Relationships Between Land Use, Land-Use Change, and Water-Quality Trends in Virginia

This research examines the relationships between land use and surface water quality trends in Virginia. Data from 168 surface water quality monitoring stations throughout Virginia were analyzed for trends for the period of 1978 to 1995. Water-quality data available at these stations included dissolved oxygen saturation (DO), biochemical oxygen demand (BOD), pH, total residue (TR), non-filterable residue (NFR), nitrate-nitrite nitrogen (NN), total Kjeldahl nitrogen (TKN), total phosphorus (TP), and fecal coliform (FC). A seasonal Kendall analysis was used to determine trends for each water-quality parameter at each station; this analysis produced an indicator (Kendall's tau) of improving or declining water quality. Median values for each water-quality variable were also determined at the monitoring stations.

Virginia land use was determined from the USGS Land Use Land Cover (LULC) data (1970s) and the Multi-resolution Land Characteristics (MRLC) data (1990s). Land-use variables included urban, forest, pasture, cropland, total agriculture, and urban change. These six variables were correlated with Kendall's tau to determine if relationships exist between water-quality trends and land use. Water-quality medians and land use were also correlated.

In general, highly forested watersheds in Virginia were associated with improving water quality over the 1978 to 1995 study period. These watersheds were also commonly associated with better water quality as measured by the water-quality medians. Watersheds with less agricultural land tended to be associated with improving water quality. Better water quality, as measured by the water-quality medians, was generally associated with watersheds possessing fewer urban acres. There were few significant relationships between water-quality medians and agricultural variables.