Assessment of nitrogen inputs and yields in the Cibolo and Dry Comal Creek watersheds using the SWAT model, Texas, USA 1996–2010

Identifying sources of nitrogen input and nitrogen yield to waterways are important aspects of watershed management. A Soil and Water Assessment Tool (SWAT) model was developed for the Cibolo and Dry Comal Creek watersheds in south central Texas, USA to assess how individual nitrogen source categories contribute to nitrogen yield. The model includes nitrogen inputs from atmospheric deposition, fertilizer, manure, a wastewater treatment plant (WWTP), and on-site sewage facilities (OSSFs). Model calibration was successful with all model performance parameters rated satisfactory or better except for percent bias (PBIAS) during the streamflow calibration period in Cibolo Creek. The high PBIAS value likely resulted from the model underestimating streamflow during two flood events coupled with potential measurement errors for the observed streamflow values during the floods. The largest contributors to nitrogen input were livestock (42.0 %), atmospheric deposition (26.8 %), deer (9.9 %), and farm fertilizer use (9.7 %). The largest contributors to nitrogen yield were atmospheric deposition (56.3 % of nitrogen yield), livestock (25.9 %), and the WWTP (7.1 %). The difference in ranking between nitrogen inputs and nitrogen yields occurred because the percentage of nitrogen input that becomes nitrogen yield varied with source. The WWTP (100 % of the WWTP nitrogen input became nitrogen yield) was highest because it directly discharged to the waterway. The WWTP was followed by feral hogs (8.0 %), atmospheric deposition (5.8 %), and waterfowl (2.1 %). These results provide a source-by-source assessment of nitrogen inputs and their corresponding yields allowing watershed managers to better assess nitrogen management strategies.