Two approaches are used to reflect the relative importance of the various impact parameters and their ability to be modeled using available data. Based on current and anticipated future land uses, it is estimated that nutrients (nitrogen and phosphorus) and sediment have the greatest potential for causing adverse impacts in the streams, wetlands, rivers and estuaries within the areas to which the model has been applied thus far. The fact that only hydrologic/nutrient transport models have been effectively tested for use in watershed assessments supported the decision that only the water, nitrogen, phosphorus and sediment loads would be simulated dynamically. These parameters may vary for other regions and the model would be adjusted accordingly.
The indexing and modeling approaches are similar because both use the watershed characteristic data from existing GIS coverages to select the appropriate input data (indices for index approach and model parameter sets for modeling approach). These data are used to calculate the combined impact of all the watershed characteristics for a given grid cell/polygon. Once the combined impact for each unique cell/polygon within a watershed is determined, the cumulative impact for the entire watershed is determined by first attenuating the constituent to the sub-basin outlets and then calculating an area-weighted ranking/index for the attenuated load generated at each cell. Constituents are attenuated based upon the flow distances (overland to nearest water body, through wetlands or depressions and within streams to the sub-basin outlet), flow rates in each related flow path and the type of wetland or depression encountered.
Wetlands are also assessed in two ways. Wetland habitat value is determined based upon the relative areal influences of surrounding land uses on wetland habit. Wetlands are also assessed for their potential for water quality treatment. This is accomplished through an analytical comparison of source loads and attenuated loads. Potential flood proneness is determined based on geographical analyses of land use, soils and topography to locate potential depressional areas and estimate the quantity of runoff contributing to each depression.
For each of the above assessments, the model produces maps of the relative ranking/index for each impact parameter across the selected area and the cumulative ranking/index by watershed/sub-basin. The model also produces summary tables and graphs of daily output. The resulting ranking/index provides a good comparative tool for assessing the spatial importance of the land use, soils, wetlands, depressions and hydrography within a given watershed and across different watersheds. The simulated results are not intended to provide precise load estimates for the individual watershed impact parameters; rather, they provide a relative index of the potential environmental impacts.
The tool bar at the left of the screen provides a means for panning, zooming and querying datasets. The query option allows the user to click on an area of the currently displayed coverage. Attributes are then displayed in the comments box at the top right corner of the screen.
The tool bar also provides a means for changing view modes and adding base maps. The mode option of the tool bar allows the user to toggle view modes from single to dual. View options include existing, modified or future (if available) land use scenarios. This is useful for side-by-side comparisons of model results. A variety of base maps are available at the bottom of the tool bar that can be overlaid onto the currently displayed coverage to provide a geographic reference. Base maps may include roads, rivers/canals, cities, counties, section/township/range, etc.
The tools provide a variety of methods to select the desired sub-basins. Selection options allow the user to create a new selection, add to a current selection, select from a current selection or remove from a current selection. Sub-basins can be selected one at a time, by box, by polygon or by circle. The user can also choose all or clear the selection. The dialog box includes a count of the number of currently selected sub-basins. Selected sub-basins appear simultaneously in the display box.
Two very useful features of the selection process involve selecting by watershed. The user can either select an entire watershed by clicking on the watershed's most downstream sub-basin or select sub-basins by clicking on the most upstream sub-basin in the watershed. The latter selects all sub-basins that the flow of the selected basin cascades through. These selection processes are made possible by accessing an INFO file that describes each sub-basin's path through other sub-basins.
Land use editing provides a means to apply changes to a specific area. By selecting Land Use from the MODIFY option of the main menu, the user is prompted to select a source coverage to edit. A first time modification will require that the existing or future land use be selected as the default land use coverage. After selecting the appropriate land use coverage, it will appear in a single view mode. The user will not be able to change view modes or select other main menu options until editing is complete. A palette of tools will appear on the right side of the screen that includes drawing options similar to those commonly found in many drawing and graphic paint programs, with one major difference. In addition to color, the user assigns an attribute identifying land use. This tool palette was borrowed and modified from an ARCTOOLS menu file provided in ARCINFO.
