Model development follows a four-step process. First, the problem to be solved and the scope of analysis are defined. Second, a description of the system is developed. This step begins by conceptualizing the broad structure of the system, followed by decomposing that structure into a series of manageable units defined by specific system sectors (e.g., agriculture, mining). For each sector a causal loop diagram (e.g., Sterman, 2000) describing the inherent structure and feedback is developed and reviewed by the cooperative modeling team. Subject experts are identified by the modeling team who are then contacted for further clarification of the system and to gather necessary input data. In the third step, the causal loop diagrams are converted into a system dynamics context, and model sectors are populated with appropriate data and mathematical relations. Step four involves model calibration against historical data followed by review. The modeling team reviews the model as it evolves through these stages.
The basic model addresses principle water supply and water demand sectors within Southwestern New Mexico. A model schematic for a representative reach is given in Figure 2 (map above). The model is structured according to seven broad sectors, surface water, groundwater, land surface processes, institutional controls, environmental, water use, and future water utilization options. Model simulations are conducted on a daily time step over a variable planning horizon. Spatially, the model is disaggregated according to river reaches as defined by active gauging stations (Figure 1). There are a total of five reaches on the Gila River and three on the San Francisco River.