Topmodel
The Topmodel is a variable contributing
area conceptual model in which the predominant factors determining the formation
of runoff are represented by the topography of the basin and a negative
exponential law linking the transmissivity of the soil with the vertical
distance from the ground level. In this model the total flow is calculated as
the sum of two terms: surface runoff and flow in the saturated zone. The surface
runoff, in the most recent versions of the model, is in turn the sum of two
components, the first generated by infiltration excess and the second, referring
to a variable contributing area, by saturation excess. Though a conceptual
model, i.e. one in which the physical reality is represented in a simplified
manner, the Topmodel is frequently described as being `physically based', in the
sense that its parameters can be measured directly in situ (Beven and Kirkby,
1979). This definition is somewhat optimistic, in veiw of the doubts and
uncertainties encountered even in defining the parameters of the `physically
based models', as already mentioned.
Topmodel performs what is called an `upward search for
conceptualisation' from the soil column level to the catchment scale. Basin
parameters are related to point estimates. The spatial variability of both soil
water content and lateral drainage is related to that of soil and topographic
characteristics by means of simple but meaningful assumptions. The model is also
attractive because of its structural simplicity and parsimonious
parameterisation. The Topmodel is one of the few conceptual models that accounts
explicitly for the saturation excess overland flow mechanism and integrates the
variable contributing area concept, both of which are essential to model the
catchment accurately.
Topmodel represents catchment topography by means of a
topographic index, ln(a/tanB), where `a' is the area draining through a grid
square per unit length of contour and `tanB' is the average outflow gradient
from the square. The index is calculated from a Digital Terrain Map (DTM) across
a grid covering the catchment. The grid must be sufficiently fine to resolve
important characteristics and slope formations. A high index value usually
indicates a wet part of the catchment; this can arise either from a large
contributing drainage area or from very flat slopes. Areas with low index values
are usually drier, resulting from either steep slope or a small contributing
drainage area. Grid squares with the same index values are assumed to behave in
a hydrologically similar manner. As a result of this assumption, the catchment's
topography may be summarised by the distribution of the index
values.
She Model
The Systeme Hydrologique Europeen
(SHE), the Institute of Hydrology Distributed Model (IHDM), and the USDAARS
small watershed model are the familiar models from this group. Because of their
inherent structure these models also make very little use of contour, soil and
vegetation maps, or of the increasing body of information in such areas as soil
physics and plant physiology. Similarly, much historical information frequently
consulted during project planning, for example crop yields over specific
periods, survival patterns of particular types of vegetation and characteristics
events occurring during floods and droughts, is not used directly. A
considerable improvement in project planning could therefore be derived from the
integration of such information into the modelling process. These observations
do not imply any criticism of conventional rainfall-runoff models in relation to
the more traditional applications in which they have clearly been successful,
for example real-time flow forecasting and the extension of short stream flow
records using longer rainfall records. However, they serve to underline some of
the potential which a new approach in hydrological modelling might be able to
fulfil. In particular physically-based, distributed models can in principle
overcome many of the above deficiencies through their use of parameters which
have a physical interpretation and through their representation of spatial
variability in the parameter values. (Storm,1989).
Structure of SHE :
SHE has been developed as a fully modular system for
mathematical description of the land phase of the hydrological cycle. The system
comprises the following models for description of water flows:
- A one-dimensional interception and evapotranspiration
model called the ET component;
- A two-dimensional overland flow model and one
dimensional river/channel flow model called the OC component;
- A one-dimensional unsaturated zone flow model, called
UZ component;
- A two-dimensional saturated flow (ground water)
model, called the SZ component; (a three-dimensional SZ component has recently
been developed).
- A one dimensional snow melt model called the SM
component.
- A two dimensional irrigation model called IR
component.
In addition to these water flow components, add-on modules have been/are
being developed for :
- A one-dimensional description of solute transport and
chemical processes in the unsaturated zone.
- A three dimensional description of solute transport
and chemical processes in the saturated zone.
- A description of soil erosion and sediment
transport.
- A description of the transport and fate of
radio-nuclear isotopes.
- A description at a catchment scale of nitrogen transport and
processes.
The physical processes considered in the SHE are schematized in
Fig.1. Each of the major hydrological processes of water movement (snowmelt,
canopy interception, evapotranspiration, overland and channel flow, unsaturated
and saturated subsurface flow) is considered. The spatial distribution of
catchment parameters, precipitation input and hydrological response is achieved
in the horizontal through representation of the catchment by an orthogonal grid
network and in the vertical by a column of horizontal layers at each grid
square. (Storm, 1989).
Point and Space Scales
The scope of hydrology is
best defined by the hydrologic cycle. Depending on the hydrologic problem under
consideration, the hydrologic cycle or its components can be traced at different
scales of time and space. The global scale is the largest spatial scale and the
watershed or drainage basin, the smallest spatial scale. Time scales used in
hydrologic studies range from a fraction of an hour to a year or perhaps many
years. The physics of the process by which rainfall is separated into surface
runoff and infiltration, and further into evaporation and ground water recharge
- i.e. the basic processes of the hydrological cycle - is best understood on the
point scale.
The basis for operational hydrology is the catchment or river
basin, or urban area. The average of the parameters observed on a point scale is
not necessarily representative of the conditions of a catchment. Hydrologists
are keenly aware that what they observe on a point scale can not be integrated
directly into area averages useable for operational hydrology, and that the
spatial redistribution of the water cycle components must be considered.
Use of Remotely Sensed Data
The remotely sensed data
(aerial photography and satellite imagery) provide spatial information about the
processes of the land phase of the hydrological cycle. The land cover maps
derived by remote sensing are the basis of hydrologic response units for
modelling units. For an understanding of the hydrology of areas with little
available data, a better insight into the distribution of the physical
characteristics of the catchments is provided by image processing techniques.
Some of the new measurement methods (photographic systems, active radar systems
etc.) could yield assessment of areal distribution or atleast to some extent
reliable areal totals or averages of hydrologic variable such as precipitation,
evapotranspiration and soil moisture. Some of the main hydrological application
field of remote sensing are:
-
Spatial rainfall patterns
-
Evaporation and soil moisture
-
Snow cover extent
-
Groundwater
-
Topography
-
Water Bodies
-
Vegetation