Hydrophysical and Ecological Modelling of Deep Lakes and Reservoirs: Summary Report of an IIASA Workshop, December 1977

A series of questions related to the geophysical and ecological modelling of deep lakes and reservoirs were formulated and distributed in advance of the workshop. This workshop report summarizes the discussion of each of the questions relating to the following topics: element cycles, nutrient uptake and grazing rates by phytoplankton and zooplankton, single versus multi-layer models, vertical mixing and diffusion, lake circulation and methods of parameter estimation

A Comparison of Water Quality Models of the Aerobic Nitrogen Cycle

The objective is to compare a sequence of biochemical water quality models of increasing complexity and diversity, in order to determine the level of complexity needed for predictive models. Primary consideration will be given to models simulating chemical, bacterial and algal components that can be compared with laboratory data. The aerobic nitrogen cycle containing seven chemical and biological components of nitrogen is chosen for the comparative study. The nitrogen components can be coupled by various linear and/or non-linear transformation functions representing mineralization and oxidation of organic nitrogen and phytoplankton-zooplankton interactions. Results of simulation runs for batch systems are compared with the same data. It is concluded that the non-linear couplings, representing bacterially mediated and plankton reactions, have a significant influence on both the system dynamics and the steady state nitrogen concentrations. Future research directions for comparative model studies are indicated

Modeling Wind-Driven Circulation in Lake Balaton

This paper reports the results achieved to date in a program of research to develop and apply mathematical computer models of water quality in shallow lakes. The portion of the research which is the specific topic of this paper is the development, testing and trial application of a transient three-dimensional model of wind-driven circulation. The results are presented in the context of an application to Lake Balaton in Hungary, a large yet very shallow lake. The paper presents a review of the mathematical formulation of the circulation problem and the major methods used in computer models of wind-driven circulation. Detailed examinations of the model assumptions and parameters are also included. A description of the application lake follows and a three-dimensional model appropriate to shallow lakes is proposed and derived. This model is examined for consistency with Lake Balaton's characteristics, and the important need for congruence with an eventual coupled biogeochemical model of the water quality is described and investigated. The requirement that the length and time scales of the hydrodynamic model and the biogeochemical model be consistent with each other and with the processes of interest in the lake is stressed. The proposed circulation model employs a Galerkin technique to compute the vertical velocity profile using a depth-dependent vertical eddy viscosity. The parameters for this model are determined by calibration using simple hypothetical seiche simulations as a standard. It is found that the function specified for the vertical eddy viscosity is a crucial determinant of the model response. The resulting model and calibration are then successfully verified with historical events on Lake Balaton. A detailed examination of the results of one of these event simulations explores aspects of the model predictions pertinent to the mass transport of water quality constituents. Conclusions of the paper include identification of the need to improve the representation of mass transport in existing models of Balaton's water quality and an agenda for future development of a coupled hydrodynamic-biogeochemical water quality model

Wind-induced Sediment Resuspension and its Impact on Algal Growth for Lake Balaton

This report includes two articles which were published concerning IIASA's joint study with the Hungarian Academy of Sciences on eutrophication management of Lake Balaton. The first one, "Dynamic Behavior of Suspended Sediment Concentrations in a Shallow Lake Perturbed by Episodic Wind Events," by Luettich, Jr. et al, deals with the understanding and modeling of physical processes influencing resuspension, while the second one, "Influence of Sediment Resuspension on the Light Conditions and Algal Growth in Lake Balaton," by Somlyody and Koncsos, builds on its achievements and estimates the impact on light conditions and algae biomass. Due to an increased internal phosphorus load and the appearance of nitrogen-fixing blue-green algae, nowadays nutrients are not a limiting factor for Lake Balaton (in spite of the significant load reduction realized since 1983). The short-term dynamics of algae biomass are primarily determined by light, as described in the second article