Alliance’s Phosphorus Transport Modeling Group Meets

By Dr. Matt Scholz

Sustainable Phosphorus Alliance

Matt Scholz

The Sustainable Phosphorus Alliance hosted its second in-person workshop of what’s been rebranded as its Phosphorus Transport Modeling Group in San Antonio, Texas, on Nov 14-15. The meeting was held nearby, and just after, the meetings of the Soil Science Society of America and SERA-17. Fourteen academics and practitioners met for the workshop with an additional two joining via phone. The meeting was underwritten in part by USDA-ARS.

The working group originally formed in 2018 to address issues of how to better harmonize field-scale hydrological models of phosphorus fate and transport, such as the Annual Phosphorus Loss Estimator Tool (APLE), with watershed-scale models, such as the Soil and Water Assessment Tool (SWAT), and how to address knowledge gaps evident across all models (e.g. specific transport phenomena not modeled). These models are often applied to validate P indices and estimate P loads into impaired waterbodies, for example. They vary not only in spatial and temporal scales, but in data requirements, sophistication, and ease-of-use. In the time since the original group formed, it has extended its purview to study how field- and watershed-scale models can inform regional models of phosphorus flows as well. For example, transport models may inform assumptions about P losses from manure application that are made in regional, substance-flow models.

All good transport models are beholden to the same laws of physics and chemistry. While they should concur with each other and with edge-of-field data when analyzing identical scenarios, especially in general direction of change, there remains a need for cross-validation studies to demonstrate model agreement. We also need to develop frameworks for integrating these models across scales. For instance, APLE’s extrapolation of total P losses from agricultural fields within a watershed should be similar in magnitude to the watershed-scale predictions of SWAT, and both models should jibe with edge-of-field data on total P losses. Where they don’t, we can identify further research needs for model improvements and integration. Furthermore, the models would ideally confirm and help provide a sound scientific underpinning to decision support tools, such as phosphorus indices. The latter can be accomplished, for example, by leveraging model simulations to estimate more accurate weights when calculating scores in a P index.

The first day of the meeting provided time for group members to update one another on recent progress on related research, and the second day developed work items to be carried forward. One of the tasks accomplished was to develop a research agenda for a postdoc who will be working in Dr. Carl Bolster’s lab at the USDA-ARS research station in Bowling Green, Kentucky, on key research themes for the group. These include such themes as how we account for legacy versus incidental P losses within models, how P hotspots might be identified using hydrologic models, and how well different models agree in these identifications. We also identified a number of areas ripe for inquiry, including: How do we better incorporate plant growth models? How do we adapt models to be more useful in P deficient systems (e.g. sub Saharan Africa)? How does fertilizer formulation affect transport? What should agronomists be measuring to improve modeling?

On day two, we outlined a group paper on best practices in phosphorus fate and transport modeling in agricultural settings. There is considerable room for user error in applying models to scenarios, and we aim through the paper to provide guidance on best practices for model use: What models are best used when? How well do specific scenarios pair with models? What questions can be asked and how long will it take to get an answer? We also discussed funding and outreach opportunities for the group.

The group was joined by 4 representatives from the Moroccan phosphate mining company OCP (foundational supporter of the Alliance) and the Moroccan l’Université Mohammed VI Polytechnique (UM6P). They were particularly interested in how such models might apply to the African context.

Collectively, this Phosphorus Transport Modeling Group set its sights on the ambitious goal of establishing the group as the “go-to” group for phosphorus fate and transport modeling questions. The Sustainable Phosphorus Alliance has built a steering committee, a web page, and other infrastructure to facilitate the group’s collaboration. Readers can look forward to future reports, including a presentation from Dr. Carl Bolster at our Phosphorus Forum 2020 event in Washington DC on April 30.

Participation in the working group is a benefit of Alliance membership and by invitation of the steering committee. Please contact us for further information.