Estimating Freshwater Acidification Critical Load Exceedance Data for Great Britain Using Space-Varying Relationship Mod
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Estimating Freshwater Acidification Critical Load Exceedance Data for Great Britain Using Space-Varying Relationship Models Paul Harris · Steve Juggins
Received: 17 June 2010 / Accepted: 31 October 2010 / Published online: 19 March 2011 © International Association for Mathematical Geosciences 2011
Abstract In this study, two distinct sets of analyses are conducted on a freshwater acidification critical load dataset, with the objective of assessing the quality of various models in estimating critical load exceedance data. Relationships between contextual catchment and critical load data are known to vary across space; as such, we cater for this in our model choice. Firstly, ordinary kriging (OK), multiple linear regression (MLR), geographically weighted regression (GWR), simple kriging with GWRderived local means (SKlm-GWR), and kriging with an external drift (KED) are used to predict critical loads (and exceedances). Here, models that cater for space-varying relationships (GWR; SKlm-GWR; KED using local neighbourhoods) make more accurate predictions than those that do not (MLR; KED using a global neighbourhood), as well as in comparison to OK. Secondly, as the chosen predictors are not suited to providing useable estimates of critical load exceedance risk, they are replaced with indicator kriging (IK) models. Here, an IK model that is newly adapted to cater for space-varying relationships performs better than those that are not adapted in this way. However, when site misclassification rates are found using either exceedance predictions or estimates of exceedance risk, rates are intolerably high, reflecting much underlying noise in the data. Keywords Acidified surface-waters · Catchment characteristics · Geographically weighted regression · Indicator kriging · Environmental risk · Nonparametric
P. Harris () National Centre for Geocomputation, National University of Ireland, Maynooth, Co. Kildare, Ireland e-mail: [email protected] S. Juggins School of Geography, Politics and Sociology, University of Newcastle, Newcastle upon Tyne, UK
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Math Geosci (2011) 43: 265–292
1 Introduction Resulting from the deposition of sulfur dioxide and nitrogen oxides, the acidification of freshwaters is an environmental threat to freshwaters across large areas of upland Britain (Mason 1993). While these acidifying compounds have natural sources in volcanic eruptions and emissions from the oceans, over 90% derive from anthropogenic activities, such as the combustion of fossil fuels at power stations and industrial plants, vehicle exhausts, and agriculture. Acidified freshwaters are hostile to aquatic life; as such, continuous assessment and informed management strategies are fundamental for their protection. One approach to protecting freshwaters involves the calculation of acid deposition critical loads. Critical loads are calculated in such a way as to indicate a freshwater site’s capacity to buffer the input of strong acid anions (Nilsson and Grennfelt 1988). Critical loads are thresholds and can be compared directly to curr
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