Quantification of urbanization impacts on discharge volume using H2U model

Urbanization tends to change in peak discharge and runoff volumes of natural catchments. In this study, the results of H2U model with the production function used for net rainfall calculation were compared to observed hydrographs. In the production function, mapped impervious area (MIA) and effective impervious area (EIA) values were tested for imperviousness parameter. A test catchment located in Tehran City was selected for this purpose whereas; part of this area was considered to compare pre and post developed conditions. Performance indicators verified that urban catchment hydrographs most closely resembled the observed hydrographs when using effective impervious area value in the production function. The comparison of pre and post development hydrographs in part of the catchment revealed urbanization leads to 95% increase in peak discharge rate and 50% decrease in unit hydrograph time base. The post developed condition runoff volume of 10 year return period rainfall event was 4.01 times more than pre developed runoff volume originated from decreasing hydraulic length and increasing impervious areas. Moreover; the results of this study revealed that the effect of Strahler order drainage network change on peak discharge rate was greater than decrease of hydraulic length.   Key words: Effective impervious area, morphological approach, net rainfall, urbanization.