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.  At first glance, it seems impossible to garden without irrigation west of the Cascades.  But there is water already present in the soil when the gardening season begins.  By creatively using and conserving this moisture, some maritime Northwest gardeners can go through an entire summer without irrigating very much, and with some crops, irrigating not at all.

Chapter 2

Water-Wise Gardening Science

Plants Are Water

Like all other carbon-based life forms on earth, plants conduct their chemical processes in a water solution.  Every substance that plants transport is dissolved in water.  When insoluble starches and oils are required for plant energy, enzymes change them back into water-soluble sugars for movement to other locations.  Even cellulose and lignin, insoluble structural materials that plants cannot convert back into soluble materials, are made from molecules that once were in solution.

Water is so essential that when a plant can no longer absorb as much water as it is losing, it wilts in self-defense.  The drooping leaves transpire (evaporate) less moisture because the sun glances off them.  Some weeds can wilt temporarily and resume vigorous growth as soon as their water balance is restored.  But most vegetable species aren’t as tough-moisture stressed vegetables may survive, but once stressed, the quality of their yield usually drops markedly.

Yet in deep, open soil west of the Cascades, most vegetable species may be grown quite successfully with very little or no supplementary irrigation and without mulching, because they’re capable of being supplied entirely by water already stored in the soil.