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The nitrogen and phosphorus concentrations in the Iskar River, Bulgaria, during a first flood event have been modeled. The term “first flood event” refers to the response to the first rainfall event after a long dry period which has a major impact on river nutrient loads. The HSPF model had been calibrated with data for 2 years (2000–2001) and verified using data for a further year for a sub-catchment of the Iskar River (from the source to Iskar reservoir). In order to test the ability of HSPF to describe nutrient pollution during a flood event, the calibrated daily time step model was run for such an event with two time steps—daily and hourly. The results of the hydrologic simulation gave a difference in water balance for the period of less than 5%. Percentage differences between observed and simulated values for the nitrogen load were calculated as 13.1% and 18% for hourly and daily simulations, respectively. Percentage differences between phosphorus measured and simulated loads were larger at 16.6% and 34.4%. The simulations performed here suggest further application of the HSPF model may be valuable in providing a better understanding and in forecasting nutrient concentrations during first flood events

The nitrogen and phosphorus concentrations in the Iskar River, Bulgaria, during a first flood event have been modeled. The term “first flood event” refers to the response to the first rainfall event after a long dry period which has a major impact on river nutrient loads. The HSPF model had been calibrated with data for 2 years (2000–2001) and verified using data for a further year for a sub-catchment of the Iskar River (from the source to Iskar reservoir). In order to test the ability of HSPF to describe nutrient pollution during a flood event, the calibrated daily time step model was run for such an event with two time steps—daily and hourly. The results of the hydrologic simulation gave a difference in water balance for the period of less than 5%. Percentage differences between observed and simulated values for the nitrogen load were calculated as 13.1% and 18% for hourly and daily simulations, respectively. Percentage differences between phosphorus measured and simulated loads were larger at 16.6% and 34.4%. The simulations performed here suggest further application of the HSPF model may be valuable in providing a better understanding and in forecasting nutrient concentrations during first flood events