SIMULATING WATER QUALITY IMPROVEMENTS IN THE UPPER NORTH BOSQUE RIVER WATERSHED DUE TO PHOSPHORUS EXPORT THROUGH TURFGRASS SOD

The Upper North Bosque River (UNBR) watershed is under a total maximum daily load (TMDL) mandate to reduce loading of soluble phosphorus (P) in impaired river segments. To address these problems, Texas A&M University researchers have developed a Best Management Practice (BMP) that removes excess nutrients from impaired watersheds through turfgrass sod. Harvest of manure-grown sod removes a thin layer of topsoil along with any residual P in this soil layer. In order to assess the impact of the turfgrass BMP on a watershed scale, the Soil and Water Assessment Tool (SWAT) was used to predict water quality changes among four scenarios in the UNBR watershed. The SWAT model was modified to incorporate turfgrass harvest routines for simulation of manure and soil P export during harvest of turfgrass sod. SWAT simulations of the four BMP scenarios predicted reductions of 20% to 36% for instream P loads in the UNBR depending on manure P rate and areas allotted to sod. In addition, total N load was reduced on average by 31% and sediment load declined on average 16.7% at the watershed outlet. The SWAT model predicted up to 176 kg/ha P was removed per harvest of sod top-dressed with 100 kg manure P/ha. Export increased to 258 kg/ha of P per harvest for the manure P application rate of 200 kg/ha. Depending on the implementation scenario, simulations indicated the turfgrass BMP could export between 262 and 784 metric tons of P out of the UNBR watershed every year.