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.
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