7. Net Production (NETPROD)

The process of ecological gross production combines solar energy and materials into organic matter and oxygen. The way the inputs are factors in gross production were shown in Section II-6. The products accumulate in storage where they are drawn off by consumption processes of plants, animals, and microbes (R).

The appearance of products in an ecological or economic system is a balance between the rate of Gross-Production and the rate of consumption. See Figure III-9. In this model production is proportional to the light resources which are varied through 4 seasons and then repeated the next year, etc. Consumption is proportional to the product storage (Q). The difference (P minus C) is the rate of net production.

The balance between gross production and consumption is called net production. The rate of appearance of products (rate of change in Q) is easily measured in the field. By calibrating a model like that in Figure III-8 model one can let the simulation show the gross production. Very bad errors come when people try to use net production as a measure of gross production, or compare the two as if they were the same.

The simulation in Figure III-8b starts with a low initial storage. The graphs include the pattern of input resource, the gross production rate, the storage of product, and the net production rate. Note the way net production is greatest during the period of growth declining toward zero as the system is close to steady state. At steady state is one with storages constant due to the balance of inflows and outflows.

This simulation calculates the hand simulation given in Chapter 8 of "Environmental Systems and Public Policy" (Odum and Odum, 1988). This model is a very simple one to compare net and gross production. See section II-7 (Figure II-8) for production and consumption with the normal material recycle included.

Examples of Net Production

Net and gross productions are the basis for all the ecological, agricultural, and forestry systems of the world. This includes ponds, lakes, rivers, oceans, fields, forests, swamps, etc. In agriculture and forestry where emphasis is yields, consumption is reduced in order to make net production as high as possible. However, wild ecosystems prevail by putting their produce into a diversity of other organisms that contribute to soils and improve gross production in this and other ways.

The principles shown here apply also to economic production such as the products of an industry. The total output of shoes is gross production whereas the net production is the balance between the shoes made and those sold and used.

"What if" Experiments

1. What is the effect of increasing the rate of consumption by increasing K2?

2. What coefficient in Figure III-8a represents the efficiency with which the sunlight S is converted into gross production? What is the effect of increasing this coefficient?

3. Can you predict what kind of graphs will results if the product storage starts off at a higher value than the steady state in the initial simulation in Figure III-8b?

COMPUTER MINIMODELS AND SIMULATION EXERCISES FOR SCIENCE AND SOCIAL STUDIES

Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611