Impact of Length of Calibration Period on the Apex Model Output Simulation Performance.

Datasets from long-term monitoring sites that can be used for calibration and validation of hydrologic and water quality models are rare due to resource constraints. As a result, hydrologic and water quality models are calibrated and, when possible, validated using short-term measured data. A previous study on the impact of the length of the calibration period on simulation performance of the Agricultural Policy/Environmental eXtender (APEX) model focused on streamflow. This follow-up study used a long-term, water quality data from the Rock Creek watershed, located in northern Ohio, to quantify the impact of length of calibration period on sensitive model parameters and performance of APEX to simulate total nitrogen (TN) and total phosphorus (TP). Five short (5-year), two medium (15-year), and one long term (28-year) calibration period scenarios were implemented. The calibrated parameters for each of the scenarios were applied to the validation period (last 5 years). The most sensitive streamflow, TN, and TP parameters and their values were presented and discussed in detail, as well as model performance results during the calibration and validation periods. Finally, computed uncertainty ranges of simulated streamflow, TN, and TP outputs were presented and discussed. These results are expected to be used by the USDA to develop an APEX model parameter database to support nation-wide deployment of the nutrient tracking tool (NTT), a user-friendly web-based computer program used to estimate reductions in nutrient losses associated with alternative practices.