Sub-irrigation and Temperature Amelioration
in Chinampa Agriculture
Philip Lawerence Crossley,
Ph.D.
The University of Texas at
Austin, 1999
Supervisor: William E.
Doolittle
Chinampas, wetland
fields in the Basin of Mexico, have been farmed and studied for centuries.
Though ambiguously described by 16th century Spanish clerics, more recent
analyses have greatly advanced understanding of chinampa agroecology. Nevertheless,
two major aspects of chinampa agriculture warrant further, systematic investigation:
sub-irrigation, widely assumed to occur but never demonstrated; and cold temperature
amelioration, a probably feature of the agriscape neglected by previous scholars.
Both may have important implications for understanding the past in areas where
vestigial wetland fields have been found, the contemporary struggles of Xochimilico
(Mexico) area farmers, and for wetland field-based agricultural development
initiatives.
In order to evaluate
the possibility and significance of sub-irrigation, the author examines soil
properties and moisture content of chinampa profiles in four areas near Mexico
City. From the field and laboratory data, the height of the capillary fringe
is estimated and compared to reported and contemporary chinampa morphology.
The author combines this analysis with review of rainfall seasonality in the
Basin of Mexico, maize rooting depth and growth phases, and implications of
willow roots prominent in chinampa exposures. Conclusions based on these observations
are weighed against farming risk, and the need to overcome catastrophic water
logging represented by chinampa construction. The author also evaluates temperature
amelioration at two sites. Using data recorded by programmable temperature
loggers, the author compares cooling trends, start times and duration of sub-zero
temperatures, and the minima recoded above two chinampas over 14 winter nights.
These data clearly indicated that during radiation frost conditions, a microclimate
develops that is considerably and consistently warmer then the air above a
similarly positioned dry land field.
The author concludes that sub-irrigation has been overemphasized as a feature of both pre-Hispanic and modern wetland agriculture, while frost risk reduction has been an under appreciated result of the creation of chinampa agriscapes. Restoration of past water levels in the chinampa canal system may be desired primarily to achieve increased frost risk reduction, not sub-irrigation. Any future wetland field-based development initiatives may also benefit from greater emphasis on factors relating to cold temperature amelioration.