The Increased Prevalence of Extreme Rainfall Events

In an earlier Field Crop News article,Soil Erosion Threat Increasing With Climate Change, we discussed that highly erosive rainfall events determine most soil erosion, although our Conservation Plans or Sediment and Erosion Control Plans express soil loss as an annual average soil erosion rate. A trial from the Carolinas documented 135 T/A soil loss in one week of high-intensity rainfall from a chisel-plowed field compared with only 1.2 T/A from the no-till treatment, showing the efficacy of this soil conservation practice. This information is very relevant for us as we head into an uncertain future.

In this article, we want to look at evidence of a trend of increasing rainfall intensity. Meteorologists studied precipitation from 1979–2014 in the northeastern U.S. (Howarth et al., 2019). They analyzed daily records from 58 weather stations sprinkled throughout the region, 14 of which were in Pennsylvania. These stations had uninterrupted records with less than 5% missing days.

We already know that total annual precipitation in the northeastern U.S. is increasing more than in any other region of the U.S. However what is more significant for soil conservation is that this precipitation is increasingly falling in heavy rainfall events. The authors compared the period 1979–1996 with 1997–2014 and found that both the frequency and intensity of high-intensity events exceeding 2 inches per day had increased. Most alarming, they found the number of events exceeding 6 inches per day had increased from 6 in the first to 25 in the second period, an increase of 317%. The number of daily events with precipitation from 2–4 inches per day increased from 594 to 719 (21% increase), and those with 4–6 inches per day increased from 56 to 89 events (59% increase).

The stations with more days with high-intensity precipitation events were concentrated in the eastern half of Pennsylvania (and New York), with decreased intense precipitation events in western Pennsylvania, east of Lake Erie. The latter was attributed to the increasing water temperature of the lake, which moderates weather extremes. The increased precipitation intensity in the eastern part of Pennsylvania was particularly evident in the months of September and October, with smaller increases in the June–August period, and was mostly due to hurricanes or tropical storms that ventured further inland compared with the past. Intense precipitation events increased less in the March–May and December–February periods.

Climatologists expect these trends to continue as the earth's climate warms. It becomes more and more likely that one of these days, we may experience rainfall exceeding 6 inches in a day in some parts of (especially eastern) Pennsylvania. It is, therefore, ever more important that our fields are protected from high-intensity rainfall, especially in the early fall.

References

Howarth, M.E., C.D. Thorncroft and L.F. Bosart. 2019. Changes in extreme precipitation in the Northeast United States: 1979-2014. J. Hydrometeorology 20: 673-689. DOI: 10.1175/JHM-D-18-0155.1