Geology Reference
In-Depth Information
soil;
g
m
is the weight density of soil at field mois-
ture content;
g
w
is the weight density of water;
and
m
is the relative saturated depth (thickness
of the saturated zone divided by soil depth above
the failure plane) (Ward
et al
., 1981).
temporally distributed basis. They may include
time-varying data for phreatic surface level,
snowpack depth, overland flow depth, soil ero-
sion or any other variable at any grid square or
channel link within the basin. Alternatively, syn-
optic or time-integrated views of the spatial dis-
tribution of any variable across the basin can be
produced (Fig. 14.2).
14.3.4 Simulation procedure
A mesh is set up which defines the spatial extent
of the basin model and which is used for repre-
senting spatial variability in the basin properties
(Fig. 14.1). The number of mesh grid squares and
the mesh resolution are limited primarily by
computational power but may also depend on the
length of the simulation and the size of the basin.
With current capabilities, a mesh of around 400
squares will comfortably allow simulations of
decades to a century with run times of a few
hours. The size of the square then determines the
basin scale which can be simulated. However, as
the fundamental model equations typically repre-
sent small-scale physics, there is a potential con-
flict with the use of large grid squares. The
maximum size considered physically reasonable
has not been defined, but mesh resolutions of
2 km have been used in a number of applications,
allowing basins of up to around 1500 km
2
to be
simulated. A trade-off would enable a larger num-
ber of squares to be used for a shorter simulation,
allowing either a finer grid resolution for a given
basin size or a larger basin for a given grid resolu-
tion. Smaller basins similarly enable finer grid
resolutions to be imposed. Adams and Elliot
(2006), for example, used 3755 squares of resolu-
tion 0.5 m to model an area of 939 m
2
.
The appropriate meteorological data, spatially
and temporally distributed, are fed into the model.
Each SHETRAN component is applied at each
grid square to generate a response (e.g. phreatic
surface rise, overland flow, soil erosion and land-
slide occurrence). These responses interact and
both surface and subsurface waters, and surface
sediments, are routed from square to square as a
function of gradient. Eventually these products
reach the river system and are routed towards the
basin outlet. Model outputs may be obtained for
any part of this procedure on a spatially and
14.3.5 Data provision
In order to represent a basin realistically, physi-
cally-based models require a wide range of data.
While on the one hand this requirement is
demanding and not always achievable through
direct measurement, on the other it is an advan-
tage of physically-based models that they can
make use of all available data, from digital terrain
models to soil property measurements to histori-
cal information (e.g. newspaper articles and pho-
tographs) on basin responses. The data required
by SHETRAN are:
(i)
precipitation and potential evaporation input
data to drive the simulation, preferably at hourly
intervals;
(ii)
topographic, soil, vegetation, sediment and
geotechnical properties to characterize the basin
on a spatially distributed basis; and
(iii)
basin response data, such as discharge
records, sediment yield, landslide inventories and
photographic evidence, for testing the model
output.
It is usually possible to obtain information on the
basin property data through field measurements,
laboratory analysis, national agencies and litera-
ture sources. For example, soil hydraulic and geo-
technical properties can be obtained directly from
field measurements and laboratory analysis of
field samples, or indirectly using pedotransfer
relationships published in the literature (e.g.
Saxton
et al
., 1986). Vegetation distributions are
increasingly available from remote sensing sur-
veys, and digital terrain data are provided by
national agencies (or increasingly from websites,
e.g. the HydroSHEDS data at http://hydrosheds.
cr.usgs.gov (Lehner
et al
., 2008) ). Runoff and ero-
sion plot studies are helpful in evaluating the
