John Wesley Powell, in the nineteenth century, introduced the notion that the 100th meridian divides the North American continent into arid western regions and humid eastern regions. This concept remains firmly fixed in the national imagination. It is reexamined in terms of climate, hydrology, vegetation, land use, settlement, and the agricultural economy. It is shown there is a stark east–west gradient in aridity roughly at the 100th meridian that is well expressed in hydroclimate, soil moisture, and “potential vegetation.” The gradient arises from atmospheric circulations and moisture transports. In winter, the arid regions west of the 100th meridian are shielded from Pacific storm-related precipitation and are too far west to benefit from Atlantic storms. In summer, the southerly flow on the western flank of the North Atlantic subtropical high has a westerly component over the western plains, bringing air from the interior southwest, but it also brings air from the Gulf of Mexico over the eastern plains, generating a west–east moisture transport and precipitation gradient. The aridity gradient is realized in soil moisture and a west-to-east transition from shortgrass to tallgrass prairie. The gradient is sharp in terms of greater fractional coverage of developed land east of the 100th meridian than to the west. Farms are fewer but larger west of the meridian, reflective of lower land productivity. Wheat and corn cultivation preferentially occur west and east of the 100th meridian, respectively. The 100th meridian is a very real arid–humid divide in the physical climate and landscape, and this has exerted a powerful influence on human settlement and agricultural development.

John Wesley Powell was a famous explorer, scientist, ethnographer, the first person of European heritage to raft down the Colorado River as well as a longtime worker within the U.S. government. He was the second director of the Geological Survey, where he oversaw ambitious land surveys to support rational development of the West in accordance with the climatic, hydrological, and geological constraints of the land. In 1879, he prepared for Congress his “Report on the Land of the Arid Regions of the United States” (Powell 1879), which argued for the requirement of a science-based approach to the development of irrigated agriculture in the arid regions and indicated the area west of the 100th meridian, apart from the wet coastal regions, to be that area. His advice, which according to DeBuys (2001) was an early appeal to “sustainable development,” was largely ignored, and in 1890, while under assault in Congress, he published three articles in Century Magazine to lay out his case. In one of these articles, Powell (1890) provides his most eloquent description of the 100th meridian:

Along the hundredth meridian from Manitoba to Mexico there is a zone of semiarid land. …The average rainfall, which varies much from year to year, is about eighteen inches on its western margin, and increases to about twenty-four on its eastern edge. Passing from east to west across this belt a wonderful transformation is observed. On the east a luxuriant growth of grass is seen, and the gaudy flowers of the order Compositae make the prairie landscape beautiful. Passing westward, species after species of luxuriant grass and brilliant flowering plants disappear; the ground gradually becomes naked, with “bunch” grasses here and there; now and then a thorny cactus is seen, and the yucca thrusts out its sharp bayonets. At the western margin of the zone the arid lands proper are reached. The winds, in their grand system of circulation from west to east, climb the western slope of the Rocky Mountains, and as they rise they are relieved of pressure and lose their specific heat, and at the same time discharge their moisture, and so the mountains are covered with snow. The winds thus dried roll down the eastern slope into lower altitudes, when the pressure increases and they are heated again. But now they are dry. Thus it is that hot, dry winds come, now and then, and here and there, to devastate the subhumid lands, searing the vegetation and parching the soil. [Powell (1890), pp 775-6]

Powell correctly notes the west–east precipitation gradient and then appears to attribute the full aridity gradient to the additional fact that the air, having crossed the Rockies, is exceedingly dry and warm, having been drained of its moisture on ascent and adiabatically warmed on descent. He correctly notes that such dry, warm air will extract moisture from the soil and vegetation. The grasslands of North America in the arid-to-semiarid plains are clearly partly a consequence of generation by the Rocky Mountains of a rain shadow to the east. Were that the only process operating, however, we would expect steady aridity east of the Rockies, so what processes allow a transformation back to humid conditions to the east of the 100th meridian?

Powell’s message regarding development of the west was that the sharp gradient in aridity centered on the 100th meridian necessitated quite different policies for European settlement (farm size, crops, irrigation, water resource development, etc.) to the west and the east. This had not been music to the ears of western politicians, who disliked talk of natural environmental limits to westward expansion and did not want to wait for Powell’s surveys and resource planning before advancing settlement and land use (Stegner 1954; Reisner 1986; Pisani 1992; DeBuys 2001). Nevertheless, the 100th meridian, passing through the Great Plains states of Texas, Oklahoma, Kansas, Nebraska, and the Dakotas, is a very real divide. And thanks to Powell [with assistance from his biographer, Stegner (1954)], it has been firmly etched into the nation’s psychogeography: a boundary with “broad and long-standing cultural and ecological significance” (Simon 2010, p. 97). It is readily visible from space, as the user of Google Earth can quickly verify, and is also plain to window seat passengers on airplanes flying across the continent.

In his classic study of the Great Plains, Walter Prescott Webb preferred the 98th meridian as the arid–humid divide but concurred with Powell about its physical and human significance:

As one contrasts the civilization of the Great Plains with that of the eastern timberland, one sees what may be called an institutional fault (comparable to a geological fault) running from middle Texas to Illinois or Dakota, roughly following the ninety-eighth meridian. At this fault the ways of life and of living changed. Practically every institution that was carried across it was either broken and remade or else greatly altered. The ways of travel, the weapons, the method of tilling the soil, the plows and other agricultural implements, and even the laws themselves were modified. (Webb 1931, 8–9)

One pertinent example of how laws were modified in the expansion of European settlement westward is the replacement of riparian water rights with the prior appropriation doctrine in the lands of the west (Pisani 1992). An example of how methods of development were modified is the stark reduction in density of the rail network at about the 98th meridian, which was established in the nineteenth century and persists today (see http://www.acwr.com/economic-development/rail-maps; last accessed 22 September 2017). As Webb (1931) points out, east of the meridian, railroads followed population, but to the west, the railroads needed to cross the plains, and this was only financially possible with investment by the national government of capital generated east of the meridian. Despite Webb, the numerical appeal of the 100th meridian being the arid–humid divide has endured. Regardless, what causes such a marked gradient in aridity across central North America, and how has it influenced the physical and human landscape? Powell (1890) provides one plausible cause of the aridity gradient. Is it correct? Will human-driven climate change transform the aridity gradient, effectively moving the 100th meridian to a new longitude? We seek to address these questions using data on climate, land hydrology, vegetation, human settlement, and the farm economy. Here, in Part I, we will address the following questions:

How is the aridity gradient realized in precipitation and soil moisture?

Given the physical climate and land hydrology conditions, how is the aridity gradient realized in terms of natural vegetation?

What are the climate dynamical processes that generate such a marked gradient in aridity across the 100th meridian?

How is the aridity gradient reflected in terms of settlement and land use, area under farms, number of farms, farm size, and crops grown?

By answering these questions, we will establish a firm understanding of how planetary-scale and local physical processes in the atmosphere–land system work to establish a sharp gradient in aridity across central North America and how this gradient has fundamentally impacted the settlement of people and the agricultural economy. To our knowledge, this is the first critical examination using extensive and varied data of the realism and expression of the 100th meridian concept.