Many of those farmers were irrigating before ethanol was an issue. The growth of irrigated crops started after WWII per the following:
itc.tamu.edu/mf2849.pdf
Rapid expansion of Kansas irrigation occurred following WWII for a variety of reasons, including political/societal will, technology, and readily available energy (Figure 1). The 1945 water appropriation act, which provides the basis of Kansas water law today, was designed to encourage development of water resources. With improvements in irrigation well drilling and pumping equipment, and the development of the Hugoton natural gas well field, irrigation acreage increased rapidly using groundwater from the Ogallala Aquifer.
System Type Acreage Trends
Irrigation system types have changed over time, switching from predominately surface flood irrigation to sprinkler irrigation, which is predominately center pivots (Figure 2a). In 1970, about 18 percent of the 1.8 million irrigated acres were sprinkler irrigated. The volatility in the reported total irrigation acreage base until 1989 was due to the association of irrigation data being reported using authorized acres as opposed to acres actually irrigated on the irrigators’ annual water-use report. Never-the-less, much of the increase in the total irrigation acreage base during the 1970s was associated with the adoption of center pivot irrigation, with an increase of nearly an additional million irrigated acres by center pivots. In 1990, about one half of all acres were center pivot irrigated. Since 1990, the total irrigated acreage base has remained relatively stable, but center pivot irrigation now accounts for nearly 90 percent of all irrigated land.
But then, irrigation is growing for another reason:
agfax.com/.../irrigation-booming-in-midwest-as-producers-try-to-boost-...
In a year like this, irrigation systems have returned 30% to 100% of their cost in one year. Murdock’s yield difference between irrigated and non-irrigated is at least 160 bushels per acre. “And who knew we’d have $8 corn at harvest time?” Murdock asked.
In Texas:
www.tshaonline.org/handbook/online/articles/ahi01
In 1913 the Texas legislature passed the first major irrigation act, which called for the establishment of the Board of Water Engineers to regulate water appropriations. A series of dry seasons (1916–18) aroused renewed interest in irrigation from storage reservoirs, and a constitutional amendment was adopted providing for the formation of water-conservation districts by landowners. Whereas the previous irrigation projects had been largely private ventures, most of the new ones were formed under the district act, and most of the former private projects were taken over by public agencies and operated as a nonprofit service to the land. The use of irrigation continued to increase throughout the first half of the twentieth century. In 1948, of the nearly 30,000,000 agricultural acres in the state, 2,884,700 acres was under irrigation. Irrigated land accounted for about 10 percent of the state's harvested acreage. Irrigation was practiced on 29,779 farms; sprinkler or overhead systems were used on more than 850. In some areas irrigation was absolutely necessary for successful agriculture. In others, rainfall was normally sufficient to sustain plant growth, but production was greatly increased by supplemental irrigation.
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The irrigation acreage supplied by groundwater pumped from wells was said to have expanded more rapidly in Texas than in any other area in the United States. Methods of irrigation established in various sections of the state depended upon the crops to be grown and the available water supply. In the rice belt the common method used was border flooding or pans. In the Valley the basin flooding method was used for citrus fruit and the furrow or row method was used for vegetables and field crops. The row method was followed also for cotton and grain crops in other sections of the state.
Irrigated land in Texas increased to 3,131,534 acres by 1950. The growth was largely a result of the rapid development of irrigation on the High Plains, which was supplied exclusively with high-quality groundwater from the underlying Ogallala sands. Irrigation of the region continued to grow spectacularly—more rapidly than any development elsewhere—to 5,894,686 acres by 1974. The High Plains was one of the largest irrigated areas in the United States and represented 65 percent of all Texas irrigation. The amount of irrigated cropland harvested increased from 10 percent in 1948 to about 25 percent during the mid-1960s, to 72 percent in 1973. In 1974 there were 8.5 million acres in Texas under irrigation. Of that total, 22 percent (more than 1,800,000 acres) was irrigated by sprinkler systems and the rest was watered by surface systems that channeled water in borders, rows, and field levees. The field levees were used mainly for rice. Many irrigation systems employed efficient pipelines or lined ditches to prevent water loss. Drip irrigation generated some interest, especially for tree crops, and was used on 4,500 acres; thereafter the method became standard for many kinds of orchard. Eighty percent of the total irrigated acreage came from water pumped from wells. Important groundwater-supplied irrigated areas included the Winter Garden Region and adjacent lands below the Balcones Escarpment, the Trans-Pecos farms of Reeves, Pecos, and Ward counties, the Marfa, Van Horn, and Dell City vicinities, and part of the Gulf Coast, as well as several north central Texas localities (where, especially during the 1960s, there was an increased use of sprinkler systems for peanut growing). Major areas supplied with surface water were the lower Rio Grande valley, portions of the Gulf Coast rice-growing area, the El Paso valley area, and alluvial lands along the lower Brazos River and other Texas rivers and tributaries.
With regards to corn and Kansas:
skyways.lib.ks.us/genweb/archives/1912/c/corn.html
About 1895 J. M. McFarland, formerly assistant secretary of the Kansas State Board of Agricuture[sic] and statistician in the United States department of agriculture, published a pamphlet showing the production of corn in the eastern part of Kansas—that is east of line drawn from the northern boundary of the state between Smith and Jewell counties to the southern boundary between Harper and Barber counties—as compared with the great corn growing states east of the Mississippi river, for the ten years 1884 to 1893, inclusive. Illinois was the only state east of the Mississippi that exceeded eastern Kansas in every one of the ten years. In 1886 Kansas was exceeded by Illinois and Indiana; in 1887, owing to a marked decrease in the acreage in eastern Kansas, it was exceeded by Ohio, Indiana, Illinois and Kentucky; in 1890, when the acreage fell off to about one-half that of the preceding year, it was exceeded by Ohio, Indiana, Illinois, Kentucky and Tennessee.
The greatest corn crop in the history of Kansas was in 1889, when the state produced 273,988,231 bushels, having over 5,000,000 acres in "waving corn fields." This great crop led Gov. Martin to say in an interview: "Corn is the sign and seal of a good American agricultural country; corn is an American institution; one of the discoveries of the continent. It was known to the Indians, and to cultivate it was one of the few agricultural temptations which overcame their proud and haughty contempt for labor. Kansas has corn and so has luck."
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The corn crops of Kansas for 1910, when over 8,500,000 acres were planted, amounted to 152,810,884 bushels, valued at $76,402,328.
