Finite element modeling of long-term phosphorus leaching through macropores in the Ozark ecoregion

Phosphorus (P) is a critical nutrient for plant growth in agriculture, but is also responsible for surface water enrichment that leads to toxic algal growth. While P loading to surface waters has traditionally been thought to occur from surface runoff, contributions from subsurface transport can also be significant. While P transport through many soil types is well-documented, the presence of highly conductive gravel outcrops and macropore networks can have a significant, yet poorly-documented effect on P movement to the aquifer. Floodplain soils in the Ozark ecoregion generally contain coarse chert gravel layers that exhibit macropore behavior. Previous research has evaluated short-term P transport in plot trials ranging from 1 m² to 100 m² across many Ozark ecoregion floodplain sites. Traditional methods of estimating P loading and soil saturation do not account for macropore flow and likely underestimate P transport to the water table. To address this concern, long-term P modeling was performed in HYDRUS-2D/3D using data collected from short-term plot experiments. Calibration was performed using single- and dual-porosity models with both homogeneous and heterogeneous gravel profiles. The dual-porosity model with heterogeneous hydraulic conductivity best matched experimental data, although the dual-porosity model with homogenous soil layers also performed well. Long-term P transport to a 3 m-deep water table was simulated using 9 years of both daily and 5 minute rainfall data with a P flux consistent with yearly poultry litter applications. Long-term simulations with 5 minute rainfall data found that 113 kg ha^-1 reached the water table over 9 years, or 21 % of P applied.