Hydrology and chemistry of the Choptank River basin

The Choptank River basin is a coastal plain catchment dominated by agriculture (52 % of land use). We summarize an 11 year data set of discharge and chemistry from a gauged subbasin. Discharge exhibited seasonal variations driven by seasonal evapotranspiration. There were double seasonal maxima of pH, NH4+, NO3-, total N, Fe, and total P concentrations in late spring and fall as the saturated zone rose and fell within the soil. Significant interannual variability in discharge was the result of rainfall variation. There were positive interannual trends in NO3- concentrations and negative interannual trends in NH4+ and PO43- concentrations. These data were combined to estimate N and P export coefficients of 3-11 kg N ha(-1) yr(-1) and 0.14-0.66 kg P ha(-1) yr(-1), driven primarily by interannual variations in discharge. These export coefficients are low compared to other coastal plain watersheds dominated by agriculture and may be responsible for the small anthropogenic effects in the Choptank estuary compared to other Chesapeake drainages.