The concept of "desert"
The meaning of the term "desert" has varied through time and across cultures. The term was sometimes used to describe any uninhabited or treeless land, whether or not it was arid, and sometimes to refer to hot and arid lands, evoking images of sandy wastelands. European colonists believed that treeless lands were not good for agriculture, thus the term desert also had the connotation of "unfit for farming."
The High Plains region is mostly semi-arid grassland and steppe. Today much of the region supports agriculture through the use of aquifer water irrigation, but in the 19th century, the area's relative lack of water and wood made it seem unfit for settler farming.