Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 73

6 Humidity and Rainfall

A place's natural vegetation is usually a reliable indicator
of its rainfall regime. Deserts, with their sparse, slow-
growing vegetation, tell the observer that the local annual
rainfall is minimal. The lush vegetative growth of tropical
and temperate rainforests points to abundant rainfall
through at least most of the year. Rainfall amounts and
vegetation have this direct relationship because for most
terrestrial ecosystems, water is the most important limiting
factor.
Water is also a primary limiting factor in agroecosys-
tems. Agriculture can be practiced only where there is
adequate rainfall or where it is possible to overcome,
through irrigation, the limits imposed by a dry climate.
In this chapter, we discuss the agroecological signi-
ficance of water in the atmosphere, both as humidity and
as precipitation. Despite this focus, the reader should keep
in mind that water in the atmosphere is only one aspect of
a larger set of environmental factors affecting plants —
those involving the atmosphere as a whole. Patterns of
movement and change in the atmosphere influence not only
rainfall patterns but also wind and variations in tempera-
ture. Combined atmospheric factors make up climate
(when we are referring to the annual average conditions)
and weather (when we are referring to the climatic condi-
tions at one moment in time).

Relative humidity can change as a result of either
changes in the absolute amount of water vapor or changes
in temperature. If the absolute amount of water vapor in the
air is high, small variations in temperature can greatly influ-
ence relative humidity. A drop of a few degrees in tempera-
ture in the evening or morning hours, for example, can push
the relative humidity to 100%. Once relative humidity
reaches 100%, water vapor begins to condense into water
droplets, and shows up as dew. The temperature at which
this condensation begins to occur is called the dew point .
In natural systems, the interaction of temperature and
the air's moisture content can be a very important factor
in determining the structure of an ecosystem. The redwood
forest community along the coast of California is a good
example. Cold ocean currents condense the moisture-laden
air over the ocean, forming fog. The occurrence of fog
almost every night during the dry summer months com-
pensates for the lack of rainfall and is believed to be the
main reason that redwoods still exist where they do. Some
studies estimate that fog and dew add at least an extra 10%
to the effective total of rainfall for redwood regions.
For similar reasons, humidity can affect agroeco-
systems. Crops grown in the redwood forest region, for
example, may benefit from the extra moisture that fog and
dew provide; farmers of crops such as Brussels sprouts,
lettuce, and artichokes use less water as a result.

WATER VAPOR IN THE ATMOSPHERE

PRECIPITATION

Water can exist in the atmosphere in a gaseous form (as
water vapor) or in a liquid form (as droplets). At constant
pressure, the amount of water vapor that air can hold
before it becomes saturated, and its water vapor begins to
condense and form droplets, is dependent on temperature.
As the temperature of the air goes down, the amount of
water that can be held in vapor form goes down as well.
Because of this dependence on temperature, humidity —
the amount of moisture in the air — is usually measured
in relative terms rather than according to the absolute
amount of moisture in the air. Relative humidity is the
ratio of the water-vapor content of the air to the amount
of water vapor the air can hold at that temperature. At a
relative humidity of 50%, for example, the air is holding
50% of the water vapor it could hold at that temperature.
When the relative humidity is 100%, the air is saturated
with water vapor, and water vapor condenses to form mist,
fog, and clouds.

Although dew and fog can contribute significant quantities
of moisture to some regions, the primary (natural) source
of water for agroecosystems is precipitation, usually in
the form of rain or snow. Precipitation contributes mois-
ture to the soil directly, and in irrigated agroecosystems,
it does so indirectly by being the ultimate source of most
irrigation water.

T HE H YDROLOGICAL C YCLE

Precipitation is part of the hydrological cycle , a global pro-
cess moving water from the earth's surface to the atmo-
sphere and back to the earth. A diagram of the hydrological
cycle is presented in Figure 6.1. The core of the hydro-
logical cycle is made up of the two basic physical processes
of evaporation and condensation. Evaporation occurs at the
earth's surface, as water evaporates from soil, bodies of

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