|Zandlo, J - MN DEPT NATURAL RESOURCES|
|Spoden, G - MN DEPT NATURAL RESOURCES|
Submitted to: Journal of Applied Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 5, 2000
Publication Date: N/A
Interpretive Summary: Runoff and leaching of chemicals from agricultural landscapes, which escalates with more frequent rainfall, threaten the quality of our surface and ground water systems. US chemical companies continue to promote application of pesticides and herbicides to soils in the autumn in the northern Corn Belt. Their rationale is based upon a belief that less frequent rainfall in the autumn should minimize runoff and leaching in the autumn than in the spring. Our analysis of 100-year records across the northern Corn Belt indicate that rainfall in the autumn is less frequent than in the spring. Growers in the western region (Dakotas) have 50% fewer rainfall events while growers in the eastern region (Wisconsin) have 25% fewer events in the autumn than in the spring. These data can serve as guidelines for determining the relative risks involved in fall versus spring applications of chemicals. However, further studies are needed to address the fate of chemicals in the soil during winter to ensure fall- applied chemicals don't leach or run off with snowmelt.
Technical Abstract: Knowledge of the frequency of precipitation events can aid in managing water resources, but little is known concerning the regional variability in frequency of daily events in the northern U.S. Corn Belt. The frequency distribution of daily precipitation events, varying from 0.25 to 102 mm, was examined at 15 climate stations located in Minnesota, South Dakota, Iowa, and Wisconsin. Data were available from 1898 to 1997. Annual precipitation increased from the NW to the SE across the northern Corn Belt. The frequency of daily precipitation events of at least 0.25 mm was fairly constant at about 20% during winter, then increased during the spring before attaining a maximum near 35% in late May or June at all stations. The frequency of events then declined through August before approaching a local maximum in September. Daily precipitation events of at least 10, 25, and 102 mm contributed about 10, 5, and less than 1% to the overall frequency in June. The frequency distribution was skewed and exhibited kurtosis at all stations. The distribution was peaked with more frequent precipitation events earlier in the year. Precipitation events in the spring, as compared with the fall, were twice as frequent at westerly stations and 30% more frequent at easterly stations. This study suggests that precipitation events in the northern Corn Belt are spatially variable with regard to the frequency of daily events and the distribution of events throughout the year.