Process models ask the question, “how does the study area operate?”. Your answer to this question involves analyzing the information you inventoried in the Representation models phase, using spatial analysis and modeling to unveil geographic processes and relationships.
Steinitz makes it clear that data collection is NOT the first step in a geodesign project. Before data can be collected, you must first have a good understanding of the key elements and relationships contributing to the problem, and criteria for improving the study area. If you collect data first, then your knowledge of the study area is constrained to the data available, rather than on the issues and elements related to solving the problem.
That said, once you have done your initial research, representation and process models are generally visualized using GIS. Key elements and criteria are translated into data. Data can now be effectively collected and analyzed now that you know what you are looking for.
For example, you have identified that slope and soil type are likely contributing to sediment loading in streams. You therefore collect data representing land form (a digital elevation model), and soils. This is a simple data inventory (representation models). To understand the dynamic between slope and soil type, you then perform a slope calculation on digital elevation model data, then combine that information with soils data to ascertain areas more prone to erosion. This then becomes an erosion assessment or analysis map (process models). Your analysis can also tell you how geographic processes may change over time. Thinking in systems, process models are about identifying the interconnections and behaviors in your study area system. The analysis produces information about essential processes in the study area.
Image: Data categories representing "criteria" for environmental quality. These maps of environmental values Parkland County, Alberta Canada illustrate various themed data that were combined and analyzed to understand critical environmental processes present in the county.(From top to bottom, left to right: important land forms, agricultural soils, wetlands, vulnerable groundwater areas, species and habitats of concern, surface waters, landscape ecology, and overall environmental quality).