Introduction: Infiltration is the most important physical properties of agricultural soils. Infiltration families are general relationships that attempt to categorize the infiltration behavior of soils. Walker et al. (2006) discussed the assumptions and procedures used to develop the original NRCS families. Those families categorize infiltration behavior according to their steady-intake rate and were developed largely from border irrigation data. As such, those families have been more widely adopted in border/basin irrigation analyses than in furrow studies. …

Considering the lack of hydrometrical stations in most small watersheds, determination of maximum flood discharge requires a suitable method to estimate runoff and peak flow thats used in flood and erosion control management plans. One of such methods is the graphical method presented by Natural Resources Conservation Service of the United States Department of Agriculture. To evaluate the applicability of this method, 19 watersheds in three regions of Iran Vis Karaj-Ghazvin region (region 1), Shomal region (region 2) and Azarbaiejan region (region 3) were selected. Among the different types of rain, type II exhibits a minimum error. The results showed this method had better applicability for estimation of peak flood in Karaj-Ghazvin and Azarbaiejan regions, but was more erroneous in Shomal region.

Introduction The Curve Number (SCS-CN) method, one of the most widely used methods for estimating runoff in hydrology, has been developed by soil conservation services. The application of SCS equation in Iran may be mixed with considerable errors due to different climatological, geological, and hydrological characteristics. In addition, the other cause of error in this method is the relationship between Ia and S (Equation 2), which is experimental and has been taken from rainwater data in small laboratories (Anonymous, 1972). This relationship can be criticized in terms of the document and was examined in various studies of its accuracy and application (Bo et al., 2011, Shi et al., 2009, Baltas et al., 2007, Mishra et al., 2006, 2004, Hawkins et al., 2002), In order to calibrate the SCS equation and evaluation in empirical relations, Pasekohak catchment was selected. Pasekohak basin includes four sub-basins where flood runoff was gauged during five flood events. Curve Number (CN) for the sub-basins was estimated according to the remote sensing, geological maps, and field works. The real hydrograph for each event was compared with SCS hydrograph in HECHMS software, and the calibration and correction of SCS coefficients were conducted. The results suggest a correction coefficient of 0. 92 for SCSLag, the average ratio of Ia/S is 0. 044, which is completely different from 0. 2 in SCS. The CN in the study area is 0. 41 of CN provided by SCS...

Kashafrud Watershed Area is the part of the Great Plains Gharehghom in the northeastern province of Khorasan Razavi with area of 8996 square kilometers and considered the main drain Mashhad plain. to determine flooding potential of Kashafrud watershed used of modified SCS Method, the GIS system according to classification of map watershed and Tamab grade 4 in the digital elevation model, mapping the distribution of precipitation, precipitation losses and land use. Meteorological data, including precipitation statistics and rainstorms (28 stations) related to the period received of the relevant organizations and reviewed. After calibration data based on a common time base for 25 years (1989-1988 to2013-2012) and remove of suspicious data, in some cases rebuild the lengthening period. Layers of information is used such as maps: weather stations location, topography, land use, soil, vegetation, Political divisions of province and divisions watershed of Tamab. receiving DEM and making any necessary modifications basin boundary and physiographic characteristics calculated such as area, perimeter, height, slope, time of concentration. value of CN calculated by using the land use map and soil hydrology groups performed. Analysis Storm determined by using the software SMADA and standard distribution function (Pearson type 3) and value of of rainfall and storm return periods of 2 to 100 years. After calculation of weighted average height of the runoff in each subwatershed (sub 86), to determine the flood zones watershed, estimates maximum flooding (Qmax) caused by rainfall effective and drawn flooding potential map of watershed. Of the total area of watershed is, 68. 25% of the normal flooding potential, 25. 5% of of critical and is 6. 25%. supercritical flooding potential. Cities Chenaran and Mashhad are flooding potential than other cities. From a total of 658 villages and population center recognized in watershed, 78% of the villages due to their placement in plain areas and low-slope are in the zone of potential flooding normal, 17% of the villages in the area with the potential flooding of critical and 5% of the villages due to the inclusion in the Highlands and steep with the potential flooding zone supercritical.

