CHAPTER VIII

REGULATING THE EVAPORATION

The demonstration in the last chapter that the water which falls as rain or snow may be stored in the soil for the use of plants is of first importance in dry-farming, for it makes the farmer independent, in a large measure, of the distribution of the rainfall.  The dry-farmer who goes into the summer with a soil well stored with water cares little whether summer rains come or not, for he knows that his crops will mature in spite of external drouth.  In fact, as will be shown later, in many dry-farm sections where the summer rains are light they are a positive detriment to the farmer who by careful farming has stored his deep soil with an abundance of water.  Storing the soil with water is, however, only the first step in making the rains of fall, winter, or the preceding year available for plant growth.  As soon as warm growing weather comes, water-dissipating forces come into play, and water is lost by evaporation.  The farmer must, therefore, use all precautions to keep the moisture in the soil until such time as the roots of the crop may draw it into the plants to be used in plant production.  That is, as far as possible, direct evaporation of water from the soil must be prevented.

Few farmers really realize the immense possible annual evaporation in the dry-farm territory.  It is always much larger than the total annual rainfall.  In fact, an arid region may be defined as one in which under natural conditions several times more water evaporates annually from a free water surface than falls as rain and snow.  For that reason many students of aridity pay little attention to temperature, relative humidity, or winds, and simply measure the evaporation from a free water surface in the locality in question.  In order to obtain a measure of the aridity, MacDougal has constructed the following table, showing the annual precipitation and the annual evaporation at several well-known localities in the dry-farm territory.

True, the localities included in the following table are extreme, but they illustrate the large possible evaporation, ranging from about six to thirty-five times the precipitation.  At the same time it must be borne in mind that while such rates of evaporation may occur from free water surfaces, the evaporation from agricultural soils under like conditions is very much smaller.

Place Annual Precipitation Annual Evaporation Ratio
                 (In Inches) (In Inches)
El Paso, Texas 9.23 80 8.7
Fort Wingate,
New Mexico 14.00 80 5.7
Fort Yuma,
Arizona 2.84 100 35.2
Tucson, AZ 11.74 90 7.7
Mohave, CA 4.97 95 19.1
Hawthorne,
Nevada 4.50 80