نوع مقاله : مقاله پژوهشی
عنوان مقاله [English]
The prediction of stream flow from a catchment outlet has been of concern to hydrologists and water resource community. To this end, the unit-hydrograph approach proposed by Sherman has evolved into one of the most powerful tools in applied hydrology. A unit hydrograph is defined as the direct runoff hydrograph (DRH) resulting from the unit depth of effective rainfall hyetograph (ERH) falling uniformly over the drainage area at a constant rate for an effective duration. The unit hydrograph (UH) at a specific point on the stream (gauging site) in a catchment is generally determined by using effective rainfall and surface runoff data observed in the gauging site. The relationship between hydrologic and geomorphologic parameters, can lead to estimate the hydrologic response of basins and is applied to the similar areas, in terms of geomorphology, that are ungauged. For ungauged watersheds and for anticipated post development conditions, synthetic unit hydrograph (SUH), based on watersheds, geomorphology and climatic characteristics, is typically used. Such conditions are common in Iran, as most watersheds lack gauging stations along its rivers and streams discharge outlets. Catchments and storm characteristics are the primary parameters that affect complex watershed responses to rainfall events. The production and behavior of runoff are functions of land use types and changes. The hydrological response of a river basin is based on the relationship between basin geomorphology (catchments area, shape of the basin, topography, channel slope, stream density and channel storage) and its hydrology. In this context, several methods have been proposed. This study has been compared the consistency, accuracy and reliability of geomorphologic, geomorphoclimatic and Rosso instantaneous unit hydrographs and SCS unit hydrograph, in estimating outlet runoff hydrograph characteristics in Manshad watershed. The Manshad watershed encompasses an area of about 60 km² and is located in central Iran in the western part of the Yazd province. A hydrometric station is located at the outlet of the watershed (Dare Station), and a raingauge station (Manshad Station) is located at the upstream of the hydrometric station. In the present study, six single rainfall- runoff events, in which snow did not melt, were selected. Both rainfall and runoff data were recorded automatically with 1 h intervals. Then, for separation of the effective rainfall from the recorded storm for each chosen event, the CN-SCS method was applied. Flow velocity was obtained by calibrating historical data or with methods appropriate for ungauged basins. Arcview GIS software was used for extracting geomorphic information on the area, slope, length of the main river basin and Horton’s geomorphologic ratios such as area ratio (Ra), bifurcation ratio (Rb) and length ratio (Rl) in the studied watershed as inputs for the models leading to the runoff simulation. Geomorphologic parameters, namely the average value of the bifurcation ratio (Rb), stream length ratio (Rl), and stream area ratio (Ra), were determined to be 5.29, 2.40 and 6.09, respectively. The Strahler’s ordering scheme was followed for ordering of the river network. The study basin was discovered to be a fifth-order basin. Then, the GIUH, GCIUH, Rosso IUH and SCS models have been developed for estimation of the DSRO hydrographs for the catchment.
Comparisons of calculated and observed hydrographs, that have been carried out by using the Mean Relative Error, the Root Mean Square Error and the Nash-Sutcliffe indices confirm the high potential of all the methods in simulating the amount of flood volume (with an error less than 12%). These results are in agreement with the results of Sorman (1995). Thus, they can be used as appropriate methods for simulating this parameter in similar catchments. In estimating peak discharge, Geomorphologic and Rosso IUH (with error of 39.03% and 42.35%, respectively) have better performances, comparing to other methods. They have weak performances in simulating time to peak and the shape of hydrograph. Also, Geomorphoclimatic and SCS methods have weak performances in estimating peak discharge, time to peak and the shape of hydrographs.
The GCIUH and SCS models had weak performance for estimating time to peak, peak discharge and the shape of outlet DSRO hydrographs and the GIUH, Rosso IUH had weak performance for estimating time to peak and the shape of hydrographs. This may relate to exposing the watershed to human activities in the form of urban development and land use changes which cause changes on watershed hydrographs.