port to water resources and an ability to predict future conditions are key to the development of efficient management and policies to protect the beneficial uses of the nation’s water resources, including drinking water and viable ecosystems. As NAWQA progresses into Cycle II with an increased emphasis on its understanding goal, the importance of model application, as recommended by previous National Research Council (NRC, 1990, 1994) committees, should not be underestimated. Understanding and prediction, embodied in water quality models, are the cornerstones of water resources management for the future.

This chapter discusses four topics related to the goal of understanding cause and effect as related to water quality. First, the important role that models play in scientific understanding is explored. Understanding evolves from the thorough evaluation of data, and this is certainly one of the primary functions of model application. Models represent conceptual and mathematical relationships between the observations that we interpret as cause and effect. They are formulated at a wide range of temporal and spatial scales to represent a variety of phenomena— from rapid chemical reactions to long-term changes in the global environment. Each scale represents a different aggregation of controlling variables, so models are often not directly transferable between scales. This results in a hierarchy of models according to scale and highlights the importance of choosing models appropriately. They are also formulated in a variety of ways, including those based on mass-balance, statistical regression, and process-based (mechanistic) models. Regardless of the means of model formulation, there are several sources of uncertainty, including measurement uncertainty in the observations that serve as input and verification, model structure uncertainty, and parameter uncertainty. The quantification of uncertainty is important in providing perspective for interpretation of model results. Despite uncertainty, models play an important role in understanding causal factors that affect water quality, and model application is one way to illuminate the degree of understanding that exists. Different aspects of these topics are reviewed later in this chapter to highlight associated strengths and pitfalls in model application.

Second, the practical aspects of the overall proposed Cycle II implementation strategy are discussed. As a starting point, Cycle I of NAWQA naturally serves as the foundation for Cycle II. For example, key components of the surface water and groundwater monitoring assessments as well as the pilot-scale monitoring in lake and reservoir sediments conducted in Cycle I are carried over and expanded in Cycle II to form the National Trend Network for Streams, National Trend Network for Contaminants in Sediment, and National Trend Network for Ground Water (discussed in Chapter 4). Cycle I activities will also help form the planned Cycle II spatial studies of effects of land-use change on stream and groundwater quality. These trend assessments (both spatial and temporal) provide both the initial information needed to design the “targeted studies” proposed for Cycle II (see more below) and a context that gives a sense of their national priority (see in discussion of trends in Chapter 4). The two new components

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