Land Use Change Historical Shenandoah Valley
Reconstructing Historical Land Use and Land Cover Information for the Shenandoah Valley
The primary goal of this research activity is to reconstruct and visualize historic land use change in the Shenandoah Valley region (~11,000 sq km) and to link these changes with modern environmental problems. The extent of agricultural land vs. forest is the initial focus and future work will look at how these uses link with environmental issues such as changes in water quality and carbon fluxes. The initial period of interest is approximately 1950-2000. Later work may push reconstructions back to the time of the American Civil War, and possibly as far back as the initial European settlement of the region in the early eighteenth century.
The "Shenandoah Valley" means different things to different people. The Shenandoah River Watershed, as mapped with HUC-8 level watershed data (view data), contains a significant portion of seven counties in Virginia and one county in West Virginia. Historical studies of the region often include additional modern counties because they were derived from earlier, larger counties. The initial analyses portrayed in the charts below are focused on eight counties in Virginia. Two counties from West Virginia will be added. While the level of development in the Shenandoah Valley is not as great as some other parts of the Chesapeake Bay watershed, the region contains 4 of the top 5 agricultural counties in the State of Virginia (based on 1997 cash receipts). Farms and farming are receiving a lot of attention because of their potential impact on the local and regional environment and because of the loss of farmland due to development.
One of the most commonly available information sources on historic human activities in the United States are the federal census records. Both population and agricultural census records can be used to identify major trends in human activities at the county level. By 1840 the modern counties in the Shenandoah Valley region had been formed so the census data reflects the same area through time. Population density (view data) in the region has changed dramatically, but not uniformly, in the twentieth century.
The number of farms (view data) reached a peak in most counties in the first half of the twentieth century, and recent changes are small compared to the historic highs and lows (It should be noted that the numbers used in these charts have not been adjusted for missing dates or categories, or changes in definitions). Farmland information in the agricultural census records has not been recorded in consistent categories through time. In addition to total farmland, two broad categories that can be fairly consistently derived for most census years are "improved" and "unimproved" land. Improved land typically includes cropland and pastures. Unimproved land typically includes woodlots, whether they were used as pasture or not. Tracking the changes in improved versus unimproved land, instead of total farmland, can give us more detail about human activities. For the eight Virginia counties included in this study so far, we can see that these categories of land (view data) have not followed the same trends through time. At the county level (view data), we can see similar trends in improved land and recent changes are small compared to the historical highs and lows.
Except for farm-level census data that exists in some nineteenth century manuscript returns, census data is aggregated at the county level. In order to obtain more spatially explicit information, other sources must be consulted. A comparison can be made between historic trends as found in county level census data and other known records (view data) in order to determine the best sources to utilize. For the Shenandoah Valley region, some of the cartographic and image sources we have identified include:
- Mid-1970's to Present: Satellite Imagery
- Late 1930's to Present: Aerial Photography
- 1963 - 1994: USGS 7.5 Minute Topographic Maps (view map) with Woodlands symbology.
- 1906 - 1947: USGS 15 Minute Topographic Maps (view map) with Woodlands symbology
Extracting historic land use and land cover information from analog sources can be laborious. Experiments are currently underway to automate some of the process of extracting relevant information and creating change maps. In the example below, two historic USGS topographic maps (1944 Parnassus 15' and 1967 Parnassus 7.5' topographic map sheets (view map, view map) of the same area were processed to produce a forest change map. After scanning and georeferencing the maps, the green "woodlands" information was extracted and then analyzed to determine where forests existed on one or both map sheets.
Some Related Research:
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