ARCSWAT Users Manual
Documented by
Ling Bian lbian@geog.buffalo.edu
Clayton Blodgett blodgett@falcon.cc.ukans.edu
1. ARCSWAT GENERAL DESCRIPTION
ARCSWAT is a public domain graphical user interface program. It is designed to link the hydrologic model SWAT (Soil and Water Assessment Tool) and the GIS package ARC/INFO. The development of the interface was supported by Kansas Water Office and the University of Kansas General Research Fund.
The SWAT model is limited in that it does not explicitly allow for the inclusion of spatial data as model inputs. Data must be processed into a form that the model can use. Procssing these data, even with the use of a GIS, is tedious and time comsumming due to the large number of model parameters reqiured to execute SWAT.
The development of ARCSWAT aims at an effective use of spatial data to enhance hydrological modeling. The interface performs the following tasks: (1) to streamline GIS processes tailored toward SWAT modeling needs; (2) to automate data communication between Arc/Info and SWAT; and (3) to provide a user-friendly data entry and editing environment for SWAT.
1.1. Description of SWAT and Arc/Info
SWAT is a public domain hydrologic model, developed by USDA Agricultural Research Service. It is a semi-empirical and semi-physical model. It has been used as a practical model to predict the effect of agricultural management decisions on water and sediment yields for large ungauged rural watersheds. Moreover, SWAT is an advanced lumped model or a semi-distributed model because it allows a watershed to be divided into a maximum of one hundred sub-watersheds.
SWAT consists of major water budget components such as weather, surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, crop growth, irrigation water transfer, groundwater flow, and channel routing. The model runs on a daily time step for short or long term predictions and operates in a semi-distributed manner to account for spatial differences in soils, land use, crops, topography, channel morphology, and weather conditions. Using GIS data, SWAT has been applied to many major river systems in the United States with promising results.
The SWAT model requires two leves of input data: basin and subbasin. The basin level requires 12 input files: .cio (control input/ouput), .bsn (basin), .cod (control code), .fig (configuration), .lwq (lake water quality), .res (reservoir), .sta (gaged station), .tmp (temperture), .pcp (precipitation), crop.dat (crop database), till.dat (tillage database), and pest.dat (pestside database). The subbasin level includes nine input files: .sub (subbasin), .wgn (weather generator), .rte (routing), .pnd (pond), .chm (chemistry), .sol (soil), .mgt (management), .mco (management code), and .gw (ground water). Each file contains 10-20 parameters.
For more detailed information about SWAT, contact Nancy Sammons, USDA, ARS Grassland Soil & Water Research Laboratory, 808 East Blackland Road, Temple, TX 76502, phone: (817)770-6512, fax (817)770-6561, e-mail: sammons@brcsun1.tamu.edu
Arc/Info is a preporitized GIS software package developed and distruibuted by Environmental Systems Research Institute, Inc. (ESRI). The package is widely used by government agencies, provate sectors, and universities. It offers advanced cacbilities in data management, spatial analysis, and scientific visualization. For more details contact
ESRI, 380 New York Street, Redlands, CA, 92373-8100, USA phone: 909-793-2853, fax: 909-793-5953, web site: http://www.esri.com
1.2. Conceptual Framework of the Interface
In the integration, Arc/Info serves as a "front end", by preparing spatialy explicit parameters for SWAT. SWAT runs in a manor similar to a batch operation. All parameter files must be prepared prior to the execution, and SWAT produces several output text files all at once.
Both SWAT and Arc/Info are mature and complex software systems with their own data structures and operation mechanisms; thus the two components are kept with minimal or no modifications. On this premise, the conceptual design of the interface includes an add-on external user interface and a shared internal database to couple the two software components.
1.3. General Operation
The interface SWAT/ARC consists of two major components: ARCSWAT and SWATWIN.
ARCSWAT is a graphic user interface written in AML (Arc Macro Language) that runs in the Unix environment. It uses Arc/Info coverages as input to generate many SWAT parameters (or intermediate values) as output. These output values can be read into appropriate SWAT input files through the internal database.
WINSWAT is a graphic user inerface that runs in Microsoft Windows (verson 3.1) on a PC. The development language is Visual Basic Professional, version 3.1. It takes the values generated in ARCSWAT and allows interactive data entry and editing to prepare all SWAT input files. When the parameter input is completed, SWATWIN can execute SWAT, that in turn produces model output files.
1.4. Hardware and Software Considerations
Both SWAT and Arc/Info have Unix and PC versions. SWAT/Arc uses the combiantion of PC SWAT and Unix Arc/Info because PC Arc/Info lacks functions critical for extracting spatial parameters for SWAT. Choosing the PC version of SWAT was based on the request of the program sponsors as well as the need of PC users. The current combination prefers network connections between PC and Unix for data transfer but it can also be done manually. AML is the logical choice to streamline the Arc/Info processes, and Visaul Basic is well suited for developing graphic user interfaces in the PC environment. The internal database is developed in an object oriented appraoch using the C++ programming language on a PC.
1.5 The End-User
The end-user is assumed to be a trained hydrologist, who is familiar with basic computer operation, and has basic knowledge of GIS. Experience or knowledge with Arc/Info is preferred but not necessary.
1.6 This Manual and SWAT Manual
This manual helps the user prepare SWAT input files. It is closely related to, but not a substitute for the SWAT manual. For specifics of the SWAT modeling process, the end-user must refer to the original SWAT manual.
2. OPERATION OF ARCSWAT
The primary function of ARCSWAT is to streamline GIS processes in order to extract spatialy explicit input parameters for SWAT.
2.1. Arc/Info Files
The base data sets include the following
MUID, will be linked by ARCSWAT to the info file COMP that comes with STATSGO. The Comp info file is required.
All coveages are assumed to be in Arc/Info format, free of topological errors, and with identical projection parameters. All the recommended scales are considered appropriate for large rural watersheds, which SWAT was designed for. All of the data involved can be obtained from public domain sources such as US Geological Survey (USGS), Natural Resource Conservation Service (NRCS),and National Climatic Data Center (NCDC).
2.2. The Main Menu
In the directory where the AMLs are located, type &r arcswat init, the main menu will appear (Figure 1).

