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Statistical models are empirical. Although they are derived from observations, the relationship described must have a basis in our underlying understanding of processes if we are to have faith in the predictive capabilities of the model (National Research Council 2000). | Section I Basic Approaches 2007 by Taylor Francis Group LLC 1 Modeling Phosphorus Movement from Agriculture to Surface Waters Andrew N. Sharpley U.S. Department of Agriculture-Agricultural Research Service University Park PA CONTENTS 1.1 Introduction.3 1.2 Types of Models.4 1.2.1 Process-Based Models.5 1.2.2 Export Coefficient Models.5 1.2.3 Statistical or Empirical Models.6 1.3 How Models Simulate P Transport.6 1.3.1 Dissolved P.6 1.3.2 Particulate P.8 1.4 Fertilizer and Manure Management.10 1.5 Spatial Data Requirements for Modeling.11 1.6 Defining Future Best Management Practices.12 1.7 How Models Simulate Fluvial Processes and Impact of P in Surface Waters.12 1.7.1 Fluvial Processes.12 1.7.2 Surface Water Impacts . 14 1.8 Summary. 14 References. 15 1.1 INTRODUCTION Phosphorus P an essential nutrient for crop and animal production can accelerate freshwater eutrophication which is the most ubiquitous water quality impairment in the U.S. with agriculture a major contributor of P Sharpley 2000 U.S. Geological Survey 1999 . Environmental concerns from harmful algal bloom outbreaks 3 2007 by Taylor Francis Group LLC 4 Modeling Phosphorus in the Environment Burkholder and Glasgow 1997 and regulatory pressure to reduce P loadings to surface waters via implementation of Total Maximum Daily Loads TMDLs U.S. Environmental Protection Agency 2000 have increased the urgency for information on the impacts of agricultural management specifically conservation practices and best management practices BMPs on P loss. Because of the time and expense involved in assessing P loss models are often a more efficient and feasible means of evaluating management alternatives. In their most comprehensive form models can integrate information over a watershed scale to identify BMPs and critical source areas where BMPs are most likely to affect watershed-scale P losses. A common limitation to model application is the lack of detailed parameterization data on soil physical chemical and .
