Z413 Clearing Forest

Original author(s)

Harmut Bossel

References and/or links for original model

Format of original model

Vensim

Converted by

Faye Warrender

Date submitted

20/04/12

Purpose

Description

The pressure to clear forests arise from human demand, and it is therefore processes are also connected to clearing and agriculture: Agriculture takes up nutrients from the soil and opens the land to erosion by wind and water. If the soil is not permanently fertilized and carefully protected from erosion., it loses its fertility, farm slow process of ecological succession. Viewed over a span of centuries, a dynamic process has occurred that can be described by a proper simulation model: As a growing population must be fed, more and more forests are cleared. For a while the fertile soil supplies the necessary food, but it gradually looses its fertility. Finally almost all forest has been cleared, but the infertile soil can only support few people any more. People migrate elsewhere, forests gradually develop again on the abandoned land, and fertility returns to the soil as forest litter is decomposed and nutrients recycle. Then the fertile forest soil attracts settlers again, the forest is cleared again for agriculture, and the cycle repeats.

Model size indicators

Stocks and flows

Simile diagram

Simile equations

net growth= NORMAL NET GROWTH RATE * population*food supply

immigration=NORMAL IMMIGRATION RATE*(food supply-1)*population+IMMIGRANTS

population= 1000

food demand=PER CAP FOOD DEMAND*population

food supply=if food_demand>0.01 then food_production/food_demand else 0

farm land demand= if population<1000 then 0 else 1-food_supply

clearing=if farm_land_demand>0 then farm_land _demand*natural_forest_area*CLEARING_RATE else 0

succession= if farm_land_demand>0 then 0 else SUCCESSION_RATE*agriculture_area*(ORIGINAL_FOREST_AREA-natural_forest_area)

/ORIGINAL_FOREST_ARE

natural forest area= 9000

cleared fraction= (1-(natural_forest_area/ORIGINAL_FOREST_AREA))*100

agricultural area= 1000

regeneration = NORMAL_REGENERATION_RATE*soil fertility*(1-soil_fertility/NORMAL PRODUCTIVITY)

degradation factor= interp (cleared_fraction,[-1,100,-1,0,10,20,30,40,50,60,70,80,90,100], [0.5,1,1,1,1.03,1.2,1.4,1.65,1.95,2.18,2.48,2.7,3])

degradation =soil_fertility* degradation_factor*NORMAL_DEGRADATION_RATE

soil fertility=NORMAL PRODUCTIVITY

food production= soil_fertility*agricultural_area

Notes on the conversion process

'Agricultural area' is the same as 'farm area'

Extra information on original model