Locating the occurrence of flood has been very difficult using common and traditional methods. And most of the time leads to errors. The soil conservation service (SCS) of the U.S has developed a method for runoff estimation due to the accuracy of the SCS method, in this study, is used to estimate the risk of floods of SCS-CN method. purpose of this study, to estimate the risk of floods is using SCS-CN. In this method, due to the hydrological groups of soil and slope area, was obtained curve number or CN. Average CN shows number 87 in this basin. Therefore, using climatological stations, the average rainfall estimated 304 mm. Results showed that due to the loss of vegetation, expansion of the city from the south, land use change and the form basin is to be Stretched, in the southern part of the basin,  the probability of flooding is greater than the rest of the basin.

In recent years, many empirical methods and mathematical models have been developed to estimate runoff, among which the SCS curve number is the most important and widely used method. In this method a parameter called Curve Number (CN) is used for transforming rainfall to runoff. Currently, different techniques such as RS and GIS in combination with filed surveying are used to estimate this parameter. These methods due to the huge amount of required data, field investigation, and computation burden are costly and time consuming. In this research based on the saturation excess concept, which is used in the structure of a semi distributed model called TOPMODEL, a new efficient method with a minimum data requirement was developed to estimate the SCS curve number. The proposed method depends on three parameters: soil porosity, average distance to the catchment water table, and the parameter that controls the effective depth of the saturated soil. The Kasilian catchment in northern Iran is used to investigate the proposed method. In this study, using lansat7 ETM+ images and performing some modification on the images the landuse map was obtained. Finally, by combining this map and the watershed soil and slope maps and also the results of an extensive filed survey, the SCS curve number map was developed for the entire watershed. Also, TOPMODEL was calibrated in the Kasilian catchment for the estimation of proposed method parameters. Results indicate that the watershed's average curve number based on RS/GIS and the proposed methods are about 59.6 and 62.8, respectively. Furthermore, the results showed that the minimum and maximum differences between the Curve Number obtained from RS and GIS techniques and from the proposed method for the whole range of the used parameters are about 0.3% and 12% respectively.

The effects of DEM resolution on the curve number values as. Based on WI_CN method is addressed in the present research. This method, due to a use of TOPMODEL’s excesses saturation concept, is considerably dependent on the topographic index and subsequently on DEM cell sizes. The results of WI_CN method application in the Kasilian watershed for different DEM cell sizes indicated that the watershed averaged curve number in DEMs with resolutions of 50 and 300 meters amount to 59.8 and 71.8, respectively, which means when DEM resolution gets coarser the difference between WI_CN method and common methods (derived CN using GIS and RS techniques) increases. For instance the maximum differences observed between the two methods (in 50 and 300 meter cell sizes) are 8.3% and 29.9%, respectively. Therefore, when making use of the saturation excess based methods for deriving curve number raster maps, especially in ungauged watersheds, DEM resolution effects should be defined with respect to data resolution.

Flood is one of the major hydroecological phenomena that in each period causes damages to human achievements. Therefore, controlling this phenomenon is the subject of many researchers' works and studies specially hydrologists who have tried to predict the flood`s possible behavior by modeling it. This paper is trying to help one of the most widely used methods to estimate the maximum discharge in one of important catchment basins in Iran. So using the Dimensionless Unit Hydrograph method that is released by the Soil Conservation Organization of America, and according to the history of rainfall in each region, the likely quantitative estimates of flood volumes can be presented in each season. Although parameters in this method are diverse and frequently correlative, the results give accurate estimates of the flood discharge that are consistent with the input data. So in this study, to calculate the maximum discharge based on SCS method, numerical modeling has been used in a Matlab program and for this purpose, the 24-hour flood data at a four-year time period has been imported. At the end, to link the precipitation of maximum discharge using a weight-bearing function, a mathematical equation with the best adaptation with each subbasin’s data was extracted which can substitute with the lengthy equations in the intended area.

Flood is among phenomenon which bring about considerable damage each year. This has been attracted greatly by hydrologists. The factors such as physiography, geomorphology and human factors can accelerate this phenomenon in basins. For using water source, flood management, damming, watershed management and the most basic hydrologic studies, peak discharge is important, so accuracy of studies and the safeties’ level of the hydraulic structures and establishments are independent to it.In this research potential flood Zilaki and Firehrud drainage basins were studied by using of SCS method and finally were compared together. So Basin Hydrograph dimensions calculated by using of 24 hour rainfall, time of concentration, Curve Number, rainfall excess, time to peak and peak discharge.Then was design Hydrograph for basins in 2, 5, 10, 25, 50, 100 period time Results show, because of physiography of Firehrud basin, It has more ability for making flood than Zilaki basin.