و مهندسی آبخیزداری ایران
پیاپی 8 (پاییز 1388)

Drought duration is defined from different viewpoints. Estimation methods of drought duration return period are different in terms of drought duration definition. This paper is aimed to perform six estimation methods of drought duration return period at Kermanshah station, as a case study. The standardized precipitation index (SPI) is used to monitor drought based on historical and synthetic series of total precipitation data in annual and monthly timescales. Probability distribution of drought duration was analyzed for time series of SPI. Different methods for estimating return period of drought duration on historical and synthetic time series of SPI were applied. Findings answered two main questions: a) if the length of historical record was enough to estimate drought return period at the station of interest? b) If the obtained return periods based on different methods have enough conformity with empirically obtained return period? Results are discussed in details in this paper.

Streamflow simulation is important as a prerequisite to engineering and environmental problems. In this research, IHACRES rainfall-runoff model was used to simulate streamflow in Kasilian Catchment which located in Mazandaran Province. This model was calibrated and evaluated in this Catchment. The error between simulated and observed streamflow values was estimated using NSE, RMSE and MAE criteria. Also significance test of difference between simulated and observed streamflow values was evaluated using t-test method in the SPSS software. The results of this research showed that the model can simulate monthly and daily data with satisfactory accuracy (0.36 annual data. On the other hand, in comparison between monthly and daily streamflows, it can be seen that simulation of monthly streamflow is more reliable than that of daily streamflow. Difference between simulated and observed streamflow values was not significant for daily, monthly and annual time intervals.

Rainfall- runoff modeling is one of the key steps of hydrology. Among rainfall- runoff models, the conceptual models are frequently used. However, calibration of the conceptual rainfall-runoff models has been a challenge for hydrologists for decades. In this study, automatic calibration of ARNO conceptual rainfallrunoff model is developed using genetic algorithm. This model is in the class of continuous semi distributed models and has been successfully used in different parts of the world. Using the employed automatic calibration tool, the calibration step of the model can be performed rapidly and simply for a given basin, without requiring an extensive knowledge of the model structure and parameters. The model was calibrated automatically for rainfall-runoff simulation of a basin in southwest Iran. The calibration results for 5 years daily observed data showed that the values of efficiency coefficient and squared correlation coefficient was 0.80 and 0.82, respectively and the validation results for 4 years daily observed data showed that the values of efficiency coefficient and squared correlation coefficient was 0.82 and 0.83, respectively. The results show that this model in conjunction with employed automatic calibration method can successfully be used for daily rainfall- runoff simulation.

Precipitation is an important climatic factor that reflects particular complexities. The statistical distribution of total precipitation of a location may vary with time. Due to the arid and semiarid climatic conditions of Iran, knowledge of the precipitation amount and variation is an important issue. Therefore in this study the characteristics and trend of precipitation in 7 rain gauging stations in the Golestan province is investigated during the 31-year period of 1974-2004. The linear regression and Mann Kendall trend analysis methods were used. The results indicated that at 5 % level, significant increasing trends have been observed in the annual precipitation of the Tamar, Tangrah, and Gonbad stations. Increasing trend has been found in the autumn precipitation in all stations, but it was significant only at the Tamar station at 5% level. Precipitation in winter and spring did not show a significant and homogenous trend across all stations. While the summer precipitation shows an increasing and homogenous trend across all stations. However it is significant at 5% level only for the Gonbad, Tamar, and Cheshmeh khan stations. The maximum daily precipitation of the Rabat Gharebil and Galikesh stations shows decreasing trend which is significant for the latter at 5% level. But it shows increasing trend in the other stations from which the Tamar and Tangrah stations have significant trends at 1% level. In case of number of days with rainfall events, all stations showed an increasing trend which is significant at 1% level except for the Tamar and Cheshmeh khan stations.

This research is about the effects of sediments on discharge coefficient of rectangular, v notch, ogee and overflow weirs. Two open channels with 7m and 5m length for low and high flow rates are used in the experiment. Tests are done for 2°, 4° and 5° slope of 7m open channel and 0.5° slope of 5m open channel in two states, including water flow and water- sediment flow. Experiments were conducted with flow rates ranging from 8 lit/s/m to 14.7lit/s/m and 1.8 lit/s/m to 11.8 lit/s/m in the 7m and 5m open channels, respectively. Sediment particles are thrown into channel according to the flow rate. A calibrated pitot tube is used for velocity measurement and a point or hook gauge with 0.02m accuracy is used for depth measurement. Each test is done three or four times and data are recorded. It can be concluded that sediments decrease the discharge coefficient and this effect is obvious in lower flow rates and smaller slopes and this trend is independent of weir type. Also, according to the results and data, some equations are suggested bycorrelation method which enables calculation of discharge coefficient in water-sediment flow from discharge coefficient in water flow.Keywords:

The aim of this research is landslide hazard assessment in a part of Haraz watershed using fuzzy multi criteria decision making. At first, landslide locations were identified using aerial photos and field study, then landslide distribution map was produced. Effective factors on landslide occurrence such as: slope, aspect, elevation, lithology, land use, distance to fault, distance to road and distance to drainage network were digitized in GIS environment as shape digital layers and then used for analysis of fuzzy sets theory. Fuzzy analysis was accomplished after determining membership function for each effective parameter in landslide occurred using Matlab 7.1 software. Then the results representing hazard degree for each pixel in case study map was transported in ILWIS software and landslide hazard zonation was produced. The results showed that identified landslides were located in the class very high (29.42%), high (32.19%), moderate (30.36%) and low (8.04%) in susceptibility zones.

The suspended sediment size is one of the most fundamental properties of sediment which affects transportation and deposition process, and their control as well. However, limited attention has been made on particle size distribution studies. The present research was therefore took place in order to study particle size distribution of suspended sediment sampled from two points in a study reach and during a flood in Educational Forest Watershed of Tarbiat Modares University comprises some 13263 ha. Towards this attempt, the sediment samples were analyzed based on stoke's law with the help of modified pipette method after sample preparation. The entire descriptive properties of sediment samples were calculated by using GRADISTAT software. The results of the analyses verified the significant spatial and temporal variation of particle size distribution of suspended sediments.

As a cansequence of Marun dam construction on the Jarrahi River, qualitative and quantitative changes of inflow is an ecological threat on wetland ecosystem. In order to prevent the long- term negative impact of the projects on the ecosystem, it is necessary to assess the environmental flow and optimum water allocation for different uses. In this research, environmental flow indices were investigated using hydrological analysis of wetland inflow at Jarrahi River. Environmental flow estimated in Shadegan wetland are 6.83, 1.54 and 3.18 m on Montana method, environmental flow is estimated 1.54 and 3.6 m respectively. The results have shown that environmental flow indices were decreased after dam construction in all hydrometric stations. 3/s based on duration curve, Aqua France law and Smakhtin methods, respectively. Base 3/s in first and second part of year.