Iowa's Statewide Land Cover Inventory
by James D. Giglierano
The Iowa Geological Survey (IGS) has used aerial and space-borne imagery on a routine basis for the last 30 years. The discipline of imaging and measuring a subject without physical contact is called "remote sensing." Techniques for counting migrating geese, assessing flood damage, geological mapping, and determining land use and vegetative cover have been developed at IGS using this remote sensing technology. In the last 20 years, these techniques have advanced beyond manual interpretation of photographs to computer-assisted processing of electronic imagery that allows enhancement of an image’s appearance; the identification of patterns of vegetation, urban areas, and water; as well as the use of combined imagery from different sources. These new digital techniques do not replace the need for manual interpretation of images, rather they supplement IGS's ability to perform natural resource inventories and mapping.
Satellite imagery is particularly adaptable to conducting land cover inventories over large areas of the earth’s surface. The Thematic Mapper instrument on the current series of Landsat satellites can image a 185-square-kilometer area anywhere on the planet every 16 days. Once images are taken, the satellite sends them to a ground receiving station by radio transmission. Electronic versions of imagery are distributed to users for manipulation and analysis on their computers. Objects smaller than 30 meters are not resolved by the Thematic Mapper sensor, but patterns of fields, forest tracts, waterways, and large artificial structures are easily seen.
Several years ago, IGS began to map the major land cover types across Iowa as a baseline
inventory for natural resource management programs of the Iowa Department of Natural
Resources (DNR). Twenty-four Landsat satellite scenes were needed to cover the
entire state, and many months of computer processing and image interpretation were
required to complete the land cover inventory. A preliminary version of the
completed mosaic for the entire state is shown above. Most of the images are
from 1992, but in some areas clouds dictated the use of imagery from 1990, 1991, or 1993.
The land cover map shows that 60% of the state is covered by row crops; 30% by
grasslands, including pasture, hay land, prairie, and wetland vegetation; and 7% by forest
lands. Urban areas, including pavement, buildings, and other large structures,
account for 1%, and water bodies cover another 1%. Too small to see on the map are
barren areas (less than 1%) that include flooded cropland and sand bars.
While land cover and cropland acreage statistics are collected for each county on a
routine basis, DNR’s land cover inventory is unique in that it visually shows the
distribution of cover types across the state’s entire landscape. Most forest
lands are concentrated in eastern Iowa along river corridors. Large areas of pasture
and hay are located in northeast Iowa, along with a broad, semi-circular swath in
south-central Iowa. Rich cropland is particularly noticeable in north-central
Iowa. Along the western border of the state, grass and trees mark the boundary of
the Loess Hills with the fertile Missouri River floodplain.
One of the most valuable uses of the land cover inventory is as a tool for evaluating changes through time. Recently, Iowa State University completed a series of county maps showing the distribution of pre-settlement vegetation taken from maps and notes of original Government Land Office surveyors in the mid-1850s. The map above shows this pre-settlement vegetation for Johnson County. Most of the county (70%) was covered with prairie vegetation, and included large tracts of forest and oak savanna. A few small fields of crops are evident throughout the county. Compare this with the 1990s land cover information for Johnson County below, taken from the statewide land cover inventory. Today, row crops cover 49% of the county, and very few prairie tracts remain. Almost all of the grassland represents residential lawns, parks, pasture, and hay fields. Since most of the vast expanse of original prairie disappeared during the last 150 years, one realizes the importance of protecting the small remaining tracts.
What will Iowa’s landscape look like 10, 20, or 50 years from now? What
patterns will be written on the land by agricultural market forces, government policies,
technological progress, or perhaps human-induced climate change? Now, more than
ever, there is a clear need for repeated assessments of Iowa’s changing landscape.
Future generations may look in wonder at the striking similarities or differences
between their own landscape and that portrayed on this present inventory of Iowa’s
land cover.
The Thematic Mapper imagery for this inventory was made possible by the U.S. Geological Survey’s GAP Analysis Project for Iowa, which is a detailed inventory of natural vegetation communities, land ownership, and the distribution of terrestrial vertebrate species. The Natural Resources Conservation Service collected information essential to "train" computer programs to identify the land cover types of every county, and to provide test data to assess the accuracy of the classification.
Adapted from Iowa Geology 1999, Iowa Department of Natural
Resources