Civil and Environmental Engineering
Henry M. Rowan College of Engineering
Hydrologic models--Calibration; Watersheds--New Jersey
Civil and Environmental Engineering
Proper calibration of hydrologic models requires both reliable observed stream flow and precipitation data. Southern New Jersey has a notable lack of observed precipitation data, in particular, at the event scale; therefore model calibration represents a significant challenge. From a design standpoint this has not been a major concern as hydrologic models have been driven using design storms and are not calibrated. However, the need for research and impact analyses in the face of climate change (changes in extreme precipitation and sea-level rise in particular) and expected population growth in the region requires calibrated models, and reliable observed precipitation data are necessary for model calibration. Several HEC-HMS watershed models were developed and calibrated using NEXRAD data for the Upper Maurice River watershed in southern New Jersey, which contains the two growing urban areas of Vineland and Glassboro, to meet three objectives: (1) to validate that NEXRAD data can be effectively utilized in this region, (2) to better understand the roles that spatial variability and scaling play in the use of NEXRAD data within a hydrologic model, and (3) to determine the possible effects of urbanization on stream behavior within this watershed. Gridded NEXRAD data were converted to virtual rain gages using the Watershed Modeling System and used as input for HEC-HMS models. Results indicate that models forced by spatially distributed basin characteristics (using multiple subbasins) and spatially coarse precipitation (fewer rain gages for the watershed) perform better in general than models driven by uniform basin characteristics and higher resolution precipitation data (multiple rain gages for the watershed) for large or extreme events (e.g. hurricanes). In addition, it was found that models driven by lower resolution precipitation (still accompanied by spatially distributed basin characteristics) also performed better than higher resolution models for smaller events, indicating that hydrologic models should be calibrated from finely distributed basin parameters and coarsely distributed precipitation for valid model application. Other results of this modeling indicate the following: NEXRAD data can be successfully utilized within this watershed if the NEXRAD input data is devoid of false positive and negative datapoints, or if these points are found and removed before use. The NEXRAD input data was determined to be the most important factor in determining any size model's accuracy. It was found that when using weighted rain gages that had been edited to remove false values as opposed to unweighted, unedited rain gages, model accuracy improved greatly. Therefore, the major determinant of model error does not seem to stem from the scale at which the model is built, but rather the quality of the input NEXRAD data used, and to a lesser extent, the use of basin characteristics that are compatible with the size of the rain event being considered. Finally, it was determined that an increase in urbanization in this watershed would likely cause a disproportional increase to the region's stormwater runoff. Model results have suggested that increasing the impervious area by as little as 5 percent would result in as much as a 70 percent increase in stormwater runoff for large or extreme events, and as much as a 37 percent increase for small events.
Siegel, Christopher, "Event-based hydrologic model calibration using NEXRAD in a data-poor region of southern New Jersey" (2015). Theses and Dissertations. 252.