CRITERIA FOR SELECTION OF SPATIAL DIMENSION IN THE APPLICATION OF ONE-DIMENSIONAL AND 2-DIMENSIONAL WATER-QUALITY MODELS

A critical aspect of modelling water quality in lakes and reservoirs i s to select a model with a spatial representation that is appropriate for the dominant mixing processes and the observed variation in water quality variables. One-dimensional modelling is frequently applied to small-to-medium size lakes and reservoirs to encompass variations that are most pronounced in the vertical as a result of seasonal or perman ent density stratification. Two- and three-dimensional models are gene rally applied to larger water bodies where both horizontal and vertica l variations are important. Horizontal variations may result from the local effects of inflows and outflows, basin scale seiching and wind m ixing. Increased dimensional capabilities in water quality models prod uce a large increase in computational time and may therefore impose se vere constraints on the length of the simulation. This problem is most frequently averted by increasing the time step, which in itself may i ntroduce problems associated with numerical diffusion. Therefore when longer simulations of water quality in a lake are required, as is ofte n the case for management applications, it is desirable to select a mo del with the least spatial dimension required to adequately encompass the observed variation in water quality variables and the most importa nt mixing processes. This study proposes a set of criteria that can be used a priori to select the appropriate dimensional representation wh en applying a water quality model to a lake. The critical factors that influence the application of a one- or two-dimensional model are the size of the lake, the volume of the inflows and the settling velocity of particulates in the inflows.