AVSWAT: An ArcView GIS Extension as Tool for the Watershed Control of Point and Non-Point Sources

Introduction

Methodology

AVSWAT, ArcView SWAT model extension, represents at the same time a preprocessor and a user interface to SWAT model.

Figure 1 shows the main interface screen once AVSWAT is loaded in ArcView.

The extension is organized in several linked tools grouped in the following eight components:

1. Watershed Delineation
2. Land Use and Soil Definition
3. Editing of the model Data Bases
4. Definition of the Weather Stations
5. Input Parameterization and Editing
6. Model Run
7. Read and Map-Chart Results
8. Calibration tool

This component is an automated sequence of steps by which the user can interactively activate some options to improve the results and define the desired configuration. The tool is based on the basic functions and procedures included within the ArcView hydrologic extension and other ESRI software (see Olivera et al., 1999; Djokic and Ye, 1999) with added customized and enhanced capabilities.
Therefore the standard methodology, based on the eight-pour point algorithm (Jenson and Domingue,1988) for delineating streams from a raster digital elevation model (DEM), is applied:
- the flow vector grid is created filling the sinks (raising the elevation of the sink until a "pour point" occurs):
- the flow accumulation grid is created by counting the number of contributing cells to each cell in the grid (cells whose flow path eventually passes through the cell). Cells which are potentially part of a stream network will have a larger flow accumulation value, whereas cells near watershed boundaries and where overland flow dominates will have a low flow accumulation value.
The AVSWAT user is provided of two prior process options:
- masking of the working area;
- burn-in the DEM using the digitized stream lines.
The first one allows to clip the DEM to reduce the processing area. The second one modifies the DEM raising the elevation values of all the cells but those that coincide with the digitized streams; there are several options of this technique (Saunders, 1999) with proved efficiency of the delineation; here a constant value is added.
The stream branches are controlled by the user specified threshold on contributing number of grid cells (flow accumulation grid) making up the branch. The default definition of the subbasin outlet points is accomplished locating the downstream edge for each branch in the stream.
The AVSWAT user is provided of two additional setting tools:
- DEM properties;
- threshold area in hectares.
The two setting tools are interconnected: the first one sets the DEM horizontal and vertical units; these play a major role for the calculation of geomorphic parameters as well as in defining the threshold as number of contributing cells calculated from the requested value expressed in hectares.
The rest of the Watershed Delineator component in AVSWAT is a unique set of tools button by which the user can interactively introduce outlets and/or inlets points, by clicking in correspondence of mouse cursor on the screen or importing a table of point locations (for example stream gauge or sampling locations), and remove any of the selecting outlets.
Once the outlet locations are specified the user defines the main watershed outlet(s) by a customized selection tool and the subbasins delineation is performed by a process tracing the flow direction from each grid cell until either an outlet cell or the edge of the DEM grid extent is encountered.
Once watershed and subbasins boundaries are determined all the geometric parameters of subbasins and stream reach are calculated by raster-grid functions and stored as attributes of derived vector themes. For example, land slopes of subbasins are automatically calculated by averaging slope values of the respective grid cells; slope, length and width are calculated for the main stream channel flowing from each subbasin inlet to the subbasin outlet, and the longest stream channel extending from each subbasin outlet to the most distant point in the subbasin.
At the end of this procedure of delineation, the user can insert the locations and data regarding point source of discharges (sewers, treatments plants or watershed inlet) and lakes.

Figure 2 shows the Watershed Delineator Dialog in AVSWAT

Land Use and Soil Definition

This component encompasses a set of tools to load and clip on the watershed area separate grid or vector layers carrying the land use and soil information. The tools allow the user to reclassify this layers using the crops (or urban types) and soil types that are defined within the model databases.
Typical data set used for US watershed are the Anderson level II classified land use/land cover layer created using the 1:250,000 scale USGS LUDA (USGS, 1990) and USDA-NRCS STATSGO (USDA, 1992) soil association data sets.
The operation of reclassification can be operated either using lookup tables or manually.
The overlay of the reclassified landuse and soil layers within the watershed basins defines the composition of landuse and soil variations within each subbasin: the user is provided of options to use either predominant or the subdivision into smaller sub-units based on the combination of all controlled percentage of land uses and soils (hydrologic response units).

