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most common dryland crop in North America. Omitted from the map are significant
areas of rainfed wheat production in more humid regions to the east of the semi-arid
zone. The map shows variation in harvested wheat area within 5-min by 5-min lati-
tude/longitude grid cells and is based on production census data (Monfreda et al.
2008 ). Because some of the semi-arid wheat is produced under irrigation, the map
also shows areas where a majority of cropland is equipped for irrigation (Siebert
et al. 2013 ). The remainder of the harvested wheat area is representative of the
extent and importance of dryland farming in North America. The map shows that
the Northern and Central Great Plains of the U.S. and the Canadian Prairies are
areas with very high density dryland farming and that the harvested area of dryland
wheat production far outweighs irrigated wheat. The map also shows an area of high
density dryland farming in the inland pacific northwest (IPNW) of the U.S. The
southern Great Plains of the U.S. and Mexico are also areas with high density dry-
land farming, but the map shows only moderate density of harvested wheat because
much of this area is in production of other crops such as sorghum (Sorghum bicolor
L.) (Baumhardt and Salines-Garcia 2006 ). Dryland crop production in the high
Fig. 1 Semi-arid lands in North America as defined by an aridity index. The index classifies land
as semi-arid when P/ETP is between 0.2 and 0.5 (P annual precipitation, ETP annual potential
evapotranspiration). The map was created using data from the CGIAR-CSI Global-Aridity
Database (Zomer et al. 2007 , 2008 )
N.C. Hansen et al.