مجله مهندسی و مدیریت آبخیز
سال چهارم شماره 1 (بهار 1391)

Soil erosion is a dynamic process with high variability in sediment size distribution. Dynamic recognition of sediment size distribution is as a base for modeling of soil erodibility and movement of nutrients، pollutants and fine particles in consequence of erosion. This study focused on sediment transport mechanisms due to sheet erosion on two different soils (cropland and sandy) using flume experiment. The experiment was performed applying different flume slopes (1. 5 and 2 %) and flow rates (75، 100، 125، 150، 175 and 200 ml/s). The result showed that hydraulic condition as well as soil type، played an important role in selective transport of sediment particles so that، in the sediment size distribution of cropland soil two peaks were observed. For the first (fine particles with size of 0. 042 mm)، the dominant transport mechanism was attributed to suspension- saltation، while for the second peak (large particles with diameter of 1. 5 mm)، bed load was known as the main transport mechanism. Moreover، increasing stream power، the selectivity of fine particles declined therefore، both fine and large particles were transported by the flow. For sandy soil، only one peak for the range of 0. 109 to 0. 175 mm was observed. In this soil، saltation was the main mechanism in transporting sediment. Increasing flow rate، the frequency of particles smaller than 0. 015 diminished، whereas particles larger than 0. 218 mm increased. For both soils at higher stream powers، the relative importance of transport as suspension and bed load decreased and increased، respectively. The findings of this research reveal that sediment size distribution and also transport mechanisms depend on hydraulic condition such as slope and flow rate.

The proper application of instantaneous unit hydrograph theory is very efficient for watershed with lack of rainfall data. Therefore، the present study has been conducted to evaluate the efficiency of Clark instantaneous unit hydrograph model with the help of geographical information system in regenerating flood unit hydrograph in Kasilian watershed in Mazandaran province. The comparative evaluation is conducted between the 3h-unit hydrographs simulated using Clark model and the one obtained from averaging the observed ones. This comparison is made with the help of statistical criteria viz. relative error، root mean square of error، coefficient of efficiency and bias. It verified high level of compatibility between two unit hydrographs using the Clark model and the hydrograph obtained by averaging the observed data.

The most important factor of drought analysis is rainfall. All resources which are supported by rainfall، such as، surface streams; groundwater and lakes، are affected by deficit of rainfall. Analysis of past rainfall statistics indicates a pattern. Forecasting future rainfall can be used in water resources planning. This opportunity can be provided by time series patterns. The aim of this research is drought forecasting of Isfahan Province، by fitness of various models with rainfall data. Studied models are included in monthly، monthly-seasonally، annual-exponential، moving average (3، 5، 7) and sinusoidal. 24 meteorology stations have been studied in the time series analysis. The at-site statistic period ranges between maximum 44 years (1957-2000) to minimum 11 years (1987-1997). By regarding ARIMA model fitness، the results indicate that the studied stations are divided to three groups as high، medium and low quality. These groups are included 9، 5، 4 stations، respectively. Although Isfahan and Kashan have been owned long-term rainfall data، they have less suitable fitness than the other stations. Therefore، by only having long-term rainfall data، it cannot be expected the better fitness of time series. Using ARIMA for annual moving average modeling can be effective in drought pattern analysis and have been presented the good patterns.

Watershed management planning and preference need variety of data like water yield and its fluctuation. Average water yield on different probabilities is needed for river discharge estimation and small and big dams utilization planning. On the other hand، annual water yield is needed for planning and management of downstream agricultural lands. In this research، 40 hydrologic stations with acceptable quantitative and qualitative data within a common base period. Watersheds homogeneity was analyzed based on water yield and influence of numeral parameters and segregated into two homogenous watersheds. The accuracy of homogeneity was tested by discriminant analysis and was 100 and 94 percent in clusters one and two، respectively. In addition to regional annual water yield analysis، the discharge probability was estimated and normal distribution was chosen as the best statistical distribution for discharges of two to 100 years return periods. Regional probable water yield equations with different return periods for homogenous regions were extracted using hydrological and numeral data such as annual discharge، area، slope and forest canopy cover. The results of present research were the probable annual water yield with different return periods which could be used in watershed management and water resources projects.

From the past، the rivers were the agent of engender of municipal، industrial and agricultural centers beside the rivers. By increasing the population، growing the technology، increasing the use of water resources and abnormal occupation، the quality condition of rivers are changed. In this respect، the analysis of the qualitative data and the investigation of local information could help us in affecting the quality of rivers for zonation and management control to the accepted level of standard for various usages. Godarkhosh، one of the country''s border rivers in Iran-Iraq border، is located in Ilam Province، with its basin of about 1202. 68 km2. The research area is Godarkhosh river basin which starts from its entrance to the province to its outlet to Iraq. Required parameters such as monthly Biological Oxygen Demand، Total Phosphate، Nitrate، Focal Coliform، Total Suspended Solids، Dissolved Oxygen، Turbidity، pH، and Temperature were measured in 10 sampling stations of Sheshdar، Tanghamam، Tasfiehkhaneh، MehdiAbad، Banvizeh، Chamab، Gardaneh Ghala، Ghebleh، Pole Felezi، and Pole Shekasteh، from autumn to winter 2010. Several studies have been performed regarding surface water quality mainly investigating the effect of unnatural water pollutants. In this research، the reciprocal effects of natural and unnatural factors on river water quality were investigated. Compilation of laboratory results with quality index information layers within the GIS environment demonstrated that Mehdi Abad sampling station with 47. 33 NSFWQI quality index is in a bad and the rest of the sampling stations with 50 to 70 NSFWQI quality index are in the average situation.

Environmental and economic impacts of soil erosion are due to improper land management in a watershed. Although it is impossible to stop soil erosion completely under natural conditions، there is a great need to control erosion for proper land use planning. According to the scarcity of literature in the area of soil nutrient evaluation with optimization models، the main objective of this research is determining the optimum level of land uses to minimize soil erosion and nutrients losses and maximize people incomes that live in a watershed. For this purpose، Abolabbas Watershed in the north-eastern part of Khouzestan Province was chosen for this research. A linear programming model was used in three different scenarios including current land use condition without land management، current land use condition with land management and standardized land use condition. Results demonstrated that the current land uses are not optimized for the least soil erosion and nutrient losses and high income. At optimized conditions، the area of forests and orchards increased by 1. 81 and 55. 7% respectively، rangelands with no changes، and irrigated and drylands decreased by 67. 5 and 31. 4% respectively. Also، results showed that land use optimization، in current land uses with no land management، decreases total soil erosion by 3. 2% and total nutrient soil losses by 2. 5 and increases total income by 29. 7%، in current land uses with land management، decreases total soil erosion by 35. 3% and total nutrient soil losses by 70. 2 and increases total income by 37. 2%، and in standardized land uses، total soil erosion by 47. 2% and total nutrient soil losses by 70. 4 and increases total income by 41. 8%. Sensitivity analysis، also، showed that the change in the area of orchards and Irrigated lands has the most effects on increasing income and decreasing soil erosion and nutrients losses in Abolabbas watershed.