Read Chapter 1 of Gardening Without Irrigation/without much‚ anyway, free online book, by Steve Solomon, on ReadCentral.com.

Predictably Rainless Summers

In the eastern United States, summertime rainfall can support gardens without irrigation but is just irregular enough to be worrisome. West of the Cascades we go into the summer growing season certain we must water regularly.

My own many-times-revised book Growing Vegetables West of the Cascades correctly emphasized that moisture-stressed vegetables suffer greatly. Because I had not yet noticed how plant spacing affects soil moisture loss, in that book I stated a half-truth as law: Soil moisture loss averages 1-1/2 inches per week during summer.

This figure is generally true for raised-bed gardens west of the Cascades, so I recommended adding 1 1/2 inches of water each week and even more during really hot weather.

Summertime Rainfall West of the Cascades (in inches)

  Location                     April        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.