Kinetics and modeling of dissolved phosphorus export from a tile-drained agricultural watershed

Agricultural runoff can be a source of P, a limiting factor for freshwater eutrophication. To develop a simple method to estimate P export from the cropland, we studied 1.2-micrometer filtered dissolved phosphorus (DP) output from four tiles draining areas ranging from 8 to 25 ha, and from a river draining a 48 173 ha watershed in east-central Illinois during 1993 to 1996. The land was under maize (Zea mays L.)-soybean (Glycine max L.) rotation. The tiles were estimated to contribute more than 86% of the river flow and 65 to 69% of the river DP export during 1995 to 1996. The DP load from tiles followed consecutive pseudo first-order kinetics in terms of tile flow (DP load depended on the amount of DP remaining in the soil matrix). The kinetic curves indicated a soluble-inorganic-P pool that was quickly depleted and replenished. In contrast, for DP export from the river at the watershed scale we observed pseudo zero-order kinetics based on river flow (DP export was independent of how much DP remained in the watershed). The contribution from numerous tiles and surface runoff to the river may have stabilized DP export at the watershed scale and therefore could explain the different kinetic orders. For the study watershed, a one parameter equation could estimate watershed-wide DP export: k' x (surface water discharge from the watershed) x (watershed area), with k' being 3.94 x 10(-6) mg P L-1 ha-1. Our approach should be tested in watersheds with different geographic and agricultural characteristics