Abstract. Flooding represents one of the most severe natural disasters threatening the
development of human society. A model that is capable of predicting the
hydrological responses in watershed with management practices during flood
period would be a crucial tool for pre-assessment of flood reduction
measures. The Soil and Water Assessment Tool (SWAT) is a semi-distributed
hydrological model that is well capable of runoff and water quality modeling
under changed scenarios. The original SWAT model is a long-term yield model.
However, a daily simulation time step and a continuous time marching limit
the application of the SWAT model for detailed, event-based flood simulation.
In addition, SWAT uses a basin level parameter that is fixed for the whole
catchment to parameterize the unit hydrograph (UH), thereby ignoring the
spatial heterogeneity among the sub-basins when adjusting the shape of the
UHs. This paper developed a method to perform event-based flood simulation on
a sub-daily timescale based on SWAT2005 and simultaneously improved the UH
method used in the original SWAT model. First, model programs for surface
runoff and water routing were modified to a sub-daily timescale.
Subsequently, the entire loop structure was broken into discrete flood events
in order to obtain a SWAT-EVENT model in which antecedent soil moisture and
antecedent reach storage could be obtained from daily simulations of the
original SWAT model. Finally, the original lumped UH parameter was refined
into a set of distributed ones to reflect the spatial variability of the
studied area. The modified SWAT-EVENT model was used in the Wangjiaba
catchment located in the upper reaches of the Huaihe River in China. Daily
calibration and validation procedures were first performed for the SWAT model
with long-term flow data from 1990 to 2010, after which sub-daily
(Δt=2 h) calibration and validation in the SWAT-EVENT model
were conducted with 24 flood events originating primarily during the flood
seasons within the same time span. Daily simulation results demonstrated that
the SWAT model could yield very good performances in reproducing streamflow
for both whole year and flood period. Event-based flood simulation results
simulated by the sub-daily SWAT-EVENT model indicated reliable performances,
with ENS values varying from 0.67 to 0.95. The SWAT-EVENT
model, compared to the SWAT model, particularly improved the simulation
accuracies of the flood peaks. Furthermore, the SWAT-EVENT model results of
the two UH parameterization methods indicated that the use of the distributed
parameters resulted in a more reasonable UH characterization and better model
fit compared to the lumped UH parameter.