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A Nutrient Management Plan Support System (NUMAPS) that assists growers in selection and application of best management practices (BMPs) related to water and nutrition management in citrus and sugarcane is being developed. NUMAPS combines expert systems with record keeping, database management, and computer simulation to offer growers the best advice on water and nutrient management using a Web-based software environment that can be readily integrated into their day-to-day production operation management. BMPs in the form of rules are compiled into a database and displayed in the software as appropriate for on-site decision making. Computer simulations of plant growth, soil moisture, and nutrient uptake enhance rule-based BMPs by providing more accurate assessments of water and nutrient utilization and potential losses via leaching and/or runoff. The system is designed to optimize yields in an economically effective manner while greatly reducing the environmental impacts associated with excess fertilizer and water applications. A new environment for developing computer simulations, created as part of a more comprehensive set of decision support system development (authoring) tools, is used in this project. This environment utilizes ontology-based simulation to design, develop, document, and deploy simulations. The environment consists of several on-line authoring tools that can be accessed through standard Web browsers. These on-line tools include a simulation component editor, an equation editor, and a symbol dictionary. Models are specified at a high level using diagrams, equations, and symbols. Computer code (Java) to run a simulation is generated automatically from the high level specification. MathML (eXtensible Markup Language - XML for mathematics) can also be generated from the model specification. Simulations are compiled and run on-line.