While the model in Figure 1 satisfies the requirement that sufficient detail be included to enable numerical modelling to proceed, it does not satisfy the criterion of simplicity. In addition, an examination of the model demonstrates the implicit requirement for clarification on the wetland characteristics, and for a statement of the disturbances to be modelled. In order to avoid the situation where one "cannot see the trees for the forest", a simpler, summarised model is more practical, particularly if the objective is to evaluate overall wetland response, rather than responses of specific components, to a particular environmental change. The conceptual model of wetland function shown below was constructed using the concept of 'pools and processes', sometimes referred to as 'stocks and flows' or 'reservoirs and rates' (e.g. Ford 1999; Safriel 1999). Four pools of carbon are recognised - inorganic C (gaseous and dissolved gases), inorganic C (solid and dissolved), detrital carbon (particulate or dissolved) and living organic carbon (primary producers, consumers and decomposers). Responses to environmental change can be depicted by changing the relative size of the pools (represented by text within geometric shapes) and the size and direction of processes (represented by arrows). One of the advantages of such conceptual models is that they can easily converted to simple numerical models as data become available using existing modelling software. The basic model shown below represents the 'reference mode' or the 'dynamic null hypothesis' (Ford 1999) of the way the system responds to different environmental conditions. Light and energy are reflected in the size of the detrital pool and photosynthetic organisms. The reference model operates in the same way across the horizontal and vertical layers of a wetland (i.e. it can be joined), only the size of the carbon pools, and the rates of flow change. Click on the individual components (arrows = processes, shapes = carbon pools) in the model below to see descriptions.
Figure 2: General wetland function model based on carbon flows.
This general model (or reference model) is designed to be underpinned by explicit assumptions. The first set of assumptions are stated to reflect the characteristics of the wetland (i.e. physico-chemical characteristics, water balance/hydrology, plant biomass/distribution, etc.). The second set reflect the nature of the disturbance (i.e. a management scenario). The result is a model which is capable of demonstrating how wetlands will change according to their inputted assumptions; a wetland response model.
