Improved Simulation of Edaphic and Manure Phosphorus Loss in SWAT

Abstract

Watershed models such as the Soil Water Assessment Tool (SWAT) and the Agricultural Policy Environmental EXtender (APEX) are widely used to assess the fate and transport of agricultural nutrient management practices on soluble and particulate phosphorus (P) loss in runoff. Soil P-cycling routines used in SWAT2012 revision 586, however, do not simulate the short-term effects of applying a concentrated source of soluble P, such as manure, to the soil surface where it is most vulnerable to runoff. We added a new set of soil P routines to SWAT2012 revision 586 to simulate surface-applied manure at field and subwatershed scales within Mahantango Creek watershed in south-central Pennsylvania. We corroborated the new P routines and standard P routines in two versions of SWAT (conventional SWAT, and a topographically driven variation called TopoSWAT) for a total of four modeling "treatments". All modeling treatments included 5 yr of measured data under field-specific, historical management information. Short-term "wash off" processes resulting from precipitation immediately following surface application of manures were captured with the new P routine whereas the standard routines resulted in losses regardless of manure application. The new routines improved sensitivity to key factors in nutrient management (i.e., timing, rate, method, and form of P application). Only the new P routines indicated decreases in soluble P losses for dairy manure applications at 1, 5, and 10 d before a storm event. The new P routines also resulted in more variable P losses when applying manure versus commercial fertilizer and represented increases in total P losses, as compared with standard P routines, with rate increases in dairy manure application (56,000 to 84,000 L ha). The new P routines exhibited greater than 50% variation among proportions of organic, particulate, and soluble P corresponding to spreading method. In contrast, proportions of P forms under the standard P routines varied less than 20%. Results suggest similar revisions to other agroecosystem watershed models would be appropriate.

Modeling the Impacts of Manure on Phosphorus Loss in Surface Runoff and Subsurface Drainage

“…This feedback loop guarantees a relatively rapid PSC and labile P increase under amendments. Thus, a dynamic PSC was necessary to maintain the equilibrium between different P pools (Collick et al, 2016). …”

“…Phosphorus in crop residues, soil, and applied amendments (i.e., manure and fertilizer) represents a consistent source of nonpoint pollution in surface runoff from agricultural lands (Bennett et al, 2001; Collick et al, 2016). The latter P source can overweigh all others when a large precipitation event follows closely after amendment application and water soluble forms of P are lost directly from the applied amendments (Withers et al, 2001).…”

Comparing an Annual and a Daily Time‐Step Model for Predicting Field‐Scale Phosphorus Loss

“…In contrast, APLE simulates P transformation and dissolution from a distinct surface layer of P using a set of empirically based equations. As a result, the amount of P vulnerable to runoff loss will generally be greater for APLE than TBET, potentially leading to greater predictions of P loss following manure application (Sen et al, 2012; Collick et al, 2016). Incorporating daily versions of the APLE manure P‐loss routines into SWAT (revision 586), Collick et al (2016) found that the new P routines better represented effects of different manure management practices on P losses at the small watershed scale (it is unclear when these modifications will be incorporated into TBET).…”

“…As a result, the amount of P vulnerable to runoff loss will generally be greater for APLE than TBET, potentially leading to greater predictions of P loss following manure application (Sen et al, 2012; Collick et al, 2016). Incorporating daily versions of the APLE manure P‐loss routines into SWAT (revision 586), Collick et al (2016) found that the new P routines better represented effects of different manure management practices on P losses at the small watershed scale (it is unclear when these modifications will be incorporated into TBET). Differences in how APLE and TBET simulate incidental P losses from surface‐applied manures, however, cannot entirely account for the differences we observed in predictions of DP loss between the two models, as we did not find a strong correlation between residuals in predicted DP loss and surface‐applied P rates (Supplemental Fig.…”

Soil Phosphorus Modeling for Modern Agriculture Requires Balance of Science and Practicality: A Perspective

“…Additionally, modeling different management practices such as tillage during periods of low rainfall can be compared to investigate methods to prevent stratification and reduce particulate P loss. Modeling with detailed weather data (e.g., to capture high‐intensity rainfall over a given time period) can identify climatic effects on soil P loss pathways (e.g., large rainfall periods corresponding to leaching events as is routinely used for nutrient management) (Collick et al, 2016; Sharpley et al, 2017; Verburg et al, 2018). Other management options such as the 4R Nutrient Stewardship system (Johnston and Bruulsema, 2014), changes to irrigation schedules, cover crops during fallow periods, and timing and depth of fertilizer application should also be tested prior to adopting new practices.…”