Figure 3 shows the tools for the Land Use and Soil definition

Editing of the model Data Bases

AVSWAT provides a set of tools to edit and update the model databases. These databases contains the model parameters regarding:
- Plant Growth
- Tillage
- Fertilizers
- Pesticide
- Urban

Figure 4 shows the dialog editor of the Plant Growth parameters data base

The databases containing the soil parameters deriving from STATSGO and the statistical data from 1130 historical weather stations (Nicks, 1985) are not editable, though the user can include new data sets in user-defined databases using customized tools.

Definition of the Weather Stations

AVSWAT allows the user to introduce locations and data sets regarding raingauges, climate and weather stations. Among the options of the tool, for a US watershed application the user can alternatively select and use the above mentioned weather stations locations with statistical data required by the SWAT weather generator. This component not only operates the geocoding of the stations, it labels the missing daily weather data records that the SWAT model will be able to simulate.

Input Parameterization and Editing

Based on the watershed configuration and the setup of the landscape and climate definitions operated with the above components, AVSWAT creates and populates project data bases to store the input parameters of the model. The requested ASCII format inputs of the models are also created.
Dialog tools grouping input parameter typologies (soil, weather, subbasin or landuse-soil subunit, stream reach, groundwater, water use, management, pond, lake) allow the user to view and edit any of the previously stored values. Each editing operation end with a checking of the allowable values; optionally the user can extend the current editing data set records to other target subbasins and/or landuse-soil combination input units.

Figure 5 shows one of the most significant input dialog regarding the management input parameters for planting, harvest, irrigation applications, nutrient applications, pesticide applications, and tillage operations. The dialog allows the user to set up crop rotations scenarios scheduling the operations by date or by heat units.

Model Run

Additionally, the user can set up the simulation control codes: evapotranspiration model, the length of simulation (beginning and ending day of simulation), type of simulation, (time step of outputs) and others, Then execute the simulation previous an optional checking of the existence and validity of all necessary input files.

Figure 6 shows the dialog for the set up and run of the simulation.

Read and Map-Chart Results

Once run the model, a set of control allows the interface to read the ASCII format model outputs and convert them in dbfs. Other tools allow the user to chart and map at subbasin and stream reach level for any of the adopted time step (daily, monthly or yearly) and for any time span subset of the simulation period.
The following are among the most important simulated variables:
- Flow discharge
- Sediment Yields
- Nitrogen and phosphorus phases
- Pesticides concentrations
- Metals concentrations
- Pathogens concentrations

Figure 7 shows an example of the simulation results and the side-by-side tabular and graphical results of the tool application.

Calibration tool

This component allows the user to operate a quick interactive calibration of the model simulations. The user can target the most sensitive input parameters of the model and, with a few clicks, set their variations (in percent of the current value or by an absolute value), activate them for target subbasins, and landuse-soil combinations and re-run the model. The calibration scenarios can be saved, modified later or exported to be used within another watershed project.

Figure 8 shows the calibration dialog.

Software Implementation and Development

Conclusions

References

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Jenson, S., and J. Domingue, (1988). Extracting topographic structure from digital elevation data for geographic information system analysis, Photogrammetric Engineering and Remote Sensing, v.54, pp.1593-1600

Nicks, A.D. (1985). Generation of climate data. In: Proceedings of the Natural Resources Modeling Symposium. USDA-ARS-30. 297-300.

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Saunders, W. (1999). Preparation of DEMs for Use in Environmental Modeling Analysis. 1999 ESRI International User Conference, San Diego, CA.

USDA Soil Conservation Service (1992). State Soil Geographic Database (STATSGO) Data Users'Guide. Publ. No. 1492, US Government Printing Office, Washington, DC.

U.S. Geological Survey (1990). Landuse and land cover digital data from 1:250,000 and 1:100,000 scale maps. Data User's Guide 4. Reston, Va.: U.S. Department of Interior.

Authors Information