Flood Damage in the United States, 1926-2003
A Reanalysis of National Weather Service Estimates

Data-Related FAQs

1. What data sets are available here?

   The following data sets are available on this WWW site:

   • Estimated flood damage in the U.S. (1926-1979 and 1983-2003, by fiscal year;
   • Estimated flood damage for each state in the U.S. (1955-1979, by calendar year, and 1983-2003, by fiscal year); and
   • Estimated flood damage, by river basin, for the U.S. (1933-1975, by calendar year).

   Little flood damage data was collected by the NWS during 1980-1982 and large errors were discovered in estimates developed later for that period. As a result, the years 1980-1982 are excluded from the reanalyzed data sets.

2. How does the NWS collect flood damage estimates?

   The NWS operates approximately 120 field offices distributed across the U.S. and its territories. In each office, a Warning Coordination Meteorologist (WCM) issues storm and flood warnings for an assigned forecast area. The WCM is also responsible for submitting monthly reports on severe storm events to the NWS, including deaths and estimates of damage to property and crops. Field office staff obtain the damage estimates from contacts in their area such as emergency managers, insurance agents, local officials, and newspapers. Crop damage estimates are obtained from U.S. Department of Agriculture agents and reports. The descriptions, deaths, and damage estimates are published monthly in Storm Data, a publication of the National Oceanographic and Atmospheric Administration (NOAA).

   A meteorologist at the NWS Hydrologic Information Center (NWS-HIC) collects the flood damage reports from the field offices and checks the damage estimates. NWS-HIC staff are in a good position to track damaging floods because they receive the first flood warnings issued by all the field offices. They also receive monthly summaries of significant hydrological events from the field offices. Hence the meteorologist is aware of most flooding events as they occur, receives narrative descriptions monthly, and can check whether estimates are received for all severe floods. In most cases, damage information is collected within three months after the flood event.

3. Where did the data for this project come from?

   For nearly a century, the NWS and its predecessor, the U.S. Weather Bureau (WB), have collected flood damage estimates through a nationwide system of field offices. Since 1983, the NWS Hydrologic Information Center (NWS-HIC) has been responsible for compiling the estimates, which are published annually by the U.S. Army Corps of Engineers (USACE) in the Annual Flood Damage Report to Congress.

   The reanalyzed data sets presented here augment published NWS data with information from NWS files and reports of other federal and state agencies, including the USACE, the U.S. Geological Survey, the Tennessee Valley Authority and numerous state emergency management agencies. The primary published sources of flood damage estimates from the NWS and WB are listed in the following table.

   Publication	Years of Flood Damage Included	Information Provided
   Report of the Chief of the Weather Bureau (WB)	1918-1933	Describes large flood events. Occasionally gives flood damage estimates for individual large events. (First national flood damage total reported in 1934.)
   Monthly Weather Review (WB, 1934-49)	1933-1947	Annual summaries describe damage in major floods. Tables give estimated damage for all major river drainages.
   Climatological Data, National Summary (WB, NOAA, 1950-77)	1948-1977	Monthly summaries describe flood damage and deaths in "notable" flood events. Annual summaries through 1975 give tables of damage in major river drainages. General summaries for 1972 and 1975 also give damage by state for each calendar year since 1955 and national flood damage and deaths by month and year since 1925.
   Storm Data (WB, NOAA)	1959-present	Monthly reports on storm events sometimes give brief descriptions of damage. Estimated damage to property and crops checked off on logarithmic scale until 1994, reported in thousands of dollars since 1995.
   Annual Flood Damage Report to Congress (USACE)	1983-present	Annual reports describe major flood events and provide table of flood damages suffered, by state. Recent reports give 10-year summary tables of flood damage and deaths, by state.

4. Why do the damage estimates here differ from those on the NWS website?

   Flood damage estimates provided here are the result of an extensive reanalysis and evaluation of data collected by the NWS, supplemented by data from other federal and state sources.

   Estimates of annual total U.S. flood damage provided on the NWS website (nws.noaa.gov/oh/hic/flood_stats/Flood_loss_time_series.htm) include three U.S. territories: Puerto Rico (since 1975), the Virgin Islands (since mid-1980s), and Guam (since 1994). In the reanalyzed data, we subtracted estimates for the three territories from the NWS totals to create a more uniform time series.

   A number of clerical errors were found and corrected. For example, the NWS website states that all damages were summed by fiscal year (Oct-Sep) when, in fact, the national data was summed by calendar year prior to 1983.

