Integration of socio-economic and bio-physical models to support sustainable development

The world we live in is complex and ever-changing. With a wide range of driving forces continuously acting within and upon it, causing actions and reactions, it is crucial to adopt a holistic view when trying to understand its behaviour. When modelling parts of our surroundings it is therefore not enough to focus merely on the individual processes affecting a region, but it is necessary to look at the system as a complex integral whole. The MedAction Policy Support System (PSS) aims to do just that by incorporating socio-economic as well as physical processes and paying much detail to the strong interlinkages between them. The MedAction PSS aims to support policy-makers in arid and semi-arid regions in understanding the impacts of autonomous developments within a region (e.g. farmer behaviour, interaction and allocation of different land use functions within the region) as well as the impacts of external influences on the region, such as economic and demographic growth or climate change. It enables its users to carry out what-if analyses of various policy alternatives. Impacts can be measured by means of a number of policy relevant indicators, such as profits in the agricultural sector, forested area, water use and availability, land degradation and land use changes. The system is composed of several sub-modules, integrated into a single model that simulates regional developments up to 30 years into the future. Each process is modelled at the appropriate temporal and geographical resolution, e.g. the land use module calculates on a yearly basis while the hydrology module has a variable time step in the order of minutes. The finest spatial resolution is 100 by 100 meters. Driving forces in this system are demographic and economic growth as well as climate change. The socio- economic growth is translated into changes in the demand of the different land use functions in the region. The allocation of these land use demands is based on cellular automata interactions, a transport network, zoning, and suitability maps for each land use category. For two of these land use categories, agriculture and natural vegetation, there is a dynamic suitability map, changing every year based on the physical aspects of the system (soil moisture, salinity, slope, temperature) which, in turn, reflect the current state and history of climate and land use. These changing physical aspects are calculated in the climate, hydrology and vegetation modules and are influenced by social processes such as water use and land management practices. Besides a dynamic suitability map for agriculture used in the land use module, there is also a dynamic suitability map for each crop type, which is used in the crop choice module. This module calculates what type of crop will grow on the agricultural cells, based on the dynamic physical suitability, the crop price, the water price and use, and the social aspects that influence farmers in their decision-making. In this paper we will focus on the integration of the socio-economic and bio-physical processes and discuss a number of problems we encountered. Furthermore we will discuss how the MedAction PSS can be used in sustainability impact assessment (SIA) studies as required by the Commission of the European Communities (2002) and discuss the importance of tightly coupled socio-economic and physical modelling in doing so.