Journal of Range and Watershed Managment

Abstract Studying temporal and spatial variations of soil erosion and the effective factors is very important for providing appropriate solutions to properly plan the utilization of watershed resources. However, few studies have attempted to examine temporal and spatial variations of soil erosion. In this study, the temporal variability of spatial participation of soil erosion has therefore been investigated in the Gonbad representative and treated watersheds in Hamedan Province with an area of 300 ha. For this purpose, the work units were designated through overlaying land cover type, soil texture, and slope maps. The soil erosion was then measured in each work unit using rainfall simulator with intensity of 50 mmh-1 during 30 minute in two main rainy months of November and October 2014. Then, three points with relatively similar characteristics and representing each work unit were determined in the specific outlines of the equilateral triangle. The soil erosion was consequently measured with five minutes interval after the beginning and ending of rainfall simulation. The results of analysis of variance verified a significant difference (P<0.01) among different work units in soil erosion in both representative and control watersheds verifying spatial variation of different parts of the watersheds in soil erosion and consequent sediment yield. While, no significant difference was found using t-test between soil erosion during November and October 2014.

Abstract Regarding the undeniable role of rainfall erositivity factor in initiating water erosion, studying its different aspects is important in optimal soil and water resources management. It is taken in to account in many soil erosion estimation models which are used for soil and water conservation. However, the impact of data length, study span, and the number of stations on variability of rainfall erosivity factor has been understudied. The present study therefore is an attempt to investigate the temporal variation of Wischmeier and Smith’s rainfall erosivity factor at different time scales and also the effect of data length, study span, and number of stations upon rainfall erosivity factor is scrutinized. Accordingly, the results of the present study with 70 stations, data span of 20 years and different study periods were compared with those obtained for another study with 18 stations and 23 years of data span. Rainfall erosivity factor of over 12,000 storm events was calculated in present study and mean values for different time scales were compared using t-Test. Results showed that the maximum and the minimum values of monthly rainfall erosivity factor in the country were different from each other. Besides, the results of t-Test showed significant difference between the calculated values of rainfall erosivity factor in some months (p<0.05) and seasons (p<0.05). Nonetheless, the difference between annual rainfall erosivity factor was not significant (p<0.05).

Abstract Understanding and quantifying the dynamics of suspended sediment transport is essential to control soil erosion and implement appropriate mitigation practices to reduce stream suspendedsediment and on-site and off-site effects of sediment transport. However, the quantification and the interpretation of sediment hysteresis loops as fluvial and hydrological responses of the watershed to storm events has been less considered. Hence, the present study was conducted in three different regions viz. the Educational and Research Forest Watershed of TarbiatModares University in Mazandaran, the Galazchai Watershed in West Azerbaijan and the Daretefi Watershed in Kurdistan Provinces to investigate the dynamic of sediment hysteresis loops on storm wise basis. The results of analyzing 67 storm event hysteresis loops verified the variety of governing condition of all three study watersheds. According to the results, all of the sediment hysteresis loops showed flushing behavior but with different rotational patterns of clockwise, counterclockwise and 8-typed and complex. Despite of differences and more diversity in precipitation pattern, compared with two other study watersheds, Daretefi Watershed sediment hysteresis loops showed less variety and mainly with 8-typed and complex forms with frequency of 61.29%. Results also revealed a frequency of 44% for both clockwise and complex hysteresis loops in the Galazchai Watershed and frequency of 53.33% for complex hysteresis loops in the Educational and Research Forest Watershed of TarbiatModares University. The results also showed that the sediment sources in the study watersheds were mainly close to the outlet with further emphasis on wash load contribution.

Abstract The hydrograph at watershed outlet can be expressed as hydrological response of watershed to geomorphological behavior. In present study with the aid of the GIS and the concept of instantaneous unit hydrograph, the watershed response was simulated. In present physical method, the IUH is derived from three parameters of time of concentration, storage coefficient and time-area curve of the basin. In the present study, the accuracy of various methods of determining time-area curve in providing Clark instantaneous unit hydrograph viz. channel profile, average velocity and Laurenson methods were therefore evaluated. The comparison of estimated and observed hydrographs by using qualitative and quantitative criteria showed that the average velocity method has had the highest accuracy in estimation of time-area method. The results of the quantitative statistics of root mean square of error, bias in peak discharge, coefficient of efficiency, and relative errors in peak discharge, time to peak and base time were respectively found to be 1.39, 0.93, 0.83, and 7.13, 33.33 and 15.38%. The results of this research also showed that the Clark model had a good efficiency in simulation of unit hydrograph in Kasilian watershed, Mazandaran Province, Iran.
 
 

Abstract The behavior of suspended sediment during flood events is not only a function of energy conditions, i.e. sediment is stored at low flow and transported under high flow conditions, but also is related to the variations in sediment supply and sediment depletion. These changes in sediment availability result in so-called hysteresis effects. Therefore, Hysteresis pattern analysis is of great importance in sediment studies in the watersheds. However, their analyses has been rarely considered. In this study, based on the discharge and sediment concentration data collected from 8 storm events occurred during March 2 011 to April 2012, event suspended sediment dynamics of 7 tributaries of the Lake Zarivar watershed was investigated using hysteresis patterns. Based on the fact that all sampling points were not active in all events, about 46 hysteresis patterns were obtained. The analysis of results showed that 16, 13, 11, and 6 events had clockwise, irregular, complex and counterclockwise patterns, respectively. Small tributaries of the Zarivar lake watershed showed the rapid responses to the variation of storm intensity and the most hydrographs of different storms were multi peak discharges and consequently high suspended sediment variations led to different hysteresis patterns. The diversity of patterns suggested that the detailed processes of sediment transport were not only complicated during one event but also varied from event to event. The reasonable and statistically significant relationship (p<0.05) between suspended sediment yield and peak discharge of each sampling point indicated that the data from all events may be statistically well described by a simple regression equation, regardless of different inter and intra-storm variations of the suspended sediment.

Abstract Todays, dynamic models are supposed as the most important tools in erosion and sediment phenomena
due to their complexities and existence of many affecting factors. Towards, the present study was
conducted in the Kojour watershed for daily sediment modeling using daily rainfall, discharge and
sediment during 2007 to 2010. The modeling process was carried out all data and the monthly and
seasonally classification data in linear and nonlinear models. The results indicated that daily linear and
non-linear models did not indicate a suitable model. The monthly and seasonally classification of the
data led to achievement of better models with determination coefficient significant at 5 percent level
and relative error less than 40 percent as compared with those obtained from no classification. It was
also found out that daily sediment of Kojour watershed was affected by discharge occurred event day
and before four days. The discharge occurred event day is the most effective factor in 80% selected
models in the study watershed. The nonlinear models were better estimation than linear models in
July, September, December and March and autumn but linear models were better than nonlinear
models in other months and seasons.