Figure 1. The main menu.
A majority of the spatially explicit parameters required by SWAT are included in six SWAT input files: Basin (.bsn), Subbasin (.sub), Precipitation (.pcp), Temperature (.tmp), Soils (.sol), and Routing (.rte). The extraction process is thus organized into six corresponding routines. The Basin routine must be executed first, with the Subbasin routine executed next. After these two, the user can proceed or skip any one of the remaining four: Temperture, Precipitation, Soil, and Routing. An error checking mechanism warns the user whenever proper procedures are violated.
Common Elements
In the main menu and submenus, there are two buttons: OK and Cancel. Help
is available at the main menu.
2.3. BASIN
The Basin Routine calculates basin area (DA in .bsn). This value will also be used by the SUBBASIN routine for further calculation. The user begins by choosing the appropriate directory and the basin coverage (Figure 2).

Figure 2. Choosing the basin coverage.
2.4. SUBBASIN
The SUBBASIN routine calculates the following seven parameters: number of subbasins (LU in .cod), and for each subbasin: proportion of each subbasin in the basin (FLU in .sub), latitude (YLT in .wgn), channel length (CHL1 in .sub), average channel slope (CHS in .sub), average slope length (SL in .sub), and average slope steepness (STP in .sub).
The user begins by selecting the appropriate subbasin coverage (Figure 3).

Figure 3. Choosing subbasin coverage.
When the subbasin map is displayed, the user has the option to process all subbasins or selected subbasins (see Figure 4).

Figure 4. Selecting subbasins.
The user needs to number each selected subbasin. The numbers will be carried to WINSWAT and used in SWAT modeling (Figure 5).

Figure 5. Assigning subbasin numbers.
The user needs to select the appropriate primary stream, all-stream, and elevation lattice in the following windows, repectively (see Figures 6, 7, and 8).

Figure 6. Selecting primary streams coverage.

Figure 7. Selecting all streams coverage.

Figure 8. Selecting elevation grid.
The user needs to identify whether an elevation lattice exist for individual subbasins (Figure 9) If they are not available, ARCSWAT will clip the elevation lattice by each subbasin.

Figure 9. Selecting elevation grid for subbasins.
Among the seven parameters, number of subbasins (LU), proportion of basin each subbasin occupies (FLU), and subbasin latitude (YLT) are caulated in a straightforward fashion. However, the remining four variables, channel length, average channel slope, average slope length, and average slope steepness, involve various methods.
To determine channel length (CHL1), the user needs to select the stream segments that connect the subbasin outlet to the most distant point in the subbasin. The longest stream is recommended (Figure 10).

Figure 10. Selecting the longest channel.
The user can re-select the stream reach if previously selected stream segments are incorrect (Figure 11).

