Depth-area relationships, such as those published by the National Weather Service in TP 40 and the NOAA Atlas 2, enable conversion of point rainfall depths to areal average depths for the same storm duration and recurrence interval. This problem of conversion is most germane to hydrologic analyses for moderate to large drainage basins, where point rainfall depths are not representative of the spatial distribution of a storm event. Historically, depth-area relationships have been developed on the basis of data from dense networks of recording gauges. However, with the ongoing accumulation of radar-rainfall records, radar-rainfall data represent an alternative to gauging data. This paper summarizes what is believed to be the first study made under the auspices of the National Weather Service ͑NWS͒ for evaluation of the potential of NEXRAD radar-rainfall data for development of geographically fixed depth-area relationships. Objectives were to evaluate the use of radar-rainfall data for development of depth-area relationships and to identify potential obstacles that might hinder use of such data. Data analyzed for this study are those recorded for the Arkansas-Red Basin River Forecast Center ͑ABRFC͒, and span the period of time from May 1993, to September 2000. Conclusions of this study are that data heterogeneities and shortness of data records are major factors limiting development of depth-area relationships on the basis of radar-rainfall data. Possible biases in radar estimates of extreme rainfall are also of concern. Depth-area curves developed for the ABRFC, presented herein, are reasonably consistent with those presented in NWS publications but should only be considered as preliminary.
The rainfall frequency atlases and technical papers published by the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) serve as de-facto national standards for rainfall intensity at specified frequencies and durations in the United States. This paper reports on progress in updating these estimates since the EWRI World Environmental and Water Resources Congress of 2006. It provides an overview of the new estimates and the methods used in their preparation as well as selected statistics. Since the 2006 Congress, NOAA has published revisions for NOAA Atlas 14 Volumes 1 through 3 covering the semiarid southwest U.S. and the Ohio River basin and surrounding states, and Puerto Rico and the U.S. Virgin Islands.
Bonnin, Geoffrey M., Kazungu Maitaria, and Michael Yekta, 2011. Trends in Rainfall Exceedances in the Observed Record in Selected Areas of the United States. Journal of the American Water Resources Association (JAWRA) 47(6): 1173–1182. DOI: 10.1111/j.1752‐1688.2011.00603.x Abstract: Semantic differences have led to a gap in the understanding of the impacts of climate change on precipitation frequency estimates. There is popular perception that heavy rainfalls have become more frequent, and that this trend will increase with global warming. Most of the literature examines this question from the point of view of climatology using definitions of “heavy,”“very heavy,” or “extreme” rainfall, which are different from those commonly used by civil engineers. This article identifies the differences in meaning used by the climate and civil engineering communities and examines trends in the observed record in the frequency of exceedances (not trends in magnitudes). Using concepts recognized as the basis for design of the Nation’s civil infrastructure, we look at trends in the number of exceedances of thresholds for a variety of precipitation frequencies and event durations used by civil engineers. We found that the estimated trends in exceedances at one‐day and multiday durations were statistically significant and increasing for the Ohio River Basin and surrounding states but the reverse was true for the Semiarid Southwest (i.e., not significant and decreasing trends). In addition, we found the magnitude of the trends was small for all but the more frequent events and also small with respect to the uncertainty associated with the precipitation frequency estimates themselves.
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