Hydrological modelling
is a powerful technique in the planning and development of integrated approach
for management of water resources.
The scope of hydrological applications has broadened dramatically over the past four decades. Although the problems of flood protection and water resourcesmanagement continue to be of importance and relevance for the security of communities and for human, social and economic development, many applied problems relating to the wider role of hydrology have come into focus.
Physically based distributed models of the hydrological cycle can in principle be applied to almost any kind of hydrological problem. These models are based on our understanding of the physics of the hydrological processes which conrol catchment response and use physically based equations to describe these processes. Some typical examples of field applications include study of effect of catchment changes, prediction of behaviour of ungauged catchment, of spatial variability in catchment inputs and outputs, movement of pollutants and sediment etc.
Hydrological modelling is a powerful technique of hydrologic system investigation for both the research hydrologists system investigation for both the research hydrologists and the practising water resources engineers involoved in the planning and developmetn of integrated approach for management of water resources.
The availability of remote sensing data and application of Geographical Information system provide very useful input data requirement for physically based hydrological models. The use of remote sensing and GIS facilitates hydrologists to deal with large scale, complex and spatially distributed hydrological processes.
Physically based Hydorlogical Models
The physically
based models are based on our understanding of the physics of the hydrological
processes which control the catchment response and use physically based
equations to describe these processes. A discretization of spatial and temporal
coordinates is made and the solution is obtainhydrological applications has broadened dramatically over the past four decades. Although the problems of flood protection and water resources management continue to be of importance and relevance for the security of communities and for human, social and economic development, many applied problems relating to the wider role of hydrology have come into focus.
Physically based distributed models of the hydrological cycle can in principle be applied to almost any kind of hydrological problem. These models are based on our understanding of the physics of the hydrological processes which control catchment response and use physically based equations to describe these processes. Some typical examples of field applications include study of effect of catchment changes, prediction of behaviour of ungauged catchment, of spatial variability in catchment inputs and outputs, movement of pollutants and sediment etc.
Hydrological modelling is a powerful technique of hydrologic system investigation for both the research hydrologists and the practising water resources engineers involved in the planning and development of integrated approach for management of water resources.
ed at the node points of this discretized representation. Physically based distributed models do not consider the transfer of water in a catchment to take place in a few defined storage as in case of lumped conceptual models. From their physical basis such models can simulate the complete runoff regime, providing multiple outputs (e.g. river discharge, phreatic surface level and evaporation loss) while black box models can offer only one output. In these models transfer of mass, momentum and energy are calculated directly from the governing partial differential equations which are solved using numerical methods, for example the St. Venant equations for surface flow, the Richards equation for unsaturated zone flow and the Boussinesq equation for ground water flow. As the input data and computational requirements are enormous, the use of these models for real-time forecasting has not reached the ‘production stage’ so far, particularly for data availability situations prevalent in developing countries like India.
Physically-based distributed models can in principle be
applied to almost any kind of hydrological problem. Some examples of typical
fields of application are:
- Catchment changes
- Ungauged Catchments
- Spatial variability
- Movement of Pollutants and
Sediments
Topmodel and SHE Model
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 TOPMODEL is frequently described as being ‘physically based’, in the sense that its parameters can be measured directly in situ. 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.
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. 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.