Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 79

FIGURE 6.4 A camellón (raised field) near Ixtauixtla, Tlaxcala, Mexico. The field is planted with rotational strips of alfalfa and
intercropped corn and beans; alder trees mark the edge of the canals dug to raise the field. The compost in the foreground is used
as a fertilizer.

frequent applications of composted animal manures and
crop residues. Much of the feed for the animals comes
from alfalfa grown on the platforms, or from residues of
other crops that cannot be directly consumed by humans
(e.g., corn stalks). Supplemental feed for animals is
derived from the noncrop vegetation (i.e., weeds) that is
selectively removed from the crop area, or periodic har-
vests that are made of the ruderals and natives that grow
either along the canals or directly in them as aquatic
species. This latter source of feed can constitute a very
significant component of livestock diets during the dry
season.
A very important aspect of this traditional agroeco-
system is the management of the complex set of canals.
Besides originally serving as a primary source of soil for
raising the platform surfaces, they also serve as a major
reservoir of water during the dry season. Organic matter
accumulates in the canals as aquatic plants die, leaves
from trees along the canal borders fall into the water, and
even weeds from the crop field are thrown into the canals.
Soil from the surrounding hillsides and the platforms is
also washed into the canals by the heavy wet-season rains.

Every 2 to 3 years, the canals are cleaned of the accumu-
lated soil and muck, with the excavated materials being
applied as a nutrient-rich top dressing on the platforms.
The canals thus play a very important role in the
sustainability of this agroecosystem. They function as a
nutrient “sink” for the farmer, and are managed in ways
that permit the capture of as much organic material as
possible. Supplemental irrigation water can be taken from
them in the dry season, and the plants rely greatly on
moisture that moves upward through the soil from the
water table by capillarity. The raised platforms provide
suitable planting surface even during the peak of the rains.
Water levels in the canals are controlled by an intricate
system of interconnected canals that eventually lead to the
rivers of the basin, but flow in the canals is very limited.
Farmers often block the flow of canals along their fields
during the dry season in order to maintain a higher water
table, and even in the wet season, water flow out of system
is minimal. Only at times of excessive rainfall, do appre-
ciable quantities of water drain from the area. Rainfall is
both an input and a tool in the management of the system,
and permits year-round cropping.