Sub-irrigation and temperature amelioration in chinampa agriculture

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 probable 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 Xochimilco (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 waterlogging 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 time and duration of sub-zero temperatures, and the minima recorded above two chinampas over 14 winter nights. These data clearly indicate that during radiation frost conditions, a microclimate develops that is considerably and consistently warmer than the air above a similarly-positioned dryland 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.