Fragile Lands of Latin America Strategies for ... - PART - USAID

John M. Treacy
Terraces are invariably owned in vertical rows so water can pass
from one to another through drop canals and elaborate systems of
stone offtake valves and chutes. Irrigators use shovels to coax flowing
water through scratch canals on terrace platforms to nurture each plant.
Terraces slope slightly forward to pass water from one terrace to the
next through gravity water drops, or to one side so water enters a drop
canal connected to terraces below. It takes one farmer from one half
hour to one hour to irrigate a regular sized bench terrace (approximately
250 m2), but with high velocity canal flow and extra laborers, work
goes much faster. It requires more time to irrigate valley floor fields,
partly because they are larger than terraces (up to 5000 m2), but also
because it takes more time to "teach" water to flow over them. Irrigation
records from Coporaque suggest a day's ration of water goes farther
for irrigating terraces than for irrigating valley floor fields, perhaps
because several terraces may be irrigated simultaneously and farmers
can manage water in a coordinated fashion.
Valley side terraces are located in belts of favorable temperatures.
Terraces on steep slopes above the Colca River's edge (3350 m) enjoy
warm temperatures engendered by low elevations. In contrast, the wide
alluvial pampas above (3500 m) often register daily temperatures 3-4
degrees celsius cooler, and are susceptible to morning frosts. Temper-
atures rise again by 1-2 degrees celsius on valley-side terraces above
the pampas to an elevation of approximately 3600 m. The effect is
produced by the drainage of cold air which glides over valley-side
terraces and settles upon valley floor areas. Constructing terraces within
the frost-free upper slopes took advantage of a micro-climatic temper-
ature inversion to create agricultural space for sensitive plants.
According to local informants, plants on terraces are also less likely
to be infested with blights. Wind turbulence, which also disrupts cold
air sinking upon terraced fields, may remove excess, blight-promoting
moisture from plant leaves.
Maize thrives on terraces year after year because of excellent soils,
studied in detail by Sandor (1987), from which the following material
is a summation. Most Coporaque terrace soils are within the Mollisol
order (Haplustolls, Argiustolls), with dark, organic-rich surface horizons
and high base cation content (Soil Survey Staff, 1975). Terrace soil
structure, consistence, microporosity, and extensive root permeation
indicate good tilth; there is little evidence of compaction. The loamy-
textured soils hold water well. The majority of the soils have organic
matter and macronutrient nitrogen levels comparable to soils in the
fertile U.S. midwest. Soils of many abandoned terraces are of similar
quality, requiring only additional nitrogen to equal the quality of
cultivated soils.