   The NWS ceased publication of annual flood damage summaries after 1975. Publication of comparable damage estimates did not resume until 1983, when USACE reports made damage estimates available again at the state and national levels (but not at the river basin level). To compile a complete time series of annual estimates required finding additional flood damage estimates for the years 1976-1982. Despite a curtailment of effort, the NWS continued to compile some damage estimates during 1976-1979, which served as a starting point for our reconstruction attempts. We compiled estimates for 1976-1979 based on information in NWS files and reports from other sources. Although we tried to reconstruct estimates for 1980-1982, there were not enough sources of information, either from NWS or other agency publications, to provide estimates of comparable quality to the overall data set.

5. How accurate are the data sets?

   Evaluation of the accuracy and consistency of the estimates led to the following conclusions:

   • The collection and processing of flood damage data by the NWS has been reasonably consistent from 1934 to the present, except during the period 1976-82. Errors are probably somewhat larger in the first few years after data collection resumed in 1983.

     Data from NWS files and other sources made it possible to reconstruct state and national flood damage estimates for 1976-79. However, little data was collected during 1980-82 and large errors were discovered in estimates developed later for that period. As a result, the years 1980-82 have been excluded from the reanalyzed data sets. Annual compilation of damage estimates resumed in 1983, but depended mainly on information from Storm Data in the first few years. Particularly in 1983-84, omissions are more likely and estimates probably contain larger errors because of the use of damage categories.

   • Individual damage estimates for small floods or for local jurisdictions within a larger flood area tend to be extremely inaccurate.

     When damage in a state is estimated to be less than $50 million (1995$), estimates from NWS and other sources frequently disagree by more than a factor of two.

   • Damage estimates become more accurate at higher levels of aggregation.

     Errors tend to average out, as long as the local estimates are not systematically biased. When damage in a state is estimated to be greater than $500 million (1995$), disagreement between estimates from NWS and other sources are relatively small (40% or less). The relatively close agreement between NWS and state estimates in years with major damage is reassuring, since the most costly floods are of greatest concern and make up a large proportion of total flood damage.

   • Floods causing moderate damage are occasionally omitted, or their damage greatly underestimated, in the NWS data sets.

     Missing NWS estimates were discovered for floods in which the state claimed as much as $50 million damage (1995$). Such omissions would have little effect on national total damage estimates, but they might be important in analyses of damaging floods at the state or river basin level.

   In summary, the NWS flood damage estimates do not represent an accurate accounting of actual costs, nor do they include all of the losses that might be attributable to flooding. Rather, they are rough estimates of direct physical damage to property, crops, and public infrastructure. See FAQ #6 for discussion of the appropriate use of the data sets. Estimates for individual flood events are often quite inaccurate, but when estimates from many events are added together the errors become proportionately smaller.

6. How can the data be used for research?

   When properly used, the reanalyzed NWS damage estimates can be a valuable tool to aid researchers and decision makers in understanding the changing character of damaging floods in the United States. Users of the reanalyzed data are advised to take the following precautions:

   • To compare flood damage over time, adjust for changes in population, wealth, or development.
   • To compare damage in different geographical areas, control for differences in population and in the incidence of extreme weather events during the period of study.
   • Use damage estimates for individual floods with caution, recognizing that estimation errors are large. Comparison of individual floods might be better done using nominal or ordinal damage levels. Look for qualitative descriptions to compare the nature and impacts of the damage.
   • Different agencies define "flood" and "flood damage" somewhat differently. Check for incompatibilities between data from different sources before seeking to combine sources or aggregate data.

   We recommend the following procedures to reduce the impact of inaccuracy and omissions in the NWS state damage estimates:

   • To determine the frequency of damaging floods in a state, establish a threshold above which damage estimates are consistently provided and report the number of floods that have exceeded the threshold. Our analysis indicates that a threshold of $1 million (in 1995 dollars) is reasonable.
   • To reduce the impact of errors and omissions in the estimates, increase the level of aggregation; this can be done either by (a) using total damages in a state or states over an extended period of years, or (b) computing damages for multi-state regions rather than using individual states. This is especially important for statistical analysis of low vulnerability states.

   With the precautions noted above, we conclude that the reanalyzed NWS flood damage estimates can be a valuable tool to aid researchers and decision makers in understanding the changing character of damaging floods in the United States. However, the NWS damage estimates are not reliable enough to be a basis for some critical decisions, such as setting precise flood insurance premiums or evaluating the cost-effectiveness of specific hazard mitigation measures.