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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Hydro-environment : Simple conceptual water quality models
Simple conceptual water quality models
Author : BEFEKADU T. WOLDEGIORGIS(1), ANN VAN GRINENSVEN(2), FERNANDO PEREIRA(3) & WILLY BAUWENS(4)
ABSTRACT
The riverine system plays vital role in the ecological functioning. Efficient and reliable method of planning is a key for
curbing the ever increasing water quality problems thereby maintaining a healthy aquatic environment and computer
modelling plays a vital role in this regard. Most river water quality models have large simulation times because they are
based on hydrodynamic simulators. This limits their applicability for operational purposes as well as planning activities
involving long-term statistical information. For instance, imperfect nature of data monitoring and the approximate
conceptualization of the reality are often causes for suspicion in model predictive capacity and hence, uncertainty analysis
is required. Therefore, fast and accurate models are crucially complementary to the detailed models in operational and
planning activities. Traditional QUAL2E model is widely in use for modelling river water quality processes and it can be
considered as an approach with less parameters than detailed models while taking the pollutant interactions into account.
This paper presents quasi-analytical solutions-based river water quality modelling approach that integrates ODEs of
QUAL2E pollutant transformation in to dynamic CSTR method as an improvement over the numerical solution approaches
used in conceptual water quality models. We compared it with the Euler method implemented in the widely applied SWAT
model and explicit as well as implicit fourth order Rung-Kuta methods implemented in conceptual water quality models
such as QUASAR and QUESTOR. The improvement mainly aimed at increasing model accuracy and solution stability
with special emphasis for simulations during the low flow periods. We evaluated the behaviors of results from the Euler
numerical solutions and the Runge Kuta4 numerical integration methods. It turned out that the numerical methods show
serious instability because of overestimations of decay processes during large residence times as well as due to abrupt
changes in the input concentrations. To overcome the instability we derived, based on simplifying assumptions, quasianalytical
solutions of inhomogeneous first order differential equations resulting from the integration of the QUAL2E
transformation formulations in to the CSTR-based mass balance equations. Finally, we tested the stability of the quasianalytical
method for real simulations and hypothetical extreme low flow scenarios. The new quasi-analytical solution
gives unconditionally stable solution even when advanced implicit fourth order Rung-Kuta schemes give unstable results.
It also gives fairly comparable results with the reference RWQM while running 130,000 times faster than it. Water quality
processes are so critical during the low flow periods that it is fair to conclude that this approach is preferable to the
numerical solutions of the ODEs of solute transformation. Besides the riverine system, this approach is well suited for
water quality modelling in conveyance systems associated with large residence times like navigation canals. Furthermore,
we demonstrated that the dynamic quasi-analytical CSTR solution of non-conservative pollutant modelling is analogous to
the classical steady linear reservoir approach having variable and residence-time-dependent reservoir constant with
proper input scaling down.
In the context of decision support system, we believe that the implementation of this approach combined with conceptual
water quantity modelling methods makes it vitally complementary to detailed models for planning and operation of water
quality problems demanding fast and fairly accurate results.
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Hydro-environment
Date Published : 19/08/2015
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