1.0 1.1 2.0 3.7
Vancouver, BC 3.3 2.8 2.5 1.2 1.7 3.6 5.8
Victoria, BC 1.2 1.0 0.9 0.4 0.6 1.5 2.8

*Source: Van der Leeden et al., _The Water Encyclopedia,_ 2nd ed.,
(Chelsea, Mich.: Lewis Publishers, 1990).

Defined scientifically, drought is not lack of rain. It is a dry
soil condition in which plant growth slows or stops and plant
survival may be threatened. The earth loses water when wind blows,
when sun shines, when air temperature is high, and when humidity is
low. Of all these factors, air temperature most affects soil
moisture loss.

Daily Maximum Temperature (F)*

July/August Average

Eureka, CA 61
Medford, OR 89
Eugene, OR 82
Astoria, OR 68
Olympia, WA 78
Seattle, WA 75
Bellingham, WA 74
Vancouver, BC 73
Victoria, BC 68

*Source: The Water Encyclopedia.

The kind of vegetation growing on a particular plot and its density
have even more to do with soil moisture loss than temperature or
humidity or wind speed. And, surprising as it might seem, bare soil
may not lose much moisture at all. 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 (