This project mainly describes how to calculate hydrologic parameters from various geographic data in GIS. Of many hydrologic models, SWAT is chosen because there is a preprocessor for SWAT called BASINS whose Avenue sources are available in the ArcView extension format. BASINS supports many different hydrologic models like HSPF, P-LOAD, QUAL2E, and SWAT; and there are many common procedures for the models. For this reason, some features are not fully implemented for a specific model input. One thing to take into account is that SWAT requires geographic parameters of HRU while BASINS generates only subbasin based parameter input.
The HRU parameters are important to assess the impact of changes of land uses and soil classes, so these input cannot be carelessly lumped to geographic parameters of subbasins which are much different from those of HRUs. The HRU is an intersection of a land use and a soil class within a certain subbasin, so each HRU has a unique combination of subbasin, land use, and soil class. Due to this characteristics of HRU, improving the default input of the BASINS interface that are subbasin-based parameters would be helpful to get better results from SWAT execution and, hopefully, it would save much time to update each .HRU file manually.
Since SWAT was introduced, there have been more than one preprocessor developed in order to interface between the model and GIS. SWAT for Windows 98 is a stand-alone interface running on MS-Windows 98/NT for easy development of the SWAT input. SWAT/GRASS is the GRASS GIS interface and ArcView SWAT is the ArcView GIS extension which is now available from the BASINS/SWAT module.
These interfaces are mainly consist of the following components [DiLuzio:2000]:
As mentioned above, these interfaces do not deal with HRU parameters, so this project focuses on development of a tool to calculate HRU parameters and to update input data before running SWAT in the BASINS project.
preSWAT and swatLoads are programmed in the Avenue language because BASINS itself is an Avenue extension for ArcView 3.x and stable compared to newly developed ArcGIS-SWAT running on ArcGIS 8.x that is still under test.
preSWAT has been developed to recalcuate slopes, slope lengths, and curve numbers for each HRU and to update tables and input files that are created by BASINS modules. This module requires the following themes created by BASINS:
These theme names are not supposed to be changed by the user so that preSWAT can automatically find required themes without user interaction. Another data required is a runoff curve number lookup table that is used to estimate curve numbers. Its format is as follows:
|Table 1. Format of the runoff curve number lookup table|
Only lucode, hyd_a, hyd_b, hyd_c, and hyd_d fields are used and lucode should coincide with the SWAT land use code:
|Table 2. SWAT land use code|
Swat-Sol, Swat-Hru, Soil Component Data, Swat-Mgt1, and Swat-Mgt2 tables are also needed to create and manipulate an HRU grid and to populate input databases. These tables are supposed to be generated by the BASINS project before running preSWAT.
Slopes are calculated using the slope and zonal statistics functions in the Avenue objects and averaged over HRUs. Slope lengths are redefined like the following:
|Table 3. Slope and slope length|
Curve numbers are computed using the Soil Component Data and the runoff curve number lookup tables [Olivera:2001]:
|CN = pct_a * hyd_a + pct_b * hyd_b + pct_c * hyd_c + pct_d * hyd_d + pct_w * hyd_w||(1)|
The curve number of 99 for water is chosen instead of 100 to avoid the domain error of some equations in the SWAT procedures.
The Lake Lewisville watershed is simulated with BASINS/SWAT and its HRU parameters are updated using preSWAT before SWAT executes. All the necessary data to run the SWAT model is easily obtained through BASINS tools like Web Extraction Tool and Data Extraction. Lake Lewisville is a part of the Trinity River basin which is shown in Fig.1.
|Figure 1. The Lake Lewisville watershed in the Trinity River basin|
BASINS extracts various data from the web. Of these data, the followings are used in this project:
Besides BASINS data extracted by the tools, the DEM and the land use grids are prepared and the NHD stream shapefile is used to burn-in streams as shown in Figs.2 and 3. To delineate the watershed, the inlet from upstream draining area and the outlet from the study area have to be added manually (Fig.3).
|Figure 2. Automatic delineation|
|Figure 3. Land use|
Once the watershed is delineated, the land use and the soil are overlaied onto the subbasins to determine HRUs. However, BASINS does not make any HRU related grid and only .HRU files are created using subbasin parameters. At this point, preSWAT is used to generate the HRU grid and update the input files (Figs.4, 5, and 6).
|Figure 4. Soil class and preSWAT menu|
|Figure 5. Calculate HRU parameters|
preSWAT requires two tables: runoff curve number (Fig.7) and soil component (Fig.8). These two tables are main factors to estimate HRU curve numbers (Eq.1).
|Figure 6. Choose runoff curve number lookup table|
|Figure 7. Runoff curve number lookup table|
|Figure 8. Soil component data|
Using preSWAT, the HRU grid is created (Fig.9) and its attributes table is calculated per HRU (Fig.10).
|Figure 9. HRU|
|Figure 10. HRU attributes table|
swatLoads summarizes the output of SWAT and the two results are compared in Fig.11. The four plots show that the result from HRU parameters is very similar to that of subbasin parameters to some extent.
|Figure 11. Comparison of the two results|
|Table 4. Landuse legend of Fig.11|
Unfortunately, in this application, there is no real data available and the model itself is simplified due to lack of input data, so it is somewhat difficult to say which one is better. However, as shown in Fig.11, the default BASINS model (subbasin parameters) underestimates loads compared to the HRU result. It may be because geographic parameters are lumped to those of subbasins in the default model, so it loses some data precision and the result can be more averagely spreaded than the result of HRU parameters.
For this project, preSWAT and swatLoads have been developed in order to generate HRU input files for the SWAT model and to summarize the output for each land use. By comparing the result with the default BASINS model which is based on subbasins, its loads are acceptably computed within reasonable ranges and it reflects geographic variety of each HRU better than the subbasin based model. Further, preSWAT reduces time and efforts required to update parameters and input files for those who want to model their watershed more exactly.
Although preSWAT can be incorporated into the BASINS project, it would be much helpful if the BASINS/SWAT module implements the feature of preSWAT and the calculation is done at one time in the consistent interface.