Environmental Engineering Reference
In-Depth Information
constant values for the rainfall losses as evapotran-spiration and infiltration. Continuous
simulation models attempt to represent the entire hydrologic system, simulating the
natural environment. They consider the whole runoff process, including surface runoff
storage, infiltration, overland routing and channel flow on a continuous basis, considering
the variation of parameters in time and space. The major advantage of models is the
possibility to apply large data sets and to simulate and predict different scenarios at
conditions closer to the naturally happening events.
Different models have been developed and applied in practice; some examples are the
EPA storm water management model SWMM, which provides for qualitative aspects to
be evaluated as well (Debo & Reese 2003, www.epa.gov/ceampubl,
http://ccee.oregonstate.edu/swmm/); the ARM transfer function model (Novotny 2003);
or the Rainfall flow erosion (RAFLER) model (Stephenson 2003). The practical
application of these models in many cases is limited because of the large amount of data
input needed, the complexity of application requiring advanced software and hardware
facilities, and the need of highly trained personnel. In some cases, the application
limitations might be related to the basic assumptions during the model development,
connected with the mathematical tools applied for the problem solution.
Considering the sources and generation of diffuse pollution loads, simple techniques
of quantification of runoff might lead to errors in the pollution loads estimation due to an
inaccurate estimation of quantitative and qualitative parameters. A more detailed
illustration of such possibility is presented in Chapter 4. An accurate pollution load
determination should be based on the evaluation of event orientated actual pollution
loads, rather than on average annual figures. Such an approach requires model application
in order to incorporate the large amount of input data regarding the rainfall events,
watershed and drainage system characteristics and water quality. A very useful tool in
this direction is the application of geographical information systems (GIS), to form the
basis for model application and data presentation.
GIS are data information systems, which deal with data referenced by spatial or
geographic coordinates. It also contains components, allowing for data management and
analysis procedures, and other functions, as the possibility for incorporation of more
sophisticated models. Information can be stored in different layers, e.g. the topographic
information is stored in one layer, the soil information on another, the ground water
aquifer on a third layer, surface water on a fourth layer, etc. Each layer represents the
spatial variation of the features, together with data for each specific item featured, e.g. the
surface water layer could show the rivers and streams of a catchment basin together with
monitoring data with respect to sampling points, as well as, qualitative and quantative
data sets for each point. The system allows for the interaction of the information between
different layers and also for processing the information by different mathematical
expressions or by different models. Incorporating different models, databases, expert
systems or other tools for data analysis into a GIS, requires a common framework
between the different components in order to allow for data exchange in both directions,
as input or output values. Thus GIS are very valuable tools because they allow for:
• Data storage and retrieval;
• Data processing, transformation and manipulation in an interactive way - spatially
within the plane of the layer under consideration, and between different layers;
• Data visualization.
