Evaluation of the Effects of Automatic Extraction Methods of Physical Parameters of River on Accuracy of Empirical Equations for the Calculation of Time of Concentration of Shallow Concentrated Flow (Case study: Deh-Bala Watershed, Yazd)

Time of concentration is the time required for all parts of the watershed area to take part in the outflow discharges. Application of Arc GIS software for extracting of physical parameters of river network in some extents has automated this approach. But it should be kept in mind that their evaluation is one of the most important necessities. The purpose of the study is to evaluate the effects of Arc GIS automatic extraction method of river network physical parameters on the empirical equations of shallow concentrated flow travel time and time of concentration. Therefore, as the channel slope that is the intended parameter and extracted using the automated method in this study, was also measured by the field surveys at 21 hydrological homogenous reaches in Deh-Bala rural watershed. Comparison of the measured and extracted slope values showed 26 to 54% error in the automated method depending on two arbitrary selected cell conditioning (500 and 1000 cells to determine outlet cell in raster DEM). Calculation of travel time using five empirical equations including Kirpich (1940), Carter (1961), Espey (1966), Yen-Chow (1983) and Bransby-Williams (1964) by the field and the automatic methods revealed that these equations show different efficiencies based on their structure and sensitivity to the parameters. Comparison of the field and automated methods based on three statistic criteria including R2, RMSE, and average of percent errors (APE) showed that for the 26-54% errors in the slope values in the automated method compared with field surveys measurements; Espey equation presented the best results both in 500 and 100 cell conditions. In the estimation of concentration time it was detected that Espey and Bransby-Williams equations presented the best results, respectively.