The land use attribute (or Fill Value) is a code that can be entered manually or selected directly from the coverage. Best Management Practices can also be applied by entering the appropriate land use code to which a BMP has been assigned. Depending on the region, a variety of BMPs can be made available in the model. Land use editing is useful for analyzing individual developments and provides the tools for regulators to assess regional impacts.
Best Management Practices provide land use managers with optional methods of treating the land to improve the quality of stormwater runoff. BMPs include fertilizer and pesticide application reductions, alternate planting schedules and crop selection, stormwater retention and detention, etc. By selecting BMPs from the MODIFY option of the main menu, the user is presented with a list of land uses and applicable BMPs. Many land uses have their own list of unique BMPs that can be applied. Figure 3 shows the BMPs available for row crops; these can be applied by clicking the corresponding check box. BMPs can be applied to as many land uses as desired. Once a selection is made, the model opens the land use INFO files and reassigns the appropriate land use codes.
A modified land use coverage will be created (or edited if already created) that includes revised land use attributes identifying the land use as having one or more BMPs. Land Swap similarly modifies attributes to effectively swap one land use for another. This is useful in assessing land use trends or incentives.
The Water Quality and Flood Proneness submodels need depressional areas which are determined based on a combination of several coverages including hydrography (streams), wetlands, soils and topography. Topographic depressions are determined by converting a vector coverage of USGS topographic contours into an grid and applying a GRID SINK function. Other potential depressions are determined through a geographic elimination process. Wetlands and certain soils that exhibit depressional characteristics are combined into one coverage and potential areas are reduced further based on land slope and proximity to streams.
The Water Quality submodel requires a series of attenuation runoff lengths. First, runoff lengths from each source cell to either wetlands, streams or depressions are calculated. Depending on which of these three features is found by the source cell first, additional attenuation lengths are found and returned to the source cell. For example, if a wetland is found, the source cell needs to know the distance from that point of contact on the wetland to the nearest stream. The source cell then also needs to know the distance from that point of contact on the stream to the sub-basin outfall. These distance calculations are accomplished with the GRID function, COSTDISTANCE, applied to the topography grid. This function returns the "least cost" path distance to cells of a specified grid based on values of another grid. By using the topography grid as the value grid, the path is determined based on the lowest elevations around the source cells.
Of course, the advantage of any GIS is the ability to overlay information and create new data sets. The Water Quality submodel requires such an overlay of land use and soils to determine the unique combinations that exist. These unique combinations are summarized and distributed to a FORTRAN program, BNZ (Cooper and Bottcher, 1993), which estimates loads for various parameters. The output from BNZ is then joined to the original land use/soils combination grid which, in turn, is combined with the attenuation distance grids. Another database containing attenuation coefficients is also joined to the coverage. The resulting coverage database includes all of the information necessary for the attenuation formula.
The Wildlife Habitat submodel estimates the animal diversity index for each wetland cell. Wildlife Aerial Influence (WAI) indices, which represent the relative negative impact of neighboring land uses on wildlife within a wetland, are joined to the land use grid and wildlife diversity indices are joined to the wetland grid. The GRID function FOCALMEAN achieves a "moving window" effect by creating a new grid with values equal to the mean WAI of the cells in a specified neighborhood block. This new value represents a factor (<1) to be applied to the wetland wildlife indices by multiplying the two grids together. The result is a wetland grid with values equal to an Adjusted Wildlife Index (AWI) that gives the relative animal diversity expected in a given wetland cell.
The Flood Proneness submodel uses the topographic depressional areas determined earlier and calculates the extent of flooding in these areas based on a given rainfall event. GRID hydrologic functions are available to determine the drainage area contributing to each depression. Modified land use/soils combination grid and topography grids are then created with extents equal to the depression contributing areas by using the GRID SELECTMASK function. A database containing CN numbers is then joined to the land use/soils combinations. The GRID ZONALSUM function is then used to determine the total volume of runoff that is generated for each depressional area. Each depression is then ranked based on its flood prone potential.
WAM provides an excellent tool for regional planners to determine and rank current areas under environmental stress, estimate future impacts of land use management decisions and set achievable pollution load reduction goals.