Simulating agricultural non-point source (NPS) pollution in the coastal plains using geographic information systems (GIS) interfaced model systems

The goal of this research was the establishment of a connection between existing agricultural watershed NPS models and Geographic Information Systems (GIS) to identify critical areas with a high pollution potential and to estimate loading rates for a variety of pollutants from the watershed. The NPS models investigated are AGNPS (AGricultural NonPoint Source) and ANSWERS (Areal Nonpoint Source Watershed Environmental Response Simulation). Interfaces to these models have been built within two GIS systems: ARC/INFO and ArcView. AGNPS and ANSWERS were interfaced with ARC/INFO GIS through AML programming. AGNPS and ANSWERS interfaced with ArcView GIS through Avenue and Microsoft Visual Basic programming. The GIS interfaces proceed in the following steps: (1) generate the model input data, (2) write and format the input data into the model input file, (3) execute the NPS models, (4) transform the model outputs into GIS formats, and (5) display simulated results within GIS.;The methodology of interactively utilizing a GIS-based interface for evaluating NPS pollution from the watershed has been demonstrated through an example application. Graphic user interfaces or a series of menus were developed using ARC/INFO AML (Arc Macro Language), ArcView Avenue and Microsoft Visual Basic programs to link GIS and NPS models. The ANSWERS and AGNPS are grid cell-based distributed parameter models and are used to analyze the resource data and determine what and where potential problems exist at the watershed scale. Both of the models are accessible through the interface. The input data for NPS modeling have been successfully created or derived in ARC/INFO GIS through the necessary manipulation of Digital Elevation Model (DEM), hydrography, soil, and land use/cover digital data layers and user-defined input values, rather than conventional methods. One particularly important function of the interface is the transformation of digital watershed data into model input parameters, thus simplify preparation of the model input data. The interfaces also provide enhanced model output to facilitate analysis of the results. Simulated outputs from the NPS model were converted into attributed GIS maps and graphics.;A suite of AML, Avenue, and Visual Basic programs were used to facilitate data input to NPS models and to visualize data output from the model resulting in a watershed simulation more accurate and less costly to implement. The preprocessors and postprocessors included in the interfaces are modular and quite flexible compared to manual preparation of NPS model input. The NPS model---ARC/INFO and NPS model---ArcView interfaces link components and functions of modeling systems in a manner that is reasonably transparent to the user for generating model inputs, executing the model and analyzing simulation results using GIS maps, tables and graphics.;Current land management/use within the watershed can be evaluated to identify critical areas in the watershed. It is also possible to develop alternative land management/use scenarios by modifying the GIS database. The interfaces can be further used to evaluate the effects of conservation systems and conservation practices in reducing or preventing agricultural NPS pollution. The graphic display of output results rather than the numerical output from the NPS model can provide an obvious indication of problems due to erosion and pollutant movement in the watershed. They help users or decision-makers to pinpoint critical areas for further study and/or control action.