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American Landscapes - Nebraska Sandhills, Submitted by Mary Ann Vinton
07.23.12
 
The Nebraska Sandhills region is one of the largest areas of mostly intact grassland ecosystems in the country and has the largest grass-stabilized sand dunes in the world. Mary Ann Vinton from Omaha is interested in how these hills have changed since the 1970s. The spread of center-pivot irrigation that taps directly into the vast underground Ogallala Aquifer has meant that agriculture has slowly taken over the landscape.

Landsat image comparison of Nebraska Sandhills region in 1972 (left) and 2011 (Credit: USGS EROS Data Center)
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map of continental United States with Nebraska Sandhills labeled The Nebraska Sandhills region is one of the largest areas of grass-stabilized sand dunes in the world. The Sandhills have remained primarily grassland since Euro-American settlement, in part, because of the semi-arid nature of the region and, more importantly, because of the fragility of the soils. When they exposed to a disturbance the soil does not recover and often makes the lands unsuitable for row crop agriculture.

An example of this disturbance is uncontrolled erosion which can cause "blowouts" where exposed below surface soils can quickly be removed by wind and water. Most of the Nebraska Sandhills region has remained as ranch land, either used for cattle grazing or for occasional cutting of hay to feed livestock. The human population has remained small and highly dispersed. Because the Sandhills as a region was never converted from its natural land covers to intense crop production, like eastern Nebraska or Iowa, it remains one of the largest mostly intact grassland ecosystems in the country.

Even though the Sandhills climate is sub-humid, the region has considerable water resources located underground. The High Plains aquifer (also known as the Ogallala aquifer) underlies the Nebraska Sandhills and the soils of the region are generally porous enough to allow rainwater to trickle down and recharge to the aquifer over the course of many years. The groundwater present in the region often reaches the surface in the low-lying valleys between the grass-stabilized dunes supporting various forms of wetlands such as wet meadows and rush-dominated marshes. These wetlands add to the ecological richness of this grassland region. Ranchers often cut the grass in the wet meadows for supplemental livestock feed.

The presence of shallow ground water has also brought land use change to parts of the Sandhills. In the 1950s, center pivot irrigation technology was developed and it allowed more acreage to be irrigated at a much lower labor cost. Center pivots use moveable, elevated piping on wheels that turn in a circle around irrigation wells and pumps. Water is pumped directly from the ground and applied to crops as the swinging arm passes over them.

Center pivot systems became much more used in the late 1960s and early 1970s, especially after spikes in grain prices driven by large purchases from the Soviet Union and other countries. For example, Nebraska had 2,735 of these systems in 1972 and by 1975 the number of center pivot irrigation systems had more than tripled to 8,517.

Peripheral areas of the Sandhills that had more level lands, shallow depth to water and somewhat less erosive soils became areas of potential land use conversion. In an Aug. 17, 1972, Landsat image, a discontinuous area of the Sandhills south of North Platte, Nebraska shows up as mostly shades of gray and green in this false color 4-2-1 band combination. The summer-dried grasslands in the center of the subset show little actively growing vegetation. Crop fields, mostly on the edge of the 1972 image, show up primarily as geometrical shapes of bright white (harvested, mostly bare soil reflecting back) and red (actively growing vegetation). Few circular features designating center pivot irrigation systems appear.

The situation had changed by the time of this Aug. 22, 2011, Landsat image that shows substantial gains in center pivot deployment across this portion of the Sandhills. The bright red circular fields, most likely growing crops such as corn and alfalfa hay, stand out interspersed among the mute grays of the native grasslands.

What these near "book end" Landsat change pair images do not show are the dynamic drivers of land change that occurred during the 39 years between these two dates. Mary Vinton asked "how regional land use responds to the ebb and flow of government incentives for commodities such as corn" and "how have surface water changed and its relationship to annual rainfall and human land use." These types of questions can be addressed more thoroughly with the temporal richness of the Landsat archive and the terrestrial monitoring potential offered by current and future Landsat imagery.

Related Links


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Roger Auch, Research Geographer
USGS EROS Data Center