Geographic Information Systems (GIS) have been successfully integrated
with distributed parameter, single-event, water quality models such a
s AGNPS (AGricultural NonPoint Source) and ANSWERS (Areal Nonpoint Sou
rce Watershed Environmental Response Simulation). These linkages prove
d to be an effective way to collect, manipulate, visualize, and analyz
e the input and output date of water quality models. However, for cont
inuous-time, basin large-scale water quality models, collecting and ma
nipulating the input data are more time-consuming and cumbersome due t
o the method of disaggregation (subdivisions are based on topographic
boundaries). SWAT (Soil and Water Assessment Tool), a basin-scale wate
r quality model, was integrated with a GIS to extract input data for m
odeling a basin. This paper discusses the detailed development of the
integration of the SWAT water quality model with GRASS (Geographic Res
ources Analysis Support System) GIS, along with an application and adv
antages. The integrated system was applied to simulated a 114 sq. km u
pper portion of the Seco Creek Basin by subdividing it into 37 subbasi
ns. The average monthly predicted streamflow is in agreement with meas
ured monthly streamflow values.