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

Data Limitations

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.

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.

For more information about the flood damage data, please visit the Data-Related FAQs.