Nutrient Transport in an Agricultural Watershed as Affected By Hydrology

Abstract

Physical and chemical properties of nutrients combined with hydrological factors can dramatically affectnitrogen (N) and phosphorus (P) concentrations in and losses with streamflow from agricultural watersheds. In particular,adsorption and/or solubility of the different nutrient forms affect concentrations in subsurface drainage water, surfacerunoff water, and sediment in surface runoff. These concentrations, combined with the volumes or masses of subsurfacedrainage, surface runoff, and sediment, then determine nutrient losses. Data from a watershed study in northeast Iowaillustrate that the soluble, non-adsorbed nature of the nitrate anion results in nitrogen losses being dominated by this formin watersheds where soils and hydrologic conditions result in a significant proportion of streamflow being subsurfacedrainage. Inorganic P in the way of orthophosphate (PO4), because of its tendency to be adsorbed or precipitated fromsolution, usually has low concentrations in subsurface drainage and higher concentrations in sediment relative toconcentrations in surface runoff water. Therefore watersheds with lower relief and more subsurface drainage generallyhave lower phosphorus losses than watersheds with steeper more erosive soils and more surface runoff water.