Figure 11. Re-selecting the longest channel.
An error checking mechanism warns the user if the selected segments are discontinuous. During this process, the headwater and outlet of each subbasin are also determined. This information will be used, in concert with the elevation lattice and the calculated channel length, to determine the average channel slope (CHS).
The average slope length (SL) and average slope steepness (STP) are calculated using a method described by Williams and Berndt (1976): l = 0.5 * DA/LCH (1); S = 0.25 * Z (LC25 + LC50 + LC75) / DA (2) where l is the average slope length; DA is the area of the subbasin; LCH is the total length of channels in the subbasin; S is the average slope steepness; Z is the elevation range in the subbasin; and LC25, LC50, and LC75 are the lengths of the contour lines generated at 25, 50, and 75 percent of the total elevation range in each subbasin, respectively.
2.5. TEMPERATURE and PRECIPITATION
The user needs to select the appropriate precipitation and temperature coverages (Figures 12 and 13).

Figure 12. Selecting precipitation coverage.

Figure 13. Selecting temperature coverage.
The temperature and precipitation data at the weather stations are interpolated by Thiessen polygons. The proportion of each Thiessen polygon in each subbasin is calculated, and stored in an output ASCII file along with the station ID for further calculations.
2.6. SOILS
The user needs to select the appropriate soils coverage (figure 14). The interface is hard coded for STATSGO soil data.

Figure 14. Selecting soils coverage.
The percentage of each soil series in each soil association polygon, the proportion of each soil association polygon in each subbasin, and the MUID are stored in an output file for further calculations.
2.7. ROUTING
This routine calculates channel slope (CHSS in .rte) and channel length (CHL2 in .rte). To determine channel length (CHL2), the user needs to select stream segments from the subbasin outlet to the basin outlet (Figure 15).

