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the SWAT model in different applications. The results suggest that the sensitivity of different
parameters might be different for different catchment areas (e.g. van Griensven
et al.
, 2006).
A variety of automatic calibration routines have been used with SWAT. However, the sheer
number of parameters also raises issues of over-parameterisation and over-fitting (see Chapter
7). Whittaker
et al.
(2010) suggest that this is not an important problem with SWAT because of
the natural regularisation of the range of feasible values that the parameters can take. They used
a multiple objective genetic search algorithm to identify over 4000 parameter values in the
Blue River application and suggested that a comparison of calibration and evaluation period
performance indicated that over-fitting was not a problem. However, the calibrated values
still surely depend on the performance measures chosen and how they are combined, and
physically there is clear potential for an increase in runoff generation in one HRU to be off-set
by a decrease in another HRU. Given a large number of HRUs, the potential for interaction
amongst parameters is great.
SWAT has also been used with a number of uncertainty estimation approaches including
PEST (Lin and Radcliffe, 2006) and GLUE (Muleta and Nicklow, 2005; Arabi
et al.
, 2007). More
details of these methods can be found in Chapter 7.
B6.2.6 An Application of SWAT
Kannan
et al.
(2007) give an interesting example of the application of SWAT (in the ArcView
SWAT2000 version) to the small Colworth catchment (141.5 ha) in England. It is concernedwith
getting the hydrological partitioning correct so as to model nutrient and contaminant transport
more correctly. Most of the catchment is covered by an arable rotation, with one section of
forest. Soil information was provided from the National Soils Resources Institute database.
Many of the fields had been drained in the 1960s with clay tile drains, with some secondary
mole drainage and sub-soiling. Raster GIS overlays of soil and land use were used to define
Figure B6.2.1
Definition of hydrological response units for the application of the SWAT model to the
Colworth catchment in England as grouped grid entities of similar properties (after Kannan
et al.
, 2007,
with kind permission of Elsevier).
