Patuxent Landscape Model: Integrated Modeling of a Watershed

Large drainage basins are composed of multiple smaller catchments. Each of these catchments contains a heterogeneous collection of land uses, which vary in composition and spatial pattern (structure) and thus differ in functions such as nutrient retention. Two problems arise from this heterogeneity that present major challenges to both research and management. First, variation in structure and function inevitably prevents true replication in intensive field studies that attempt to relate landscape function to landscape structure. Second, variation among land uses within watersheds makes it difficult to directly extrapolate among spatial scales. Even though drainage basins can be broken down hierarchically into smaller catchments based on topography, “scaling up” from intensive catchment studies is not a linear additive process because of differences among catchments and interactions between adjacent land uses. Management of water quality over large drainage basins must address both problems with innovative methods synthesizing data from intensive experimental studies on a few watersheds, then extrapolating important generalizations to larger drainages using modeling techniques. The Patuxent Landscape Model (PLM) was designed to serve as a tool in a systematic analysis of the interactions among physical and biological dynamics of the Patuxent watershed (Maryland, USA) (Fig. 8.1), conditioned on the socioeconomic behavior in the region. A companion socio-economic model of land use dynamics in the region is developed to link with the PLM to provide a means of capturing the feedbacks between ecological and economic systems (Fig. 8.2). By coupling the two models and exchanging information and data between them, the socio-economic and ecological dynamics can be incorporated. Whereas in most ecosystem models the socio-economic development is fed into the model in the form of scenarios or forcing functions, a coupled model can explore dynamic feedbacks, adjusting the socio-economic change in response to the ecological perturbations. To run the ecological and economic modules in concert, we need to account for specifics of both modules in their design and make assumptions about how the in8

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