Figure 15. Selecting channel length..
The user can re-select the stream segements if previously selected ones are incorrect. An error checking mechanism warns the user if the selected segments are discontinuous. During this process, the locations of subbasin outlet and the basin outlet are also determined. Those data will be used, in concert with the elevation lattice and the calculated channel length, to determine the average channel slope (CHSS).
When ROUTING routine is complete, select OK to clean all intermediate coveages, grids, and variables, and produce output files.
3. THE OUTPUT FILES
ARCSWAT produces two output ASCII files: arcswat.dat and arcswat.sol. If the files already exist, the command screen prompts the user for additional file names.
3.1. arcswat.dat
The following is an example of arcswat.dat, demonstrating the content and format of the file. The numbers at the begining of each line are added in this document for descriptive convenience. The capitalized abbreviation refer to either the input file extension (see section 1.1.) or the parameters name (see sections 2.2. to 2.6.).
1: COD, LU, 2
2: BSN, DA, 5137.18016
3: 1
4: PCP, 5
5: 14068202, 0.5833858324863
6: 14724802, 0.0541097856222
7: 14176502, 0.3370643940133
8: 14159302, 0.0020507908114
9: 14176702, 0.0233447255548
10: TMP, 5
11: 14068202, 0.6591011776356
12: 14176902, 0.2594612518915
13: 14159302, 0.0020059648452
14: 14176702, 0.0773582288017
15: 14159302, 0.002108974921
16: WGN, YLT, 39.7430529581
17: SUB, FLU, 0.1101166494969, CHL1, 67.55496484375
18: CHS, 1.58389542867, SL,391912.2956727
19: STP, 6.783539850816
20: RTE, CHSS, ,CHL2, 142.4102543945
21: 2
22: ......
23: ......
line1: COD, LU, number of subbasins (2)
line2: BSN, DA, basin area
line3: subbasin number # (1)
line4: PCP, number of precipitation polygons (5) in subbasin
number # (1)
line5: polygon1 ID, percetage of area of the polygon1 in the
subbasin
line6: polygon2 ID, percetage of area of the polygon2 in the
subbasin
line7: polygon3 ID, percetage of area of the polygon3 in the
subbasin
line8: polygon4 ID, percetage of area of the polygon4 in the
subbasin
line9: polygon5 ID, percetage of area of the polygon5 in the
subbasin
line10: TMP, number of temperature polygons (5) in subbasin number
# (1)
line11: polygon1 ID, percetage of area of the polygon1 in the subbasin
line12: polygon2 ID, percetage of area of the polygon2 in the subbasin
line13: polygon3 ID, percetage of area of the polygon3 in the subbasin
line14: polygon4 ID, percetage of area of the polygon4 in the subbasin
line15: polygon5 ID, percetage of area of the polygon5 in the subbasin
line16: WGN, YLT, latitude of the subbasin
line17: SUB, FLU, proportion of basin each subbasin occupies, CHL1, channel
length
line18: CHS, average channel slope, SL, average slope length
line19: STP, average slope steepness
line20: RTE, CHSS, average channel slope for routing, CHL2, channel length
for routing
line21: subbasin number # (2)
line22: ......
line23: ......
3.2. arcswat.sol
The file arcswat.sol contains the output associated with soils parameters. Its basic format is as following:
SOL, number of soils associtions in the first subbasin (10)
MUID1, fraction of the soils association polygon in the subbasin, number
of soils series in the soils association
polygon (15)
s5id1, percent of the soils series in the soils association, soils series
name
s5id2, ......, ......
......
s5id15, percent of the soils series in the soils associationn, soils series
name
MUID2, fraction, #
s5id1, ......, ......
......
s5id#, ......, ......
MUID3, ......, ......
......
MUID15, fraction, #
s5id1, ......, ......
......
s5id#, ......, ......
SOL, number of soils associations in the second subbasin (2)
MUID1, ......, ......
......
MUID10, ......, ......
......
Example file:
SOL,10 //subbasin1 SOL, number of soils association
KS116,0.282391695907,15 //MUID1, fraction of the subbasin, number of soils series
IA0070,7,KENNEBEC //s5id, percent of association, soil series name
IA0135,2,BREMER
IA0203,35,GOSPORT
KS0106,14,SOGN
IA0018,2,JUDSON
IA0070,4,KENNEBEC
MO0083,2,KNOX
MO0083,3,KNOX
IA0019,14,LADOGA
KS0072,6,MARTIN
KS0089,2,OSKA
NE0076,3,PAWNEE
NE0076,1,PAWNEE
IA0033,2,SHARPSBURG
KS0118,3,WELDA
KS146,0.3381060066788,17 //MUID2, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0544,16,CLARINDA
