I was surprised that Arkansas was not an answer. Your source is about soil lost or land affected. As for numbers of people affected there may be different answers. According to https://en.wikipedia.org/wiki/Okie "By 1950, four million individuals, or one quarter of all persons born in Oklahoma, Texas, Arkansas, or Missouri, lived outside the region, primarily in the West", and "The migrants included people from Oklahoma, Arkansas, Missouri, Iowa, Nebraska, Kansas, Texas, Colorado and New Mexico, but were all referred to as "Okies" and "Arkies.""