Tuesday, November 14, 2006 - 3:25 PM
196-9

Autumn Rainfall Patterns in Northeastern Oregon.

Steven Petrie and Karl Rhinhart. Columbia Basin Ag Res Ctr., OSU, PO Box 370, Pendleton, OR 97801

Seeding at the optimum time is one key to producing the greatest yield of any crop. Successful seeding of winter wheat and other crops is predicated on having sufficient moisture in the seed zone to foster rapid and uniform germination and emergence.  Tillage-based fallow is used in dryland cropping in northeastern Oregon to create a “dust mulch” consisting of soil with about 4% moisture which retards evaporation from the soil below the dust mulch. The objective of this work was to estimate the likelihood of receiving sufficient rainfall to wet this dust mulch layer adequately for consistent germination and emergence. We searched the weather records at the Pendleton Station of the Columbia Basin Agricultural Research Center, located near Pendleton in northeastern OR, to determine the frequency of receiving 1.0 – 1.5, 1.5 – 2.0, and more than 2.0 cm of rainfall within any 3-, 5-, and 7-day period in September and October at the Pendleton Station.  Conditions in September were rarely optimum for seeding; 2.0 cm of rain or greater was received within 3 days in only 9 of 77 years and within 7 days in only 17 of 77 years. We found only 47 of 77 years in which 1.0 – 1.5 cm of rainfall fell within 7 days, the minimum we determined to be adequate to promote uniform germination. We used the information to generate graphs that show the cumulative probability of receiving 1.0 – 1.5, 1.5 – 2.0, and more than 2.0 cm of rainfall within any 3-, 5-, and 7-days for any date in September and October.  For example, by September 30, there is about a 15% chance of having received more than 2.0 cm of rain within 3 days and about a 28% chance of having received 1.5 – 2.0 cm of rain within 3 days.

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