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30 I I I I I I I I I I I I I I I I I I I were in error by more than a factor of 1.5; and half of the IDEs were in error by more than a factor of 2 (with many off by more than a factor of 4). Given the methods used by NWS field offices to obtain flood damage estimates (described in Section 2), it is unlikely that the NWS estimates are much better than the IDEs examined here. Thus, when an annual flood damage estimate for a state is less than about $30 million, one should not expect the NWS estimate to depict actual losses accurately. However, the above analysis does not indicate systematic bias in the individual estimates, and errors tend to average out when the estimates are summed. From the above results, we conclude that aggregation of many damage estimates in floods that have caused high levels of damage ($300 million or more in 1998 dollars) provides reasonably good estimates of total damage. However, estimates at a low level of aggregation ($30 million or less) often are in error by factors of 2 or more. Such small estimates should be used with great caution: Direct comparisons of individual estimates are likely to be misleading. B. Comparison of Damage Estimates from NWS and States Appropriate data are not available for comparing NWS estimates with actual flood damage costs. However, comparable estimates are available from independent state sources to do an assessment of typical estimation variability. Every state in the U.S. has an emergency management agency. In July 2000, we wrote to the head of the emergency management agency in each state asking for historical data on flood damage in their state. The letter was followed by a phone call to the appropriate administrator if a response was not received within three weeks. Twenty-one states responded2, but many of them could provide damage information only after 1990 and only related to losses covered by FEMA. Five states either had published historical summaries of flood damage or were able to compile flood damage estimates from their files covering at least 20 years which were based on criteria similar to those used by the NWS. (1) California: A report (Montane 1999) describes disasters from 1950 through 1998 including for each disaster a brief description, general location, estimated damage, number of deaths, and whether a presidential disaster declaration was issued. We selected the disasters that involved flood, heavy rainfall, or severe storms for this comparison. (2) Colorado: The state has formally collected flood data since 1937. A report (McLaughlin Water Engineers, Ltd. 1998) summarizes flood history and provides damage estimates for major floods since 1864. (3) Michigan: A report (Michigan Dept. of State Police 1999) summarizes the 14 floods during 1975-1998 that resulted in a disaster declaration by either the governor or the president. Damage estimates are given for all of the floods that received a presidential declaration and four that received only a gubematorial declaration. (4) Virginia: Damage estimates in presidentially-declared flood disasters during 1977-1999 were provided by Michael Cline, State Coordinator of the Virginia Dept. of Emergency 2States that responded were AL, CA, CO, FL, GA, HI, IL, IN, LA, MA, MI, MN, MO, OH, OR, SC, TX, VA, W A, WV,WY.