I now know it is next to
impossible to anticipate moisture loss from soil without first
specifying the vegetation there. Evaporation from a large body of
water, however, is mainly determined by weather, so reservoir
evaporation measurements serve as a rough gauge of anticipated soil
moisture loss.
Evaporation from Reservoirs (inches per month)*
Location April May June July Aug. Sept. Oct
Seattle, WA 2.1 2.7 3.4 3.9 3.4 2.6 1.6
Baker, OR 2.5 3.4 4.4 6.9 7.3 4.9 2.9
Sacramento, CA 3.6 5.0 7.1 8.9 8.6 7.1 4.8
*Source: _The Water Encyclopedia_
From May through September during a normal year, a reservoir near
Seattle loses about 16 inches of water by evaporation. The next
chart shows how much water farmers expect to use to support
conventional agriculture in various parts of the West. Comparing
this data for Seattle with the estimates based on reservoir
evaporation shows pretty good agreement. I include data for Umatilla
and Yakima to show that much larger quantities of irrigation water
are needed in really hot, arid places like Baker or Sacramento.
Estimated Irrigation Requirements:
During Entire Growing Season (in inches)*
Location Duration Amount
Umatilla/Yakama Valley April-October 30
Willamette Valley May-September 16
Puget Sound May-September 14
Upper Rogue/Upper Umpqua Valley March-September 18
Lower Rogue/Lower Coquille Valley May-September 11
NW California April-October 17
*Source: _The Water Encyclopedia_
In our region, gardens lose far more water than they get from
rainfall during the summer growing season.
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