Recent Land Use Changes in the Rhode River Watershed
by Don Weller
You've probably seen a news story about land use change. You may have read about Maryland's "Smart Growth" program, which seeks to control the conversion of farms and forests into suburbs and cities, or you may have heard Vice President Gore's calls for smart growth and livability. Perhaps urban "sprawl" was a hot issue in your local election? As a friend (or even neighbor?) of SERC, you may also be interested in land use changes in SERC's study areas in the Rhode River watershed.
Land use change is important because land use affects everything else in the environment. Changes in land use alter soil properties, applications of fertilizers and other materials, the distribution of plant communities, and available habitats for animals. Land use affects the amount and composition of stream water, so land use changes also impact aquatic ecosystems. Studies by SERC scientists and others have shown that land use strongly affects the delivery of sediment and plant nutrients (nitrogen and phosphorus) to Chesapeake Bay, where overloads of these materials have contributed to the Bay's ecological distress.
SERC scientists have been monitoring land use and stream discharge in the Rhode River watershed for several decades (Fig. 1). The 13 square mile watershed is a complex patchwork of forests of various ages, wetlands, agricultural land (cropland, pastures, and fallow fields), and residential land (Fig. 2). This article describes changes in land use for the Muddy Creek subwatershed from 1957 through 1993. Muddy Creek is major source of freshwater to the Rhode River, and its 9 square mile watershed is about two-thirds of the Rhode River drainage (Fig. 1).
SERC botanist Dan Higman first described land use patterns in the Muddy Creek watershed in the 1970s. He worked from aerial photographs taken in 1957, 1972, and 1976. Dan used optical instruments to copy outlines of land use patches from photos to plastic overlays, and then he manually measured the area of each patch. More recently, we began tracking land use with a computerized geographic information system (GIS), and we digitized detailed land use maps for 1990 and 1993 from high-resolution aerial photographs. The earlier and later maps were both "ground truthed" by using on-the-ground observations of each land use patch to verify or correct its classification. We also recorded the numbers of houses present at each time. We used the maps from different years to measure changes in the areas occupied by eight land uses: forest, croplands, grasslands, fallow lands, old fields, yards and roads, marsh, and water.
Cropland was the land use category showing the greatest change in area, dropping from 32% of the Muddy Creek watershed in 1957 to 20% in 1972 and then to 6% in 1993 (Fig. 3). Forest was the next most dynamic category, increasing from 41% in 1957 to 58% in 1993. Grassy areas increased from 9% to 18%, mostly after 1972, while old fields dropped from 12% in 1957 to 4% in 1993. Total agricultural land (cropland, grass, and fallow fields) dropped from 42% in 1957 to 26% in 1993. For the northern third of the Muddy Creek basin, pasture land declined from 17% of the area in 1976 to 9% in 1993, a loss of almost half of the 1976 pasture. Yards and roads increased 2% in 1957 to 10% in 1993, and the number of houses rose from 139 (15 per square mile) in 1957 to 349 (39 per square mile) in 1993.
On a relative basis, yards and roads in the Muddy Creek drainage quadrupled their 1957 area, while fallow land, grassland, and forest have increased by 2.4, 2.0, and 1.4 times, respectively. These increases are balanced by the drop in cropland to one-fifth of 1957 level and a drop in old fields to three-tenths of the 1957 area. Marsh and water did not change much.
The loss of cropland and active pasture represents a major decline in the importance of agriculture in the Muddy Creek watershed. The watershed remains rural, but forests, horse farms, non-commercial "farmettes," housing, and roads now occupy much of the land formerly used for food and fiber production. Both Anne Arundel County and the entire state of Maryland have also lost agricultural land and gained developed land in recent years. According to agricultural statistics, "land in farms" was 61% of Maryland's total land area in 1957, falling to 48% in 1972, then to 35% in 1993. According to the Maryland Office of Planning, 7% of the land in Anne Arundel County was converted to developed land between 1973 and 1990, 5% from forest and 2% from agriculture. Compared to the County and State, the Muddy Creek basin has lost more agricultural land, but this loss has been accompanied by a gain rather than loss of forests.
Simple loading models predict that the land use changes from 1972 to 1993 should have reduced both nitrogen and phosphorus discharges by roughly one-fourth, primarily because of the loss of cropland. However, long-term measurements of nitrogen and phosphorus discharge in streams have not shown such reductions. SERC scientists continue to monitor land use change and stream discharge as they try to explain this surprising observation. What do you think they will find?
Dr. Don Weller is a quantitative ecologist and Principal Investigator of SERC's Ecological Modeling Group.
Fig. 1. Map of the Rhode River and Muddy Creek watersheds.
Fig. 2. Aerial view of part of the Muddy Creek watershed.
Fig. 3. Land use changes in Muddy Creek watershed from 1957 through 1993.
This is an article written for the SERC newsletter for Summer 1999.