Calculation of Areal Reduction Factors Using NEXRAD Precipitation Estimates

In general, larger catchments are less likely than smaller catchments to experience high intensity storms over the whole of the catchment area. Therefore, the conversion of point precipitation into area-averaged precipitation is necessary whenever an area,large enough for rainfall not to be uniform, is to be modeled. However, while point precipitation has been well recorded because of the availability of rain gauge data, a real precipitation cannot be measured, and its estimation has been a subject of research for the last decades. With the understanding that the Next Generation Radar (NEXRAD) precipitation data distributed by the U.S. National Weather Service (NWS) are the best data with spatial coverage available for large areas, this report addresses the estimation of areal reduction factors (ARFs) using this type of data. The study site is the 685,000-km2 area of the state of Texas. Storms were assumed to be elliptically shaped of different aspect ratios and orientations. It was found that, in addition to the storm duration and area already considered in previous studies, ARFs depend also on the geographic region and the precipitation depth, which is associated with the storm frequency for a given duration. Researchers also studied storm shape and orientation.