Thursday, April 28, 2011

Pacific Interdecadal Climate Oscillation (PDO) and Flood Plain Drainage - a Possible Linkage?

La Nina has been in the news the last few days. Snow level dropped to 500 feet last evening and I noted a few clumps of ice in the rain yesterday afternoon. The slopes of the Northwest Cascade Range have fresh snow this afternoon down to approximately 1,000 feet. Cliff Mass noted (are-springs-getting-worse) that between February 1 and April 15 there were only two days that reached 55 in Seattle, the fewest days of 55 plus weather during the same period ever.

This got me thinking about a problem I have been hearing about the past few months regarding farming in northwest Washington and Whatcom County in particular. The excellent glacial and alluvial soils combined with long hours of summer sun make for some rich and profitable farm land. However, cool,wet springs pose challenges to farmers trying to get crops planted to take advantage of the summer sun.

If it was simply cool weather, it would not be so bad, but the big problem is drainage. For farmers along river flood plains this presents a particularly difficult problem as drainage must be directed towards the rivers. Many western Washington farm areas are located on alluvial plains adjacent to rivers flowing off of the Cascade Range. While big floods that make the news and damage property take place during heavy mild rain storm events during the winter, the average flow on rivers flowing off the Cascades is highest during snow melt in late spring. With rivers high it is very difficult to drain farm fields and if the snow melt window of time is shortened due to late cool spring weather some rivers will flood farm fields along the river during this period and back up low land tributary streams that are critical for field drainage.

The continued very cool weather and still being added snow pack in the Cascades will likely pose some very difficult choices for farmers on flood plain areas subject to flooding or poor drainage from high river levels. The period of snow melt could be rather intense this year if a sudden warm up arrives in late May or early June flooding planted fields or preventing large swaths of land from being planted at all.  

Farmers along the flood plains of Nooksack River have been expressing concerns about drainage along the river and have presented anecdotal stories that the problem has been getting worse and spring planting times have been later along the lower flood plain. One theory that has been put forward is accumulation of sediment in the river is slowing drainage. The problem with this suggestion is that there is no data on the Nooksack to demonstrate that it is in fact the problem beyond anecdotal stories. What data is available suggests that river agradation is not a problem as agradation rates are very low overall on the lower Nooksack at this time. There is some data indicating that the Puyallup River near Tacoma has been agrading and causing drainage problems and flooding.

Another possibility is that the springs have been cooler and wetter than in the past. A paper by Mantura and others (1997) linked salmon decline with a periodic climate shifts in the Pacific they called the Pacific interdecadal climate oscillation (PDO). The PDO is a the result of several processes that combine to cause periods of cooler weather and water on a decadal basis. This pattern has significant impacts on salmon populations due to effects on ocean conditions. The pattern also impacts river flows another important factor for fish, but also an important factor for northwest farmers. Possibly it is the PDO that is causing flood plain farmers from getting onto their fields not sediment accumulation in the river. The follow up would be to see if there is a linkage to stream gage data similar to the pattern found by Mantura and others (1997) for larger northwest rivers and if spring temperatures and precipitation patterns might be shifting towards a less favorable condition for flood plain farmers.

PDO from 1900 to 2010 from Wikimedia Commons

The above chart of the PDO index suggests that a warm PDO index trend over the past three decades may shift to a cooler pattern and pose a problem fro some farms that was previously not nearly as common during their lifetime.

PDO since 1000 via Wikimedia Commons

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