|John Wesley Powell, 1834–1902 (Wikipedia)|
For instance, on US 20 in Nebraska, Valentine is definitely in the West, but anything east of Ainsworth feels like the Midwest.
Driving west across South Dakota, the Missouri River makes an easy marker. From downtown Pierre, I see the dry hills to the west and feel at home. (It helps that I lived as a kid in western South Dakota.)
Another tradition is just to use the 100th meridian of longitude as the marker. John Wesley Powell, Civil War veteran and visionary Western geographer, made this one popular. (In this New York Times article, the writer ventures among the natives along the 100th meridian.)
Some climate researchers, however, are now saying that the arid/wet boundary is shifting eastward. "Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part I: The Story So Far" is an article published by the American Meteorological Society.
Its abstract (summary) states,
The aridity gradient [east and west of the 100th meridian] 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.This boundary has moved before. An archaeologist friend pointed out to me that in part of the Middle Archaic period (3000–1000 years ago), trees extended farther east onto the plains. Think of of the "pine ridge" country of the Palmer Divide (eastern Douglas and El Paso counties, Colorado) extending clear to Kansas! "Those were the good times," he mused.
Part II of the article makes this prediction for the 21st century:
It is first shown that state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project generally underestimate the degree of aridity of the United States and simulate an arid–humid divide that is too diffuse. These biases are traced to excessive precipitation and evapotranspiration and inadequate blocking of eastward moisture flux by the Pacific coastal ranges and Rockies. Bias-corrected future projections are developed that modify observationally based measures of aridity by the model-projected fractional changes in aridity. Aridity increases across the United States, and the aridity gradient weakens. The main contributor to the changes is rising potential evapotranspiration, while changes in precipitation working alone increase aridity across the southern and decrease across the northern United States. The “effective 100th meridian” moves to the east as the century progresses.
|The Anderson Creek fire burned almost 400,000 acres in Oklahoma and Kansas in March 2016.|
In "Why is Oklahoma Burning," weather writer Bob Henson discusses the recent Rhea Fire, which burned more than 242,000 acres.
May 2015 was the state’s wettest single month on record, and 2015 was its wettest year. “The November-December 2015 period was the wettest on record as well, and the sixth warmest. So the growing season extended into winter to some extent that year,” said McManus. The result was an unusually lush landscape going into the first part of 2016 that dried out quickly in the weeks leading up to the Anderson Creek fire.
Likewise, the summers of 2016 and 2017 were on the moist side, said McManus. “We also had a pretty severe ice storm during January 2017 that left lots of big fuels on the ground waiting for that spark,” McManus said. Later that year came the the state’s second-wettest August on record. “August would normally be a time we'd get rid of some growth in our typical summer burn season,” said McManus.
The landscape of the Southern High Plains has been extraordinarily dry over the last six months. The western third of Oklahoma has seen little more than 2” since October—only about 20% of average—and most of the Texas and Oklahoma Panhandles have received much less than 1”, making it the driest six months on record in some locations. Any moistening of the landscape has been all too brief, which has left the landscape highly vulnerable to a spell of fire-friendly weather.Some good photos there too.