COMPARISON OF SUBSURFACE AND SURFACE RUNOFF PHOSPHORUS TRANSPORT CAPACITIES IN ALLUVIAL FLOODPLAINS

There have been numerous studies on phosphorus (P) contributions from surface runoff, but studies comparing the contribution of surface versus subsurface P are limited, as subsurface transport is often considered negligible. Previous work has shown that the transport of P in the gravelly subsurface at two sites in northeast Oklahoma can be significant, especially in preferential flow paths (PFPs), hypothesized to be buried gravel bars. The objective of this project was to quantify subsurface P losses based on field data, and compare with surface runoff P losses derived from Pasture Phosphorus Management Calculator (PPM Plus) simulations. Ozark ecoregion study sites adjacent to the Barren Fork Creek and Honey Creek, neither of which have received litter applications or extensive cattle production in the past decade, were instrumented with observation wells. Groundwater levels and P concentrations were monitored for several months. Using a P transport capacity equation and Monte Carlo simulations based on appropriate statistical distributions derived from these data, the mean subsurface P load traveling along with the groundwater through the non-PFP flow domain and a single PFP was estimated to be 0.12 kg yr-1 and 0.02 kg yr-1 for the Barren Fork Creek and Honey Creek field sites, respectively. Monte Carlo simulations for surface loads were performed using PPM Plus based on current site conditions (i.e., no fertilization or cattle grazing), resulting in average total P surface runoff loads of 0.46 kg yr-1 for the Barren Fork Creek site and 0.67 kg yr-1 for the Honey Creek site. Simulations were also performed based on typical intensive pasture management for the region with poultry litter application and cattle grazing. These simulations resulted in average total P surface runoff loads of 14.0 kg yr-1 at the Barren Fork Creek site and 9.8 kg yr-1 at the Honey Creek site, two orders of magnitude greater than the estimated subsurface P transport capacities on low intensity agricultural fields. Subsurface P contributions with a single PFP was significant compared to surface runoff loads for the low intensity agricultural fields. These results indicated that the subsurface P capacity of alluvial floodplains in the Ozark ecoregion was at least 0.01 to 0.10 kg yr- 1, although the capacity may be higher in cases with greater numbers of PFPs and where the subsurface is connected to a larger P source. Further work on subsurface P transport should address sites with P application and the factors that influence P leaching through the topsoil