Thomas G. Chastain
Annual rainfall can be tabulated as calendar year (January –December) or crop year (September – August), but given the seasonal distribution of rainfall in the summer dry, winter wet climate of western Oregon, crop year precipitation is more relevant with regard to crop production and is considered here. The wet cycle in our Mediterranean climate begins in autumn and ends in spring so crop year precipitation better captures the seasonal nature of our region’s rainfall patterns (Fig. 1).
With the end of the 2014-15 crop year today, the Willamette Valley is running a three-year rainfall deficit of 12.0 inches in the current drought. To put that number in perspective, that figure represents 29% of a single crop year precipitation that the area is lacking, and that shortage has reduced non-irrigated yields for many of the local seed crops. The combination of unseasonably warm temperatures and extended drought has put added stresses on seed production enterprises in the Willamette Valley.
An examination of crop year precipitation over time reveals that the only trends in rainfall evident for the area are the extended dry periods in the 1920s and 1930s (Dust Bowl years) and that there seems to be more variability in precipitation in recent decades (Fig. 2).
Since the area has had 3 consecutive dry crop years, what are the prospects for a 4th dry crop year in a row and further extending the drought? The likelihood of a 4th dry crop year is not strong if past records are considered and if forecasts for changing conditions in the Pacific Ocean that herald the possibility of increased rainfall are realized. Nevertheless, there are 5 instances of past multi-year droughts extending beyond 3 crop-years with the most recent long-term drought lasting 5 crop years (1987-1992). The longest multi-year drought to date stretched over 7 crop years, from 1943-1950.
In other words, the drought may end for western Oregon or it may not.