Sensitivity of Simulating Hydrologic Processes to Gauge and Radar Rainfall Data in Subtropical Coastal Catchments

This research evaluated the impact of three rainfall datasets on hydrologic process simulations in two coastal catchments located in Alabama. In this study, rain gauge time series recorded by the U.S. National Oceanic and Atmospheric Administration (NOAA) and Geological Survey (USGS) along with radar precipitation data derived from NOAA National Weather Service (NWS) were input into the Hydrological Simulation Program—FORTRAN (HSPF). Automatic parameter calibration was performed using daily streamflow data recorded at USGS Fish and Magnolia River gauge stations from 07/01/2002 to 12/31/2008. HSPF parameters were optimized using the Model-Independent Parameter Estimation (PEST) program. Model parameter ranges were refined by incorporating physical characteristic of the study areas and after analyzing observed streamflow time series. This approach, in turn, helped PEST optimization tool to find the most physically-related set of parameters that can be transferred to any watershed with similar characteristics and minimum parameter calibration. On average, annual USGS and radar rainfall values were around 480 mm and 250 mm, respectively lower than NOAA precipitation records. Overall, it is found that the NOAA precipitation input data resulted in better daily flow simulations than results from radar and USGS rainfall time series. Streamflows derived from USGS rainfall time series showed the worst model performance at both catchment outlets because of missing data, low amounts, and temporal delay of peaks. This study found that annual actual evapotranspiration values were closed among rainfall time series and varied from 900 to 958 mm. Deep percolation values for Magnolia and Fish River, regardless of rainfall source, ranged from 66 to 192 mm/year. Major discrepancies were found at storm runoff values. Gauge rainfall time series yielded the closest streamflow values compared to observed flow time series at both watershed outlets. Rainfall derived from radar yielded consistent and acceptable runoff results in Fish and Magnolia River models. In both case studies, the high spatial variability of rainfall storm events was not adequately captured by any of the rainfall datasets and yielded high uncertainty in model results.