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358 | IAASTD Global Report

Table A.5.1 Overview of major uncertainties in IMPACT Source: Based on MA, 2005

Model Component

Uncertainty

Model structure

•   Based on partial equilibrium theory (equilibrium between demand and supply of all commodities and production factors)
•   Underlying sources of growth in area/numbers and productivity
•   Structure of supply and demand functions and underlying elasticities, complementary and substitution of factor inputs.
•   Water simulations and connection between Water and Food modules

Parameters

Input parameters:
•   Base year, 3-year centered moving averages for area, yield, production, numbers for 32 agricultural commodities and 115 countries and regions, and 281 Food Producing Units
•   Elasticities underlying the country and regional demand and supply functions
•   Commodity prices
•   Drivers

Output parameters:
•   Annual levels of water supply and demand (withdrawals and depletion), both agricultural and nonagricultural, food supply, demand, trade, international food prices, calorie availability, and share and number of malnourished children

Driving Force

Economic and demographic drivers:
•   Income growth (GDP)
•   Population growth

Technological, management, and infrastructural drivers:
•   Productivity growth (including management research, conventional plant breeding) for rainfed and irrigated areas
•   Rainfed and irrigated area growth
•   Livestock feed ratios
•   Changes in nonagricultural water demand
•   Supply and demand elasticity systems

Policy drivers:
•   Commodity price policy as defined by taxes and subsidies on commodities, drivers affecting child malnutrition, food demand preferences, water infrastructure, etc.

Initial Condition

Baseline: 3-year average centered on 2000 of all input parameters and assumptions for driving forces

Model operation

Optimization in Water Simulation Model using GAMS

 

that are assumed to reflect the cost of producing primary biomass. The biomass model also describes the conversion of biomass (such as residues, wood crops, maize and sugar cane) to two generic secondary fuel types: biosolid fuels (BSF) and bioliquid fuels (BLF). The solid fuel is used in the industry and power sector, and the liquid fuel in other sec­tors, in particular, transport.
     The output of TIMER is affecting the biophysical sys­tem of IMAGE through land use changes (for bioenergy) and emissions (from the energy sector). Changes in food production are taken from IMPACT. The land cover model of IMAGE simulates the change in land use and land cover

 

in each region driven by demands for food, including crops, feed, and grass for animal agriculture, timber and biofuels in addition to changes in climate (Bouwman et al., 2005; Eickhout et al., 2007). The model distinguishes 14 natural and forest land cover types and six land cover types created by people. A crop module based on the FAO agroecologi-cal zones approach computes the spatially explicit yields of the different crop groups and grass and the areas used for their production, as determined by climate and soil quality (Alcamo et al., 1998). In case expansion of agricultural land is required, a rule-based "suitability map" determines which grid cells are selected. Conditions that enhance the suitabil-
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