Modeling phosphorus in rivers at the global scale: recent successes, remaining challenges, and near-term opportunities☆
Introduction
Phosphorus (P) is an essential, often limiting, macronutrient in freshwater systems [1] that, under certain conditions, can also limit primary production in terrestrial and coastal ecosystems [2, 3, 4]. By mining phosphorus and using it primarily as a fertilizer, humans have more than doubled the rate at which bioavailable P is supplied to the terrestrial biosphere [5, 6, 7, 8]. Widespread mobilization of geologic P in agriculture has been necessary to feed a large and burgeoning global population. However, P runoff and leaching from agricultural fields and animal production facilities [9], and flowing through inadequately treated sewage systems [8], has incurred substantial environmental costs. These include increased frequency and severity of hypoxic events, harmful algal blooms, changes in primary productivity and ecosystem function, often leading to decreased biodiversity, impaired water quality, and increased greenhouse gas emissions [10, 11, 12, 13]. Understanding and mitigating the effects of P overenrichment of waters globally, including the evaluation of the global Sustainability Development Goals (SDGs), requires the use of global models that quantitatively link land use, global population growth and climate to aquatic nutrient loading and biogeochemical cycling. Such global P models are useful instruments to evaluate global hotspots and future trends of aquatic P loading under global climate and socioeconomic changes, and can therefore help to provide insight in where and to what extent better management and mitigation measures are needed. A variety of models have been developed for use at comparatively small (0.01–1000 km2) scales in regions where model input and evaluation data are available, and these are reviewed elsewhere [14, 15, 16]. Here we focus exclusively on global P loading and transport models, which have emerged just within the past two decades, following early pioneering work [17, 18, 19]. Such global models are also useful at local and regional scales for several reasons. They provide context for local observations and modeling efforts, helping people and governments understand where regions fall in terms of the severity of their present-day and potential future P loading problems. Global P models can also provide reasonable estimates of P loading and sources in data-poor regions where applying complex, locally or regionally calibrated models is not possible. Also, when evaluated at the local-regional scale, Global P models provide a mechanism to test, evaluate, and, ultimately, improve understanding of how P sources and transformation control P delivery to and through rivers and watersheds. Finally, global models, when applied in a regional context, can facilitate transboundary analyses that are often not possible using local water quality models.
Below, we: first, briefly describe, compare, and contrast three ‘families’ of published global total phosphorus (TP) transport models, second, highlight major insights attributable to the development and application of these models, third, discuss important areas for future model enhancement, and fourth, identify important research avenues deserving near-term attention that can be addressed with either existing or somewhat enhanced global P transport models.
Section snippets
Global P models: key characteristics, similarities and differences
There are currently three peer-reviewed ‘families’ of global models capable of predicting river total P (TP) loading and transport: IMAGE-GNM, WaterGAP, and Global NEWS. The specific version of each model reviewed here is the most recent one capable of predicting river TP loading and transport: IMAGE-GNM-TP, WaterGAP3.2, and Global NEWS-2-TP for each modeling family, respectively. NEWS-DIP-HD, a high-resolution (half-degree) version of NEWS-2-DIP [20] is also discussed. Each of these models is
Understanding where river P fluxes are high (and rapidly changing)
Global NEWS-2-TP and IMAGE-GNM-TP models provided the first global, spatially explicit estimates of global P delivery to coastal zones [21,29, 30, 31,32••]; Global NEWS-DIP-HD and IMAGE-GNM-TP provided the first-ever estimates of DIP and TP delivery to (and through) the surface freshwater component of watersheds [20,32••]. WaterGAP3.2 and IMAGE-GNM provided the first-ever estimate of TP loading to the world’s largest lakes [22••]. Understanding spatial patterns of P loading to freshwaters and
Challenges & opportunities
Despite progress over the past two decades in modeling P transport to and through aquatic systems, a number of challenges and opportunities remain. One puzzling outcome (and research opportunity) resulting from the comparison of several P models is that there is substantial uncertainty regarding even the total global rate of P delivery to the coastal zones. Global NEWS-2-TP and IMAGE-GNM-TP produce estimates of global P export that differ by more than a factor of two (9 Tg P year−1 for Global
Conclusion
Within the past two decades, global P loading and transport models have granted useful insight into: P loading hotspots, P sources, and drivers of P delivery to surface waters historically, currently, and under future scenarios. However, there is still much room for improvement in model performance and representativeness, and key questions remain to be answered. Opportunities to achieve model improvements include: first, increased temporal resolution, including representation of ‘event’ fluxes,
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This paper evolved from a workshop titled ‘Water Quality: a new challenge for global scale modelling’ held at Wageningen University 18–21 September 2017. Funding for this workshop came from the OECD-CRP, the support of which is gratefully acknowledged. Harrison received support from an NSF INFEWS grant (NSF EAR1639458). A.F.B. and A.H.W.B. received support from PBL Netherlands Environmental Assessment Agency through in-kind contributions to The New Delta 2014 ALW projects no. 869.15.015 and
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