## Modelling for Estimation and Projection of Carbon Stocks in Land Use Systems

Models are simplified versions of a system used to estimate and project certain features or functions or outputs of a system. In order to study a system scientifically a set of assumptions about how it works is often made. These assumptions, which usually take the form of mathematical or logical relationships, constitute a model (Law and Kelton 2000). Models are used to make projections of carbon stocks in forests, plantations, grasslands and cropping systems. Models are used to make separate projections for biomass and soil carbon stocks in different pools. Further, models are also available to project above-ground and below-ground biomass separately. Models are often based on several assumptions about data and quantitative relationship between input variables and output values. Thus, model outputs are often characterized by uncertainty due to assumptions made about the relationships between variables.

Why modelling is necessary If the relationships that make up a model are simple enough, it may be possible to use mathematical methods such as algebra, calculus or probability theory to obtain exact information on questions of interest; this is called an analytical solution. However, most real-world systems are too complex to allow realistic models to be evaluated analytically, and these models are studied by the means of simulations. Models can be used for making projections of future carbon stocks in carbon mitigation, roundwood production or grassland development projects and are particularly useful in making projections of changes in carbon stocks at the project development stage; models are also useful in estimating carbon stocks based on measurements for land categories or project activities when data are limited. For example, by using only a single parameter such as diameter at breast height (DBH), which can be easily measured, it is possible to estimate the standing biomass of trees in a forest or plantation at any given point. Process-based models are also used in estimating changes or gain and loss in carbon stocks as part of a national greenhouse gas inventory and, in particular, a carbon inventory.

15.1 Types of Models and Application in Estimating and Projecting Carbon Stocks

Several types of models are used in estimating changes in carbon stocks and in growth rates. These models vary in data requirements, process adopted, outputs generated and their application. In general, all the following models can be used in determining the stocks in or growth rates of carbon pools. The models already in use for such purpose are listed below and their features and applications are presented in this chapter:

(i) Biomass equations (or regression models) for biomass and soil carbon projection

(ii) PROCOMAP for project-level carbon stock projection

(iii) CO2FIX for estimating biomass and changes in soil carbon stock

(iv) CENTURY and ROTH for dynamics of soil carbon

The features, outputs and their applications are summarized in Table 15.1. The details of data needed and steps in adopting the models are described in the following sections.