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 freshw ater eutrophication. To develop a simple method to estimate P export f rom the cropland, we studied 1.2-mu m filtered dissolved phosphorus (D P) output from four tiles draining areas ranging from 8 to 25 ha, and from a river draining a 48173 ha watershed in east-central Illinois du ring 1993 to 1996, The land was under maize (Zea mays L.)-soybean (Gly cine max L.) rotation. The tiles were estimated to contribute more tha n 86% of the river flow and 65 to 69% of the river DP export during 19 95 to 1996, The DP load from tiles followed consecutive pseudo first-o rder kinetics in terms of tile flow (DP load depended on the amount of DP remaining in the soil matrix), The kinetic curves indicated a solu ble-inorganic-P pool that was quickly depleted and replenished. In con trast, for DP export from the river at the watershed scale we observed pseudo zero-order kinetics based on river flow (DP export was indepen dent 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 differen t kinetic orders. For the study watershed, a one parameter equation co uld estimate watershed-wide DP export: k' x (surface water discharge f rom 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 differe nt geographic and agricultural characteristics.