directly measured by current meter surveys or are estimated by rating-curve extension or indirect measurement techniques (Rantz and others, 1982). Jarrett (1987) stresses the need to assess the reliability of extreme flood data, particularly data collected before 1950.

Flood frequency analysis of systematic flood data typically assumes that the data are independent and identically distributed in time (i.e., temporally uncorrelated and stationary). Local human activities, such as land use changes or reservoir construction, or climate change (regional or global) can make this assumption untenable. There are relatively few gaged streams on watersheds that have not been affected to some degree by human activities. At the same time, there are relatively few cases where human impacts on flood magnitude and frequency have been carefully documented. Lins and Slack (1999) evaluated flood data from watersheds that are considered to be relatively unimpacted by local human activities and did not find compelling evidence of climate-induced non-stationarity for floods. Note, however, that it is very difficult to detect climate-induced non-stationarity in flood data because of the high variability (Jarrett, 1994).

Precipitation Data

The National Weather Service is responsible for maintaining a network of meteorological stations in the United States. The current network includes about 300 primary stations staffed by paid technicians and over 8,000 cooperative stations operated primarily by volunteers (NRC, 1998b). As of 1975 there were about 3,500 non-recording precipitation gages with records of 50 years or more (Chang, 1981). Precipitation data are published in Climatological Data and Hourly Precipitation Data by the National Oceanic and Atmospheric Administration (NOAA). Digital records can be obtained from the National Climatic Data Center, from regional climatic centers, and various vendors. Note that many digital records do not include data collected prior to 1940.

Another source of extreme precipitation data for the United States is a catalog of extreme storms maintained by the U.S. Army Corps of Engineers, the U.S. Bureau of Reclamation, and the National Weather Service. This catalog includes information on over 300 extreme storms including the 1862 storm in California and the Pacific Northwest. For each storm in the catalog, official climatic data as well as rainfall bucket survey data (if available) were compiled and storm characteristics analyzed. It should be noted that extreme storms and floods are not as well documented today as they were in the past, in spite of technological improvements that greatly facilitate such documentation.

Precipitation data are subject to large errors. The most serious problem is the undermeasurement at all operational precipitation gages by amounts that depend primarily on the type of gage (including wind shield), exposure, wind speed, and whether the precipitation is rain or snow. Precipitation measurements during snowfalls are particularly biased. For example, during a snowfall a wind-shielded gage typically undermeasures precipitation by about 40% in a wind of 25 km/hr (Larson and Peck, 1974). In the case of a systematic rainfall record, the problem may be exacerbated if the location and type of precipitation gage is changed during the



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