مجله علوم و مهندسی آبخیزداری ایران
پیاپی 23 (زمستان 1392)

Surface sealing formation in soils of arid and semi-arid areas is one of the most important factors of soil permeability reduction, formation of runoff and interrill erosion. The objective of this study was to investigate the impact of surface sealing on runoff and interrill erosion rates. This experiment was conducted in a small flume with dimensions of 1×0.5 meter under four rainfall intensities (21, 38, 50 and 81 mm/h) and three different slopes of 5, 15 and 25 percent. Three soils with contrasting soil texture namely as S1 (sandy loam), S2 (clay loam) from farmland use of Ilkhichi, and a clay soil (S3) from farmland use of Caleibar area, East Azerbaijn, Iran were selected in the current research. Relative Sealing Index (RSI), the need time to reach to the final infiltration rate (tlim), runoff and sediment rate were measured in each combination of slopes and rainfall intensities. The results showed that S1 with RSI= 2.23 and tlim= 6 min was the most sensitive soil and S3 with RSI= 8.9 and tlim= 102 min was the least sensitive soil to surface sealing. For soil S1 as a sensitive soil, the amount of runoff at rainfall intensity of 50 mm/h decreased due to increment of slope, but for the rest of gradients and soils runoff rate was not affected by increasing slope. The rate of sediment loss in S1 and S2 Soils decreased due to surface seal formation during rainfall, but there was no clear trend of sediment loss in soil S3.

Rainfall characteristics are different spatially and temporally and this issue has impact on runoff. So, considering the spatial and temporal distribution of rainfall is one of the most important factors in the simulation of rainfall-runoff. Spatial and temporal distribution of rainfall is not same in a basin due to its topographic and climatic variations, but to simplify the calculation of storm movement in sub basins, this issue is not considered in most studies and is led to the under or over estimate of the peak discharge of runoff. So, in this research, at first spatial distribution of rainfall was calculated in each sub basin using the recorded data of rainfall in daily rain-gauge stations of Amirkabir dam basin and Inverse Distance Weighting (IDW) method. Temporal distribution of rainfall was examined in two cases. At the first case, that is a widespread method for regions without station, the used rainstorm in all sub basins considered as same as the nearest stable rain-gauge station and in the second case, time of rainfall occurrence in sub basins has been estimated based on the distance of the center of gravity of sub basins from the nearest rain-gauge station to the study area and the time of rainstorm in the other rain-gauge stations and then was shifted based on the hyetograph of rainfall in sub basins. To evaluate the impact of temporal shift of rainfall on peak discharge, simulated runoff was compared in two stable and moving storm cases. Results of rainfall simulation for five selected storm events showed that temporal shift of rainfall (second case) significantly has improved the estimation of peak discharge and time to peak of the rainfall compared to the first case.

Drought is a natural hazard which affect on the social and economic status of the country based on short, medium and long term. Timely and accurate monitoring of droughts is the main purpose of this research in planning to reduce the losses caused by drought. Any delay in the forecasting of droughts, is an obstacles to the correct determination of drought for the affected region. The exact assessment of the drought should be based on determination of three meteorological, agricultural and hydrological drought and then integration of the three droughts will able us to define the occurrence and extent of the drought in one area. In this study, the standardized precipitation index (SPI) and the standard discharge index (SDI) and the normalized difference vegetation index (NDVI) as a representative of vegetation (grassland and dryland) was used for a long period and the information layer was used in a GIS environment. The results showed that different types of drought are not matched in terms of time and space. The results also show that during the period of years with above average rainfall, agricultural drought (grassland and dryland) will dominate and in years with less than average precipitation (drought) conditions, rangeland and dryland will behave more accurate than normal conditions. The research has shown that due to the spatial nature of the drought in large areas, using appropriate spatial and temporal resolution of satellite images, is essential in the better assessment of the drought.

