User:Abdrehman Ahmed

Distributed Process Description Based Models

Another approach to hydrological processes modeling was the attempt to produce models based on the governing equations describing all the surface and subsurface flow processes in the catchment. A first attempt to outline the potentials and some of the elements in a distributed process description based model on a catchment scale was made by Freeze and Harlan (1969). The calculations require larger computers to solve the flow domain and points at the elements of the catchment. Distributed models of this type have the possibility of defining parameter values for every element in the solution mesh. They give a detailed and potentially more correct description of the hydrological processes in the catchment than do the other model types. The process equations require many different parameters to be specified for each element and made the calibration difficult in comparison with the observed responses of the catchment.

In principle parameter adjustment of this type of model is not necessary if the process equations used are valid and if the parameters are strongly related to the physical characteristics of the surface, soil and rock. In practice the model requires effective values at the scale of the elements. Because of the heterogeneity of soil, surface vegetation establishing a link between measurements and element values is difficult. The Distributed Process Description Based Models can in principle be applied to almost any kind of hydrological problem. The development is increased over the recent years for the fact that the increase in computer power, programming tools and digital databases and the need to handle processes and predictions of runoff, sediment transport and/or contaminants.

Another reason is the need of the models for impact assessment. Changes in land use, such as deforestation or urbanization often affect only part of a catchment area. With a distributed model it is possible to examine the effects of such land use changes in their correct spatial context by understanding the physical meaning between the parameter values and the land use changes.

Recent examples of distributed process based models include the SHE model (Abbott et al., 1986), MIKE SHE (Refsgaard and Storm, 1995), IHDM (Institute of Hydrology Distributed Model; Calver and Wood 1995), and THALES (Grayson et al. 1992), etc.