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A GIS based nonpoint pollution simulation model

Eugenia Naranjo, VKI Intitute for the Water Environment

Nonpoint pollution is a difficult, often controversial issue because by definition, it comes from diverse, hard to identify sources. However, the use of Geographical Information Systems provides an extensive approach to evaluate land use and other mapping characteristics to explain the spatial distribution of nonpoint source contamination.

A simple method to assess nonpoint pollution has been developed for integrating GIS, databases, as well as transport and decay of pollutants in a watershed. The output from the model can be linked to an existing water quality model (Mike-11).

Nonpoint pollution models are basically a description of the hydrologic rainfall-runoff transformation processes with attached quality components. Therefore, mean pollutant concentrations are included in the model based on predefined land uses. The model uses a grid laid over the landscape, accounting for the pollutant loading and runoff derived from each cell. By tracing the flow of water from cell to cell, the movement of pollution over the landscape and into the river system is simulated. With a digital terrain model of the watershed a flow direction network would be generated and therefore the downstream values of average annual pollutant loads will be determined. Transport and decay of the pollutants through the watershed are modeled in the program as a first order decay process. The decay of the pollutants is assumed to depend on the distance from each cell to the nearest downstream point in the river. The main outputs from the model are the following:

• Areas that contribute to the highest nonpoint source pollution loads in any watershed.
  
• Total pollution loads (spatially distributed) generated by human activities in the watershed
  
• Pollution loads that reach the river system
  
• A MIKE-11 Water Quality input file.
  

The model works as an ArcView 3.0 extension and uses the Spatial Analyst module.