Everyone knows temperature is important in farming—how many home-gardeners have lost their tomato plants to a late spring or early fall frost!—but did you know that just a few feet in elevation can have a huge effect on temperature, spelling the difference between a successful crop and a failed harvest? Farmers in the Mesa Verde region have long been aware of the phenomenon known as "cold-air drainage," that is, the tendency for cooler air to "drain off" of canyon rims and settle in the canyon bottoms below. For the ancestral Pueblo farmers, cold-air drainage was a critical factor in growing the corn and other crops upon which their very survival depended.
Many varieties of corn require roughly 120 frost-free days, from planting to harvest, to mature. So in an area where the average length of the growing season ranges from 110 to 150 days, even very small variations in the number of frost-free days can have dramatic effects on agricultural productivity. Pueblo farmers during ancient times undoubtedly considered the interrelated factors of cold-air drainage and growing season when choosing where to locate their farm fields.
As part of the Pueblo Farming Project, Crow Canyon researchers are measuring variations in temperature in two study transects—one located at the head of Crow Canyon (on the Center’s campus) and the other at the head of Goodman Point Canyon, where we are currently conducting excavations at several ancestral Pueblo sites. Both transects begin on the canyon rim, dip down into the canyon bottom, and continue up the opposite rim. In the spring of 2008, researchers placed a total of 10 temperature monitors at intervals along the transects. The monitors record temperature readings every hour, and we plan to leave them in place for several years. In addition, a large weather station is providing measurements of the accumulated precipitation on the Crow Canyon campus. The data recorded by the temperature monitors and weather station should help us understand how ancient people chose locations in which to grow corn, beans, and other crops.
So what have we learned so far? Well, we have already seen that, indeed, cold-air drainage does occur at Crow Canyon—and to an extent that it could have dramatically affected agricultural productivity. Although the results are preliminary, data downloaded from three of the monitors on campus indicate that the growing season in the shallow canyon bottom, where cold air tends to pool, can be at least one-and-a-half months shorter than the growing season in farmable areas adjacent to the canyon rims (see figure, below). It is likely that a similar phenomenon occurs at Goodman Point.
This raises an interesting question: Why did ancient farmers, who surely knew about the risk associated with cold-air drainage, choose to locate one of their farm fields in the shallow canyon bottom on Crow Canyon’s campus? We became aware of this field only after we discovered an ancient agricultural checkdam in the very location where we chose to place one of our own garden plots as part of the Pueblo Farming Project. Of the four plots, this one has been the most productive, possibly because it has the best soil and retains moisture. It is likely that this same location was chosen by Pueblo people for a “secondary field” planted as a buffer against potential crop losses in fields located on the warmer, but drier, canyon rim.
At different times, the size of the areas affected by cold-air drainage may have expanded or contracted due to shifts in the overall climate of the region. Monitoring the character of cold-air drainage at Goodman Point and on the Crow Canyon campus will help us understand how people responded to and viewed climatic shifts through time across the Mesa Verde region and beyond.
Pueblo Farming Project Gardens Update
The growing season for the Pueblo Farming Project gardens ended with a hard frost on October 12. Several light frosts occurred in October, with the first frost of the season recorded at the bottom of the drainage (near the Crow Canyon campus) on October 2. At the end of October, Pueblo Indian farming consultants will work with Crow Canyon staff to harvest the gardens. Watch for harvest details in the next issue of Crow Canyon's eNews.