Moisture is essential for life processes and most of the earth's water circulates as vapor within the atmosphere or below the surfaces of land and ocean. This circulation is called the hydrological cycle and is driven by solar energy through evaporation and precipitation. The amount and distribution of rainfall varies globally and influences what types of plants can grow in different regions, from hydrophytes that prefer wet conditions to xerophytes adapted to aridity. Water is critical for plant metabolic functions like photosynthesis, yet transpiration is the main process by which soil moisture and minerals are transported throughout plants.

2. Moisture is as important in climate as is temperature.
Water is essential for life processes.
Most of the earth's water is found circulating within the
atmosphere from the surface to a height of about 15 km, and to
a similar distance below the surfaces of land and ocean.
The earth's moisture exists in liquid, gaseous (vapor), and solid
(ice and snow) phases.
The conversion from one phase to another is temperature
dependent.

3. The circulation and dispersion is called the hydrological cycle.
This cycle is sustained by solar energy.
Evaporation from bodies of water, land, and plant surfaces
constitute the vapor phase.
When this source is transformed into a liquid or solid phase, it
falls to the earth's surface as rain, snow, or hail.

4. The annual precipitation averaged over the earth is about 1000
mm and varies from a high of more than 12,000 mm to a low of
0.5 mm in certain areas.
The amount of rainfall is closely related to air pressure and
winds.
It is directly associated with moisture sources such as oceans or
sources lacking moisture such as deserts.
Heavy rain occurs along the equatorial low-pressure belt which
is due to the abundant moisture from inflowing ocean air.
The heaviest rainfall is usually near coasts or mountainous
regions where moisture-laden air is cooled by assent.

5. Heavy rains commonly occur in the tropics between 20°
latitudes north and south of the equator.
The high-pressure areas between the 20° and 40° latitudes in
each hemisphere are known as the horse latitudes and are
generally regions of low rainfall.
The Sahara, Arabian, Turkestan, Gobi and Sonora desert are
examples of arid regions in these latitudes.
Summer monsoons bring warm moist ocean air into the Indian
subcontinent and southeastern Asian countries.

6. This is due to periods of low pressure in the interior of Asia.
Between the 40° and 60° latitudes, precipitation is fairly heavy.
Near the poles, high pressures again create conditions for low
precipitation.
It is not only the amount of rainfall but also the distribution of
rainfall that is important.
About 500 mm of rain falls in south central Canada.
This is sufficient for a good crop without irrigation because a large
portion of the moisture occurs during the summer growing
season.
In contrast, about the same amount falls in central California, but
nearly all of that is in the winter. Thus, irrigation is necessary to
grow summer crops.

7. Moisture in the atmosphere is measured as absolute humidity
or as relative humidity.
Absolute humidity is the amount of water present in a unit
volume of air.
Relative humidity (RH) is the amount of water present in air as a
percentage at the same temperature and air pressure.
Dew point is the air temperature at which water vapor is at the
saturation point.
The dew point varies with the amount of water vapor in the air
and the temperature.

8. Relative humidity is a important factor in plant growth and
development because it has a strong influence on transpiration.
Low humidity tends to increase transpiration; high humidity has
the opposite effect.
High humidity has other effects, such as favoring an increase in
the incidence of many diseases and insect populations.

9. Arid < 250 mm
Semiarid 250-500 mm
Sub humid 500-1000 mm
Humid 1000-1500 mm
Wet >1500 mm

10. Minute hygroscopic particles are effective in the condensation of
water vapor into water droplets called fog.
Fine salt particles or smoke are important in fog formation from the
condensation of water vapor.
When high-moisture-containing air masses close to the earth are
cooled, fog can form.
Fog also forms when cold air comes into contact with warm waters.
Moisture from fog can collect in appreciable amounts on plant
leaves.

11. This is especially important along coastal regions where seasonal
rains are few, and thus plants often obtain enough moisture
from fog to survive.
Foggy conditions reduce moisture loss from transpiration.
Dew can result when heat is lost, usually by radiation at night
from surfaces such as the ground or leaves.
The air adjacent to the particular surface is cooled to the point
of saturation, and water vapor condenses on the cooled surface.
The temperature at which this occurs is called the dew point,
and if above O°C, the condensed water vapor is liquid (dew).
However, if the surface temperature is below O°C, frost or ice is
formed by sublimation of the vapor.

12. In some areas, as much as 25 mm is deposited annually and is
significant because the water can be directly absorbed by
leaves.
A single occurrence of "dewfall" can be as much as 0.3 mm.
Under dry conditions, dew on the leaves increases cell turgor,
which is important for growth.
Excess dew can drip from plants and be absorbed by the soil.
At or near freezing temperatures, the condensation of water as
dew releases heat, which helps to prevent freezing, because
when water vapor condenses to liquid, 540 cal of energy are
released from each gram or milliliter of water.
The occurrence of dew can also be a disadvantage because it
creates a high-humidity environment favorable for infection by
pathogens.

13. Hydrophytes are water-loving aquatic plants that normally grow
in water or swamps.
Some vegetable examples are taro, water convolvulus, water
chestnuts, lotus root, and watercress; generally, most are
tropical or semitropical plants.
Mesophytes are the most common of the terrestrial vegetables
and prefer to grow in well-drained soils.
Such plants wilt easily if stressed from low moisture.
Xerophytes are the opposite of hydrophytes in preferring a drier
habitat and are able to endure relatively long periods of low
moisture without exhibiting wilt or damage.
Some xerophytic species can lose from 50 % to 75% of their
moisture content without wilting; cacti are an example.

14. Water is the major constituent of plant tissues; it is the medium
in which cell metabolic processes occur and the medium for
transport between cells in plant tissues and organs.
For these functions, relatively little water is actually required,
but its availability is essential.
It is estimated that less than 1% of the water that passes
through the plant is utilized in the photosynthetic process.
Nevertheless, in water stressed plants, photosynthesis and
growth are very much reduced.
In fact, the photosynthetic rate is reduced well before the
available soil moisture is depleted.

15. Most of the water absorbed by the plant is used in transpiration
through minute leaf openings called stomata.
As a result of transpiration, soil moisture and minerals are
absorbed by roots and transported to other portions of the
plant.
Transpiration has a cooling effect on leaves, which is especially
important in climates where air temperatures are very high.
Evapo-transpiration is a term used to describe the combined
evaporation from the soil surface, transpiration, and cuticular
loss of water from plants.

16. This is expressed as the rate of water loss from a given area and
is useful in measuring and estimating the water requirement of
growing plants.
Factors influencing evapo-transpiration include temperature,
relative humidity, wind, plant leaf area, and the soil surface.
For most mesophytic vegetable species, little or no moisture
stress during the entire growth period usually results in high
yield and quality.
Except for hydrophytes, flooding should be avoided, as this
condition restricts oxygen to the roots and adversely affects
growth.