Development of a decision framework to identify appropriate spatial and temporal scales for modeling N flows

Egon Dumont 1, Carolien Kroeze 1, Evert Jan Bakker 2, Alfred Stein 2 and Lex Bouwman 3
1 Environmental Systems Analyses Group, Wageningen University 
Ritzema Bosweg 32a, Wageningen, The Netherlands
Tel.: + 31 317 484812; Fax: + 31 317 484839
egon.dumont@wur.nl; carolien.kroeze@wur.nl
2 Mathematical and Statistical Methods Group
Wageningen University, Bornsesteeg 47, Wageningen, The Netherlands
Tel.: + 31 317 484085; Fax: + 31 317 483554
alfred.stein@wur.nl
3 Netherlands Environmental Assessment Agency
A. van Leeuwenhoeklaan 9, Bilthoven, The Netherlands
Tel.: + 31 30 2742745; Fax: + 31 30 2744479
lex.bouwman@mnp.nl

Abstract
Decision makers need to know the effects of alternative watershed management strategies on export of nitrogen (N) to coastal waters. They use this knowledge to reduce undesirable effects of excess N in coastal waters. Various models exist to predict the effects of watershed management on export of N. This paper describes a decision framework to identify the appropriate spatial and temporal scales for using N flux models. The framework is developed for existing models that predict N export from large watersheds and the contribution of N sources and N sinks to this N export. With  this framework, modelers can identify the appropriate scale for model predictions and independently scalable model parts. The framework bases the appropriateness of model scales on indicators, which are to be specified by the modeler and which are associated with four criteria. The four criteria require modeling scales to correspond with (A) data, mitigation options, and scenarios, (B) model assumptions, (C) available resources for modeling, and (D) requirements of prediction users. A successful application of the framework is illustrated for a global model of dissolved inorganic nitrogen export from watersheds to coastal waters. Ranges of appropriate scales are determined for model output and five independently scalable model parts, which model the (1) surface N balance, (2) point sources, (3) N flux in sediments and small streams, (4) retention in dammed reservoirs, and (5) riverine retention. Appropriate model scales were found, if the four criteria were not set too strict. We conclude that the decision framework can contribute substantially to selecting the appropriate modeling scales in a balanced and comprehensive manner.

Keywords: nitrogen cycling, surface water quality, decision support, models, scale

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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