Using crop rotation models (1665-1820), census data (1850-2000), maps (1850, 1900), aerial photography (1938), and remote sensing images (MSS 1976, TM 1988 and 1996), four periods were distinguished in the history of land cover in the Choptank basin. The first, extensive conversion (1662-1760) was characterized by tobacco cultivation, which demanded little land in crops but large areas in forest fallow, causing low net rates of deforestation and broad occupation of the land in farms. The second, intensive conversion (1760-1800) occurred when cropland expanded due to wheat cultivation, and most undisturbed and fallow forest disappeared. During the third, semi stationary period (1800-1950) agricultural land (cropland + grassland + forest fallow) was relatively stable, although cropland decreased somewhat due to improved management practices. The last period, afforestation and early urbanization (1950-1998), occurred when the land in farms decreased by ~25% (~15% as forest regrowth and ~9% as developed areas) due to fertilizer applications and increasing crop yields. Spatial variations in land cover classes within the basin were influenced by soil drainage properties and distance from water.

The effect of land cover change on nutrient yields was estimated using the Generalized Watershed Load Function model (GWLF). This model was parameterized using the results of land cover, with spatial and temporal adjustments for human populations, soils, and land use. Changes in nutrient export were driven mainly by land cover modification (population density, fertilizer applications). During 1850-1950, under extensive use of the land with slow increases in crop yields, export of N and P was low (1-2 kg N and 0.1-0.2 kg P ha-1 y-1). However, from 1950 to 2000, with increasing use of synthetic fertilizer, there were rapid increases in crop yields, and export of N and P increased rapidly (10x for N and 2x for P) due to increases in ground water NO3 and surface soil N and P content. Land cover change determined the temporal variations in nutrient yields, while soil characteristics regulated the magnitude of the yields.