Many studies have focused on temporal and spatial distributions of erosive wind speeds, however there exist a few studies for probability of erosive winds in different return periods. As there are not enough wind speed measurements or the data sets are short and insufficient for reliable estimations of wind speeds in different return periods, we used the method of L-moments to transfer at-site erosive winds data to the region. The results of homogeneity test (Hi) showed removing discordant sites, the entire region could be considered as homogeneous. According to goodness-of-fit test measure, Generalized logistic distribution (GLOG) gives the best overall results. The cluster analysis (CA) using Wards method showed four regions of erosive winds in Iran such as Northwest, coastal, central and Zagrous foothills regions. The homogeneity test of L-moments showed that most of these regions are homogeneous. The evaluation of influence of elevation on the type of the best fit distribution showed that 3-parameter distributions gave the best results for low elevated regions. This may be due to complexity of wind speed regimes of these regions.

The assessment of soil erosion hazard can identify critical regions in a watershed and prioritize management plans and soil conservation measures. This study aims to present a new approach for mapping potential soil erosion hazard using fuzzy logic and GIS in the Gharnave watershed, Golestan Province. Three main factors affecting soil erosion processes including slope (%), soil erodibility and rainfall erosivity were prepared as input layers of the fuzzy model. The slope factor map (%) was obtained from a Digital Elevation Model (DEM) with a spatial resolution of 30m, provided by ASTER global DEM data. The rainfall erosivity factor raster map (R) was generated using interpolated annual R values at 14 stations and utilizing the Ordinary Kriging (OK) Geostatistical method and Gaussian semi-variogram model. Then, the fuzzy maps of the three layers were made by defining appropriate membership functions. These maps are combined by applying fuzzy rules in the MATLAB software. This led to a fuzzy based soil erosion hazard map. In addition to that, a potential soil hazard map using the RUSLE model was created as a reference map for the accuracy assessment purpose. The accuracy assessment undertaken between the two maps showed an overall accuracy of 58% and a Kappa coefficient of 0.76 for the very high (VH) hazard class. This implies that there is an acceptable agreement between the results obtained from fuzzy logic and RUSLE models.

Zanjan River is one of the main branch of Ghezel Ozan River which most of cultivated lands in Zanjan province are located around it. The depth of sediment in some area around the rive is about 30 meters and due to flood condition on this river, water have eroded riversides and have destroyed agricultural farms and caused the river to become meandrous. Due to economic and environmental aspects in river engineering works, a suitable method for protection of riversides is bioengineering which uses plant roots to stabilize riversides. Thus, this study has focused on mechanical strength of plant roots in this area to protect riversides from water erosion. In this research, the native plants of riversides were studied and the tensile resistance of their roots was measured in laboratory. The distribution and depth of penetration of roots were determined and shear resistance of soil with and without roots was measured and compared. The results of this experiment show that tamarisk and elm trees increased the shear strength of soil up to 30 – 50% in riversides. With respect to characteristics of these trees, it is possible to use these trees to stabilize riversides of Zanjan River. Tamarix, Populous Euphratica and willow trees with local reduction of water speed increased sedimentation in riversides with respect to characteristics of these trees it is possible to use them in biotechnical projects of riversides.

The present research studied the performance evaluation of watershed management measures in Ekbatan Dam watershed. Therefore, runoff, sediment and precipitation data for time interval of 1963-2005 were collected and divided into two periods of pre and post of implementation of measures, i.e. 1963-1984 and 1984-2005. The various statistical methods include double mass curve, moving average, discharge-duration curve, hydrologic regime and sediment evaluation of reservoir used for each periods and by comparison of them, efficiency of watershed management activities studied. All of the used methods approved the positive performance of watershed management activities in the watershed hydrologic condition.

The appropriate design of channels and prediction of stable cross sections make reductions on economical cost and preventing damages on riverside engineering works as well as agriculture lands. Researchers have conducted several procedures to achieve some design equations for prediction of stable channels. One of the procedures is so called mechanical procedure. In present work, according to introducing a cosine form of stable section, width and depth prediction relations are developed. Two sets of data points including 93 experimental and 126 field data taken from sandy bed channels are used to evaluate the equations presented herein. The obtained equations indicate that mean depth and width of channel related to bank-full discharge gives a relation as 0.415 in power of discharge, and it gives a relation as 0.17 to power of mean flow velocity. The evaluation results show that the predicted equations have good agreements with experimental and field data.