1. Trends in Iowa Precipitation: Observed and Projected Future Trends
Christopher J. Anderson, PhD
Scientist, Assistant Director Climate Science Initiative
Iowa State University
Eugene S. Takle
Director, Climate Science Initiative
Professor of Atmospheric Science
Department of Geological and Atmospheric Sciences
Professor of Agricultural Meteorology
Department of Agronomy
Ames, Iowa 50011
A Watershed Year: Anatomy of the Iowa Floods of 2008
Lessons Learned – Preparing for the Future
Cedar Rapids, Iowa 
21 June 2010

2. The Two Questions Most Frequently asked about Changes in Rainfall
What role might have climate change played in recent heavy rainfall events?
Will similar heavy rainfall events happen in the the future?
The two things I can tell you about changes in rainfall is that, first, your sense that rainfall is heavier than it has been is supported by measurements and statistical analysis. Your observations are very astute and right on target. Second, Iowa is not alone.

3. Iowa resides in a region where trend in high rate precipitation is remarkable and has garnered attention within the scientific community. The image on the left hand side is the cover from the synthesis of scientific information published in 2008 that was produced by the US Climate Change Science Program that was established by the Bush Administration in The image in the right hand side is an image from a synthesis of many years of reports that was produced by the US Global Climate Change Program through a scientific advisory panel and represents the Program’s synthesis of credible climate change information.
Changes in rainfall statistics are occurring over broad regions, indicating broad shifts in rainfall mechanisms. Even SW US, where drought has been predominant, heavy rainfall amount has increased.
Karl, T. R., J. M. Melillo, and T. C. Peterson, (eds.), 2009: Global Climate Change Impacts in the United States. Cambridge University Press, 2009, 196pp.
“One of the clearest trends in the United States observational record is an increasing frequency and intensity of heavy precipitation events… Over the last century there was a 50% increase in the frequency of days with precipitation over mm (four inches) in the upper midwestern U.S.; this trend is statistically significant “

4. State-Wide Average Data
Annual precipitation is computed as the average over 33 stations within the state of annual sum of precipitation. The 33 stations were selected as they were the stations with the fewest changes in the characteristics of their surroundings.
Precipitation changes may be expressed as a trend. It may also be expressed as frequency of outlier years.
The trend in state-wide precipitation has a 19% increasing trend over the period.
The frequency of annual precipitation exceeding 40” has increased, consistent with the region-wide result reported in the synthesis documents. In fact, this frequency is 4-times greater in the second half compared to first half in the 136-year record. On the flip side, extreme dry periods are absent in the recent history. In particular, Iowa has not received less than ~30” since The only other period of similar length with absence of dryness occurred from 1910 to 1929, during which extremely wet years did not approach the recent extremely wet years.

5. State-Wide Average Data
37.5”
Annual precipitation is computed as the average over 33 stations within the state of annual sum of precipitation. The 33 stations were selected as they were the stations with the fewest changes in the characteristics of their surroundings.
Precipitation changes may be expressed as a trend. It may also be expressed as frequency of outlier years.
The trend in state-wide precipitation has a 19% increasing trend over the period.
The frequency of annual precipitation exceeding 40” has increased, consistent with the region-wide result reported in the synthesis documents. In fact, this frequency is 4-times greater in the second half compared to first half in the 136-year record. On the flip side, extreme dry periods are absent in the recent history. In particular, Iowa has not received less than ~30” since The only other period of similar length with absence of dryness occurred from 1910 to 1929, during which extremely wet years did not approach the recent extremely wet years.
31.5”
19% increase

6. State-Wide Average Data
Totals above 40”
8 years
2 years
Annual precipitation is computed as the average over 33 stations within the state of annual sum of precipitation. The 33 stations were selected as they were the stations with the fewest changes in the characteristics of their surroundings.
Precipitation changes may be expressed as a trend. It may also be expressed as frequency of outlier years.
The trend in state-wide precipitation has a 19% increasing trend over the period.
The frequency of annual precipitation exceeding 40” has increased, consistent with the region-wide result reported in the synthesis documents. In fact, this frequency is 4-times greater in the second half compared to first half in the 136-year record. On the flip side, extreme dry periods are absent in the recent history. In particular, Iowa has not received less than ~30” since The only other period of similar length with absence of dryness occurred from 1910 to 1929, during which extremely wet years did not approach the recent extremely wet years.

7. Cedar Rapids Data 57% increase 6.6 days 4.2 days
Individual days with high rainfall rate, here defined as 3.0 cm (about 1.25”) in one day, are critically important to flood risk and runoff.
These high rain rate days have increased in frequency with an unprecedented number of 13 days in 1993 and Note, however, the timing of these events is critical, as the frequency in 2008 was not unusual but instead was about the new average.
57% increase
6.6 days
4.2 days

8. Years having more than 8 days
Cedar Rapids Data
Years having more than 8 days 11 2
Individual days with high rainfall rate, here defined as 3.0 cm (about 1.25”) in one day, are critically important to flood risk and runoff.
These high rain rate days have increased in frequency with an unprecedented number of 13 days in 1993 and Note, however, the timing of these events is critical, as the frequency in 2008 was not unusual but instead was about the new average.
The number of years with 8 or more days exceeding 3.00 cm has increased substantially. Since the 1950s, these years occur about once every five years.
57% increase
6.6 days
4.2 days

9. What role might have climate change played in recent heavy rainfall events?
More water vapor is in the air throughout the Midwest.
Warmer surface temperature in Gulf of Mexico and Caribbean
Increase in agricultural production and irrigation
Pacific sea surface temperature pattern is conducive to a wet period in the recent 15 years.
More storm systems steered into rather than north of the Midwest
May not be a result of climate change but rather a continuation of historical climate variability.
The importance of soil conditions: wet soils promote more rainfall, more rainfall keeps soil conditions wet.
May amplify the moistening from warmer surface temperaure in Gulf of Mexico and Caribbean
The relative importance of these factors and their connection to climate change are matters of research and debate.
These are factors that we know are important; however, it should stressed sternly, overmphasized that the relative contribution of these factors is still a matter of research and scientific debate.
Warmer surface temperature in the Gulf of Mexico and Caribbean is important because it releases more water into the atmosphere which is then transported into the Midwest. More than half of the Spring-Summer rainfall originates from the surface of the Gulf of Mexico and Caribbean.

10. Will similar heavy rainfall events happen in the future?
In the next 5-10 years precipitation will be more like the past 30 years than the 50 to 100 years before that.
Between 10 and 20 years from now, days with heavy rainfall similar to the past 30 years will occur intermittently during periods of drought.
Beyond 30 years from now, greater highs and lows of rainfall will emerge.
Seasonal rainfall will likely increase in spring and decrease in summer
Maximum daily rainfall likely to continue to increase

11. For More Information
For current activities on the ISU campus, regionally and nationally relating to climate change see the Climate Science Initiative website:
Contact: