Gardening Without Irrigation: or without much, anyway | Page 6

May June July Aug. Sept. Oct. Eureka, CA 3.0 2.1 0.7 0.1 0.3 0.7 3.2 Medford, OR 1.0 1.4 0.98 0.3 0.3 0.6 2.1 Eugene, OR 2.3 2.1 1.3 0.3 0.6 1.3 4.0 Portland, OR 2.2 2.1 1.6 0.5 0.8 1.6 3.6 Astoria, OR 4.6 2.7 2.5 1.0 1.5 2.8 6.8 Olympia, WA 3.1 1.9 1.6 0.7 1.2 2.1 5.3 Seattle, WA 2.4 1.7 1.6 0.8 1.0 2.1 4.0 Bellingham, WA 2.3 1.8 1.9 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 (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.
Soil's Water-Holding Capacity
Soil is capable of holding on to quite a bit of water, mostly by adhesion. For example, I'm sure that at one time or another you have picked up a wet stone from a river or by the sea. A thin film of water clings to its surface. This is adhesion. The more surface area there is, the greater the amount of moisture that can be held by adhesion. If we crushed that stone into dust, we would greatly