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ARS Home » Midwest Area » Columbia, Missouri » Cropping Systems and Water Quality Research » Research » Publications at this Location » Publication #314180
Title: Estimation of phosphorus loss from agricultural land in the Heartland region using the APEX model: a first step to evaluating phosphorus indices

Author
item LORY, JOHN - University Of Missouri
item NELSON, NATHAN - Kansas State University
item Baffaut, Claire
item SENAVIRATNE, G.M.M.M. - University Of Missouri
item VAN LIEW, MIKE - University Of Nebraska
item BANDHARI, AMMAR - Kansas State University
item MALLARINO, ANTONIO - University Of Iowa
item HELMERS, MATT - University Of Iowa

Submitted to: Waste to Worth Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/4/2015
Publication Date: 3/30/2015
Citation: Lory, J., Nelson, N., Baffaut, C., Senaviratne, G.A., Van Liew, M., Bandhari, A., Mallarino, A., Helmers, M. 2015. Estimation of phosphorus loss from agricultural land in the Heartland region using the APEX model: a first step to evaluating phosphorus indices. Waste to Worth Conference. Available: http://www.extension.org/72813.

Interpretive Summary: Accurate phosphorus loss estimation from agricultural land is important for development of best management practices that protect water quality. Phosphorus indices are a key tool to minimize Phosphorus loss from agricultural fields but there is insufficient water quality data to fully test them. Models have potential to generate appropriate data but must be compared to measured data as validation prior to their use. The Agricultural Policy/Environmental Extender (APEX) model is a simulation model designed to simulate edge-of-field water, sediment, and nutrient losses. Additional research is needed to determine the level of model calibration required for it to accurately simulate P loss. The objectives of this study were to determine the accuracy of runoff, sediment and Phosphorus loss estimations by APEX after two calibration strategies: i. a watershed-specific parameterization based on full calibration/validation comparing measured data with simulated results of the model for runoff volume, sediment load and Phosphorus load, ii. a minimal parameterization approach based on best professional judgment (BPJ) consistent with using APEX when measured runoff, sediment and Phosphorus data are not available for model calibration. The analysis was conducted for 19 watersheds at four sites representing a range of hydrologic conditions and including grazing, tilled row-crop, and no-till row-crop management systems. Full calibration provided excellent fit for runoff and total Phosphorus and marginal fit for sediment. In contrast, the BPJ resulted in unacceptable estimates of sediment and Phosphorus load, and marginal fit for runoff volume. These results emphasize that failure to calibrate APEX with runoff and water quality data (the BPJ approach) will result in poor estimates of annual sediment and total Phosphorus loads. The next phase of this project is to use appropriately calibrated APEX model to generate the long-term estimates of Phosphorus loss needed to evaluate P indices in IA, KS, MO and NE.

Technical Abstract: Purpose. Phosphorus (P) indices are a key tool to minimize P loss from agricultural fields but there is insufficient water quality data to fully test them. Our goal is to use the Agricultural Policy/Environmental eXtender Model (APEX), calibrated with existing edge-of-field runoff data, to refine P indices and demonstrate their utility as a field assessment tool capable of protecting water quality. In this phase of the project our goal is to use existing small-watershed data from the Heartland Region (IA, KS, MO and NE) to determine the level of calibration needed for APEX before using the model to generate estimates of P loads appropriate for evaluating a P Index. What did we do? APEX model is designed to simulate edge-of-field water, sediment, and nutrient losses. Our analysis included data from 19 watersheds at four sites in the Heartland Region representing a range of hydrologic conditions and including grazing, tilled row-crop, and no-till row-crop management systems. We evaluated two strategies to optimize settings of model parameters: i. a watershed-specific parameterization based on full calibration/validation comparing measured data with simulated results of the model for runoff volume, sediment load and P load, ii. a minimal parameterization approach based on best professional judgment (BPJ) consistent with using APEX when measured runoff, sediment and P data are not available for model calibration. Model fitting for strategy (i) was done using event data in each watershed. The two parameterization strategies were evaluated based on the fit of “annual” totals where data at each location were summed by year (total of 97 site-years). The Nash-Sutcliffe model efficiency and regression methods were used to quantify model fit. Full calibration provided excellent fit for runoff and total P (NSE>0.8 for each) and marginal fit for sediment (~0.3). In contrast, the BPJ resulted in unacceptable estimates of sediment and P load, and marginal fit for runoff volume (NSE~0.4). These results emphasize that failure to calibrate APEX with runoff and water quality data (the BPJ approach) will result in poor estimates of annual sediment and total P loads. Future Plans. We are testing a regional parameterization strategy as another possible way to extend the APEX model to locations where there is no runoff and water quality data. The next phase of this project will then use appropriately calibrated models to generate the long-term estimates of P loss needed to evaluate P indices in IA, KS, MO and NE.