The Land of the Stone Hoe

The Hohokam irrigated the desert in central Arizona for a thousand years before they disappeared in the fifteenth century. They constructed and maintained hundreds of miles of canals and ditches. What follows is an analysis of how the canals were designed and constructed.

For more on Southwestern water issues see Rainfall in the Anasazi Region.

Canal Design:
Excavate 4 square units.
Angle of Repose -- 30 Degrees -- 2 to 1.
Canal is 2 units deep and 10 units wide.
Canal capacity is 12 square units.
Water depth of 1.5 units.
Canal carrying capacity of 8 square units.
Assuming a unit of 1 yard and a flow rate of 1 yard per second (2 M.P.H.) this canal could carry 216 cubic feet per second.

An acre needs about 4 feet of water to make a crop. Figuring a 110 days growing season, the one yard basic unit ditch described above can deliver enough water for nearly 10,000 acres.

Canal construction:
One person can dig one cubic yard per hour.
A two-person team can excavate one cubic yard per hour.
When the basic unit is a yard, four two-person teams (8 people) digging can extend the canal 30 feet per day, that's about 870 feet every lunar month.
8 people can construct 1 mile of major canal in 176 days.
Assume a team of 10 can build 2 miles of major canal in a year.
A single 10 person team could have built 100 miles of Hohokam Canal in 50 years.

Double the basic unit to 2 yards and four 10 person teams (40 people) are needed to construct at the same rate and the canal would carry 864 cfps (that's about 600 million gallons a day). This super canal would be 60 feet wide and carry water at a depth of 9 feet. Some of the ancient canals are nearly this size.

Conclusion:
It's obvious that the Hohokam had little problem fulfilling their canal and water needs. Actual construction was probably preceded by a small pilot ditch to establish grade. A fall of 1 foot per thousand would insure a good flow but prevent bank erosion. A canal under construction was likely flooded periodically to soften the ground to facilitate digging.