Updating SWAT plus to Clarify Understanding of In-Stream Phosphorus Retention and Remobilization: SWAT plus P.R&R

Efforts to reduce riverine phosphorus (P) loads have not been as fruitful as expected or hoped. One reason for the failure of these efforts appears to be that models used for watershed P management have understated and misrepresented the role of in-stream processes in shaping watershed P export. Here, we update the latest release of the Soil and Water Assessment Tool (SWAT+), a widely used watershed management model, to better represent in-stream P retention and remobilization (SWAT+P.R&R). We add new streambed pools where P is stored and tracked, and we incorporate three new processes driving in-stream P dynamics: (a) deposition and resuspension of sediment-associated P, (b) diffusion of dissolved P between the water column and streambed, and (c) adsorption and desorption of mineral P. The objective of this modeling work is to provide a diagnostic tool that enables researchers to challenge existing assumptions regarding how watersheds store, transform, and transport P. Here, in a first diagnostic analysis, SWAT+P.R&R helps reconcile in-stream P retention theory (that P is retained at low flows and remobilized at high flows) and a discordant data set in our validation watershed. SWAT+P.R&R results (a) clarify that the theorized relationship between P retention and flow is only valid (for this point-source affected testbed, at least) at the temporal scale of a single rising-or-falling hydrograph limb and (b) illustrate that hysteresis obscures the relationship at longer temporal scales. Future work using SWAT+P.R&R could further challenge assumptions regarding timescales of in-stream P legacies and sources of P load variability.

Automated summary

Efforts to reduce riverine phosphorus (P) loads have been hindered because existing watershed management models fail to accurately account for in-stream processes that influence P export. A new version of the Soil and Water Assessment Tool (SWAT+), known as SWAT+P.R&R, has been developed to address this limitation by incorporating new streambed pools and processes related to P retention and remobilization. The modeling work with SWAT+P.R&R challenges existing assumptions about P storage, transformation, and transport in watersheds, and provides insights on the relationship between P retention and flow at different temporal scales.