TheImprovement ofTropical and Subtropical Rangelands

SIT8 EVALUATION 99
Climate is difficult to characterize, owing to frequent deficiencies in
the length and consistency of necessary meteorological records.
The climate of a site is most easily described from records of
the United Nations World Meteorological Organization or from data
collected by national weather offices. Unfortunately, many weather
stations from which this information is obtained are often poorly
distributed, especially in semiarid and arid lands ofdeveloping countries.
PrecipitGtion PGttern. The amount and distribution of rainfall is
important because of its role as a source of soil moisture. Survival
and subsequent growth of plants is, of course, closely tied to the
availability of water in the soil mantle. Rainfall, in itself, is usually
of little direct significance to plants, although there can be some
absorption of water through the leaves and, occasionally, the bark.
Although soil moisture is mostly derived from rain, not all of
the precipitation that falls on a site is equally effective in raising
the soil moisture content. The slower, more gentle a rainfall event,
the greater the penetration, or percolation, of water into the soil.
However, a series of precipitation events that totals only several
millimeters may add little to the soil moisture content, because the
individual events are too widely separated and too gentle to have
a cumulative effect. The more severe a drought, especially in dry
climates, the greater the quantity of rain required subsequently to
alleviate the drought.
Reliable measurements of rainfall are most commonly acquired
from networks of rain gauges. There are many types (for example,
standard, recording, and totalizer), and dimensions of rain gauges,
but they all consist essentially of a funnel with a vertical collar that
delivers water to a collecting reservoir. Only precipitation records
obtained from gauges located away from eddies caused by physical
obstructions should be used in a site evaluation. As a general rule,
obstructions overhead should be no closer to the gauge than twice
the height (from the ground) of the receiver funnel.
TemperGture Regime. Heat from solar radiation controls the temperature
regimes near the surface of the earth. The temperature
at a site is inftuenced by incoming solar radiation that, in turn, is
modified by secondary heat transfers from terrestrial radiation and
air movements. Temperatures of either high or low extremes can be
detrimental to the establishment and growth of plants. Hot temperatures,
in combination with drying winds, can be damaging to