OVERVIEW OF LOICZ BIOGEOCHEMICAL BUDGETING PROCEDURE
Budgeting the fluxes of materials to and from a system may be undertaken by many different procedures, but there are inherent similarities among these procedures. Basically, a budget describes the rate of material delivery to the system ("inputs"), the rate of material removal from the system ("outputs"), and the rate of change of material mass within the system ("storage"). Some materials may undergo internal transformations of state which lead to appearance or disappearance of these materials. Such changes are sometimes referred to as "internal sources or sinks" (Figure 1).
Figure 1. Generalized diagram characterizing material budgets.
It is also useful to describe such a budget in terms of a simple equation:
where "dM/dt" represents the change of mass of any particular material in the system with respect to time. It is often assumed that dM/dt = 0; that is, the system mass is assumed to be at steady state. While this assumption is not necessary, it simplifies a discussion of equation (1) and will be used in the math laid out here. More detail can be found in Gordon et al. (1996).
Many so-called budgets deal with only one set of the fluxes in equation (1) (e.g., inputs); in some cases, only a subset of the inputs (e.g., inputs from land into a bay, without consideration of the oceanic inputs) is considered. In the terminology applied here, such a description without an attempt to estimate each of the terms in equation (1) is not a budget. Many different approaches can be taken to budget a particular system. The LOICZ Biogeochemical Modelling Guidelines (Gordon et al., 1996) advocate that a single general (and, we believe, widely applicable) approach be used for building budgets to describe the coastal marine environment, in order to maximize comparability among the budgets. Basically, this approach has three parts:
- How fast does water move through the system of interest?
- How fast do the nutrient elements carbon, nitrogen, and phosphorus move with the water?
- What can be inferred about system performance by discrepancies between the movement of water and the movement of nutrients?
Consider a coastal water body which is of interest. The rate of water exchange between that system and adjacent systems is estimated by one of several procedures. The simplest procedure to describe water exchange in many coastal marine systems is the construction of combined water and salt budgets for those regions. Water flows into the system from land; water is gained and lost through precipitation and evaporation. If it is assumed that the volume of the system (averaged over time, to remove short term variations like tide height) remains constant, then net water outflow balances water inflow. In addition to this budget for water itself, a salt budget can also be established. Water mixes back and forth between the system of interest and adjacent systems. Each of the water inputs and outputs described by the water budget and described by the mixing has a characteristic salinity. Water and usually salt in a system can be assumed to have no internal sources or sinks. That is, the inputs and outputs just outlined account for the water and salt budgets. The term for internal sources and sinks therefore becomes 0 in equation (1), and the budget simply describes water exchange. This combined water and salt budget does not provide a dynamic, quantitative understanding of the processes controlling the characteristics of water exchange in a particular system, but it is often a quick and simple way to describe the exchange. More details on the arithmetic of water and salt budgets and some examples of the procedure for describing water exchange can be found by clicking here. Definitions of key variables may be found by clicking here.
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Last Updated 12 May 2009 by DPS