Landuse boundary east of Amarillo, Texas: grasslands and center-pivot spray irrigated cropland.

The excellent Ken Burns documentary, “Dustbowl” (see previous post) featured personal accounts of many individuals who experienced the disaster first hand. The film emphasized a mostly non-technical, human perspective that, I thought, did a good job of placing us into the shoes of those who endured incredibly tough circumstances – within the United States.

In the second half of the film, Burns introduced Hugh Hammond Bennett and other soil scientists who proposed improved soil management thechniques, such as planting wind rows and contour plowing to help conserve moisture and reduce erosion.

In the years since the Dustbowl days, soil erosion control methods have improved but irrigation is the major technical adaptation installed to cope with severe drought is irrigation. The Dustbowl film did not really address this and, frankly, the tapping of the Ogallala Aquifer to support agriculture in the American Plains is worthy of another Burns documentary.

Irrigation has long been viewed as the solution to drought problems in the American West.  After the Civil War, Major John Wesley Powell, who lost an arm at the Battle of Shiloh, became a field geologist and led the United States Geographical and Geological Survey of the Rocky Mountain Region. His landmark text and maps Report on the Lands of the Arid Region was published in 1879. In the Report, Powell did not anticipate the use of groundwater, but expressed high expectations for irrigated western agriculture supplied by surface flows:

There are two considerations that make irrigation attractive to the agriculturist. Crops thus cultivated are not subject to the vicissitudes of rainfall; the farmer fears no droughts; his labors are seldom interrupted and his crops rarely injured by storms. This immunity from drought and storm renders agricultural operations much more certain than in regions of greater humidity. Again, the water comes down he mountains and plateaus freighted with fertilizing materials derived from the decaying vegetation and soils of the upper regions, which are spread by the flowing water over the cultivated lands. It is probable that the benefits derived from this source alone will be full compensation for the cost of the process…

A stranger entering this Arid region is apt to conclude that the soils are sterile, because of their chemical composition, but experience demonstrates the fact that all the soils are suitable for agricultural purposes when properly supplied with water. It is true that some of the soils are overcharged with alkaline materials, but these can in time be “washed out.” Altogether the fact suggests that far too much attention has heretofore been paid to the chemical constitution of soils and too little to those physical conditions by which moisture and air are supplied to the roots of the growing plants.

To his credit, Powell recognized, but apparently failed to convince enough others, that only a small fraction of the Arid Region could be feasibly used for irrigated agriculture. What he had in mind were flat valley bottoms adjacent to streams.

In the top figure, the aerial photo east of Amarillo, TX, the circular shapes are center-pivot spray irrigated croplands. Typical crops are cotton, sorghum, and corn. The green area to the north and west is grassland. The partitioning of the land into these two usages comes with recognition of the limitations of site specific soil and landscape conditions. The whitish area to the west is an actively eroding soil surface.

Spray irrigation in north Texas is mostly supported by groundwater from the Ogallala Aquifer. In the second figure, the drawdown-recharge map produced by the USGS shows areas of depletion, recharge and no significant change (click on the image for a larger view). The map was created from data ending in 1995, so it’s somwhat outdated.

Unless precipitation trends change, it looks like the areas of depleted groundwater reserves will ultimately have to shift operations from irrigated crops to light grazing, or left idle.

As we learned from the Dust Bowl, “rain” doesn’t necessarily “follow the plow.”