With rapid intensification of agricultural catchments in Northern Thailand a suite of environmental issues has surfaced. The Integrated Water Resources Assessment and Management (IWRAM) project was instigated in response to these issues. The project developed a Decision Support System (DSS) for the exploration of biophysical and socio-economic impacts of water resource use options. The IWRAM-DSS is comprised of a 'Biophysical Toolbox' that can be implemented alone and an 'Integrated Toolbox' that links socio-economic models with the Biophysical Toolbox to explore economic trade-offs and impacts of various scenarios. The Biophysical Toolbox is comprised of three modules - the CATCHCROP crop model, a hydrologic module based upon the IHACRES rainfall-runoff model, and a Universal Soil Loss Equation (USLE) approach modified to suit conditions in Northern Thailand. This paper describes the Biophysical Toolbox and runs forest conversion, land management, and climate scenarios to demonstrate the potential of this tool in exploring the environmental effects of land and water management options.