Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 80

A GROECOSYSTEMS A DAPTED TO S EASONAL R AINFALL

of the wet season, with fire being used to clear the slash,
and the crop (called milpa de año ) being harvested in
September. The second crop (called tolnalmil ) is planted
just following the second rainfall peak in late October or
November for harvest at the beginning of the dry season
in late February. The second crop depends greatly on the
presence of residual soil moisture extending into the dry
season, and since the crop is planted during the wet sea-
son, any slash on the surface at planting is not burned.
Different local varieties of corn are used in each planting
system.

Outside of the wet tropics, a common rainfall regime is
one in which one or more wet seasons are interspersed
with relatively long dry seasons. In these areas, crops are
often planted at the beginning of the rainy season, grow
and develop while there is moisture in the soil, and become
ready to harvest at the end of the wet season or the begin-
ning of the dry season.
This kind of wet-season cropping takes many forms.
In much of the midwestern heartland of the U.S., for
example, spring wheat, corn, and soybeans are planted
in the late spring and depend on convective summer
rainfall to develop. In Mediterranean climates around
the world, the mild, wet winters and dry summers are
appropriate for grain crops such as oats, barley, and
rye grown in winter, with the land being left fallow
or grazed during the summer unless irrigation can be
provided.
A seasonally rainfed cropping system of considerable
importance is the Mesoamerican corn/bean/squash poly-
culture system. Adapted to a wide range of rainfall inten-
sities and amounts, this intercropping system is found
throughout much of Latin America (Pinchinat, et al.,
1976; Davis, et al., 1986; Laing, et al., 1984). These three
crops are planted in many different arrangements,
sequences, and patterns, sometimes only two of them
together, and at other times all three. But regardless of
the combination, it is the arrival of the rainy season that
determines planting.
If shifting cultivation practices are used, clearing and
burning takes place during the dry season. Sometimes
farmers wait to burn until after the first rains of the wet
season dampen the lower layers of the slash. Since these
first rains are most often interspersed with periods of sun,
the upper layer of the slash is dried enough between rains
to carry a fire, while the newly acquired moisture below
prevents excessive heat from reaching the soil. Crop seed
is then planted into a mulch made up of nutrient-rich ash
and a protective layer of unburned organic matter. This
practice achieves the dual goals of nutrient supply and soil
erosion protection. Soil protection is important in many
areas where this crop system is used, since the early rains
of the season occur most often as intense, convective
showers.
Once the rains begin, crop seeds germinate and
develop quickly, covering the soil and protecting it against
the continued rains. The amount of time it takes for the
crop to mature (from 4 months to 6 months) depends on
the length of the wet season.
In areas such as the wet lowlands of Tabasco, Mexico,
two corn crops can be planted because the wet season is
longer and characterized by a bimodal distribution, with
one rainfall peak in June/July and another in Septem-
ber/October. One crop is planted in May at the beginning

DRYLAND FARMING

In many parts of the world, rainfall during the cropping
season does not meet the needs of the crop, either
because the area does not receive enough rainfall to
offset the moisture lost through evapotranspiration, or
because the cropping cycle does not coincide with the
wet season. The type of agriculture developed in such
climates — when irrigation is not an option — is termed
“dryland agriculture or dry farming .”
Dryland agriculture is defined as crop production
without irrigation in semiarid regions of the world, where
annual rainfall is mostly between 250 and 500 mm. But
total rainfall is only one influence on dryland agriculture;
annual and seasonal variations in temperature and the type
and distribution of rainfall are key factors as well. The
traditional agriculture in most dryland regions is pastoral
in nature, with cultivated crops limited to small areas
farmed by hand tools or animal power. Today, mechani-
zation has added a new dimension to dry farming, but the
types of tillage, seeding management, and harvest proce-
dures remain much the same. In many countries, hand
labor still plays a major role.
The most important aspects of dry farming are (1)
the use of some type of cultivation system that promotes
the penetration of rainwater into the soil profile and
its storage there, and (2) the frequent use of summer
fallows or rest seasons to allow replenishment of water
reserves depleted by cropping. Other practices can be
important as well. Cultivation of the surface soil during
the cropping cycle is used to control potential water-
using weeds and to create a “dust mulch” of pulverized
surface soil that reduces the proportion of large pores
and therefore reduces evaporation. Drought-resistant
cultivars are often planted to reduce moisture use.
Altogether, these practices allow a much higher pro-
portion of the moisture from rainfall to be channeled
through the crop rather than to pass from the soil to the
atmosphere.
The most highly developed modern dryland agricul-
tural systems, at least in terms of intensive management
and technology, are in Australia, Canada, and the U.S.