IA0033,14,SHARPSBURG
MO0094,11,HIGGINSVILLE
IA0020,8,MACKSBURG
IA0020,11,MACKSBURG
KS0089,1,OSKA
KS0072,2,MARTIN
MO0001,2,GRUNDY
IA0019,2,LADOGA
IA0070,1,KENNEBEC
IA0070,1,KENNEBEC
MO0067,3,POLO
MO0020,1,LAGONDA
IA0296,1,COLO
IA0141,1,NODAWAY
KS0089,2,OSKA
KS161,0.1890942246391,19 //MUID3, fraction, number of soils series
IA0033,23,SHARPSBURG
KS0021,6,CLARESON
OK0096,1,ERAM
KS0023,11,CLIME
KS0106,4,SOGN
OK0004,4,DENNIS
OK0096,8,ERAM
OK0096,2,ERAM
KS0142,11,KENOMA
KS0145,3,LEBO
OK0015,2,SUMMIT
OK0008,1,LULA
OK0015,8,SUMMIT
KS0114,2,VERDIGRIS
KS0119,1,WOODSON
KS0058,19,KENOMA
KS0058,7,KENOMA
KS0058,3,KENOMA
KS0054,4,IVAN
KS0062,3,LABETTE
KS142,0.0915667656721,14 //MUID4, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
KS116,0.0017584911875,15 //MUID5, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,7,KENNEBEC
IA0135,2,BREMER
IA0203,35,GOSPORT
KS0106,14,SOGN
IA0018,2,JUDSON
IA0070,4,KENNEBEC
MO0083,2,KNOX
MO0083,3,KNOX
IA0019,14,LADOGA
KS0072,6,MARTIN
KS0089,2,OSKA
NE0076,3,PAWNEE
NE0076,1,PAWNEE
IA0033,2,SHARPSBURG
KS0118,3,WELDA
KS194,0.065689825265,6 //MUID6, fraction, number of soils series
IA0033,23,SHARPSBURG
MO0011,44,WABASH
KS0095,23,READING
IA0070,15,KENNEBEC
IA0018,11,JUDSON
KS0019,6,CHASE
IA0070,1,KENNEBEC
KS201,0.0311521525056,16 //MUID7, fraction, number of soils series
IA0033,23,SHARPSBURG
OK0011,30,PARSONS
KS0058,27,KENOMA
OK0004,12,DENNIS
OK0064,3,CATOOSA
OK0167,3,APPERSON
KS0114,4,VERDIGRIS
KS0020,3,CHEROKEE
KS0120,3,ZAAR
OK0096,1,ERAM
MO0032,6,BARDEN
MO0032,1,BARDEN
KS0050,1,HEPLER
OK0002,1,COLLINSVILLE
MO0010,2,DEEPWATER
KS0007,1,BATES
OK0015,2,SUMMIT
KS142,0.0001668672246,14 //MUID8, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
KS142,0.0000489369043,14 //MUID9, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
KS142,0.0000292786398,14 //MUID10, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
SOL,8 //subbain2 SOl, number of soils associations
KS116,0.1267554759209,15 //MUID1, fraction, number of soils series
IA0033,23,SHARPSBURG //s5id, percentage, soils series name
IA0070,7,KENNEBEC
IA0135,2,BREMER
IA0203,35,GOSPORT
KS0106,14,SOGN
IA0018,2,JUDSON
IA0070,4,KENNEBEC
MO0083,2,KNOX
MO0083,3,KNOX
IA0019,14,LADOGA
KS0072,6,MARTIN
KS0089,2,OSKA
NE0076,3,PAWNEE
NE0076,1,PAWNEE
IA0033,2,SHARPSBURG
KS0118,3,WELDA
KS116,0.0026929217955,15 //MUID2, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,7,KENNEBEC
IA0135,2,BREMER
IA0203,35,GOSPORT
KS0106,14,SOGN
IA0018,2,JUDSON
IA0070,4,KENNEBEC
MO0083,2,KNOX
MO0083,3,KNOX
IA0019,14,LADOGA
KS0072,6,MARTIN
KS0089,2,OSKA
NE0076,3,PAWNEE
NE0076,1,PAWNEE
IA0033,2,SHARPSBURG
KS0118,3,WELDA
KS142,0.3510941902822,14 //MUID3, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
KS116,0.1340264021893,15 //MUID4 fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,7,KENNEBEC
IA0135,2,BREMER
IA0203,35,GOSPORT
KS0106,14,SOGN
IA0018,2,JUDSON
IA0070,4,KENNEBEC
MO0083,2,KNOX
MO0083,3,KNOX
IA0019,14,LADOGA
KS0072,6,MARTIN
KS0089,2,OSKA
NE0076,3,PAWNEE
NE0076,1,PAWNEE
IA0033,2,SHARPSBURG
KS0118,3,WELDA
KS142,0.2729071047375,14 //MUID5, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0070,8,KENNEBEC
KS0119,6,WOODSON
IA0070,2,KENNEBEC
KS0072,2,MARTIN
KS0072,22,MARTIN
KS0212,2,MARTIN
KS0072,3,MARTIN
KS0106,6,SOGN
KS0115,19,VINLAND
KS0089,11,OSKA
KS0104,3,SIBLEYVILLE
KS0115,7,VINLAND
KS0115,5,VINLAND
NE0076,4,PAWNEE
KS146,0.072561966746,17 //MUID6, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0033,23,SHARPSBURG
IA0544,16,CLARINDA
IA0033,14,SHARPSBURG
MO0094,11,HIGGINSVILLE
IA0020,8,MACKSBURG
IA0020,11,MACKSBURG
KS0089,1,OSKA
KS0072,2,MARTIN
MO0001,2,GRUNDY
IA0019,2,LADOGA
IA0070,1,KENNEBEC
IA0070,1,KENNEBEC
MO0067,3,POLO
MO0020,1,LAGONDA
IA0296,1,COLO
IA0141,1,NODAWAY
KS0089,2,OSKA
KS194,0.036952970072,6 //MUID7, fraction, number of soils series
IA0033,23,SHARPSBURG
MO0011,44,WABASH
KS0095,23,READING
IA0070,15,KENNEBEC
IA0018,11,JUDSON
KS0019,6,CHASE
IA0070,1,KENNEBEC
KS146,0.0030331849373,17 //MUID8, fraction, number of soils series
IA0033,23,SHARPSBURG
IA0033,23,SHARPSBURG
IA0544,16,CLARINDA
IA0033,14,SHARPSBURG
MO0094,11,HIGGINSVILLE
IA0020,8,MACKSBURG
IA0020,11,MACKSBURG
KS0089,1,OSKA
KS0072,2,MARTIN
MO0001,2,GRUNDY
IA0019,2,LADOGA
IA0070,1,KENNEBEC
IA0070,1,KENNEBEC
MO0067,3,POLO
MO0020,1,LAGONDA
IA0296,1,COLO
IA0141,1,NODAWAY
KS0089,2,OSKA