The system ENVUSE in Figure IV-9a is a general model for examining economic uses of environmental resources. As the diagram shows, the yield depends both on the environmental inputs from the left and the purchased goods and service inputs to the process from the right. Economic success depends on the two prices diagrammed on the right, the price PE of the commodity that is sold and the price (PI) of the inputs that have to be bought. Higher use of environmental inputs relative to purchased inputs reduces costs and allows the farmer to compete with others with less environmental inputs.

The environmental systems of the earth generate renewable products that are incorporated by human activity into the economy for which money is received and paid out for other inputs from the economy necessary to the processing. In agriculture, for example, the interaction of the environmental inputs of sun, wind, water, nutrients, soil structure, etc., generate plant production which is managed by the farmers and sold for the money that is used to buy necessary inputs of seed, fertilizer, pesticide, tractors, and labor.

Relationships and Equations of the Model

The relationships for action within the model are shown in the diagram in Figure IV-9a and stated as equations. The relationships and mechanism represented by the diagram and equations may be expressed in words as follows:

Economic production is a product of the availabilities of the environmental resources (Q) and the economic assets (A) based on purchases. Products are sold as they are accumulated and the money received is temporarily stored as money on hand and then paid out for the economic inputs, for interest and repayment of loans. The simulation plots a graph of the environmental storages, economic assets, money and debt account. The money accumulated is an indicator of the economic success.

Money received for sales is dependent on the prices at the time. Prices of the commodities in the model increase with scarcity until a limit is reached. See equation for PE. The amount of purchased inputs depends on the externally determined price (PI) of these inputs. Relative increase of prices of purchased inputs reduces growth and what can be supported.

Setting JL determines the money borrowed from outside. This investment accelerates purchases and growth, but later reduces what can be supported as money has to go for interest and repayment of principal. Equations for debt account (D) keep track of loans and repayments. In this model when the enterprise earns enough money, it stops borrowing and pays back the principle and interest.

Examples of Economic Use of Environmental Resources

The model ENVUSE is appropriate for agriculture, forest yields, parks, aesthetic contributions, mining, and other economic interactions with environment. The storages affecting the response time from within the system are environmental stocks like stored soil reserves and forest woods. Examples of the storage of economic assets are buildings and tractors. Outside items that control the system and its production include price of inputs, rate of investment, and interest rate.

"What If" Experimental Problems

1. How well would the economic system survive if the environmental inputs are cut in half? Run the program. Make the change: 320 I = 0.5 and run the program.
   


2. If the price of goods and services rises each year, how does this change the economic system? Add a line: 475 PI = PI + 0.2*DT.
   


3. If the economic system is started with much more borrowed capital, what happens to economic assets and money on hand? To start with more money and more debt change: 350 M = 2 and 370 D = 2.
   


4. What happens if the interest rate (IT) on the debt is twice as high? Make IT = 0.2.

Letters and equations for the simulation program ENVUSE:

Storages (state variables):
Q = Stock of environmental produce
A = Assets of the economic user system
M = Money on hand
D = Debt account

Sources (outside influences):
I = Environmental sources, sun, rain, etc.
JL = Annual loans to the economic activity
IT = Interest rate on loans from outside
PI = Price of purchased inputs

Equations and change equations: Unused environmental inputs
R = I - K0*R
and therefore: R = I/(1 +K0)

Environmental Resource Storage
DQ = Kl*R -K2*Q -K3*A*Q

Economic production
P = K4*A*Q

Price of commodity
PE = (L1 + Q)/(L2*Q) a limit at 3

User assets
DA = K5*M/PI -K6*A -K7*A*Q

Money on hand
DM = PE* K4*A*Q +JL -K9*M -L2*M -K5*M -IT*L-Z*K9*M

Debt account
DL - JL - Z*K9*M where repayment is when Z = 1

Price of commodity
PE - L4*Q/(L6 + Q) when switch Y = 1

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
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Enrique Ortega
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007