Numerical modelling is a technique used by water resources practitioners that incorporates the most up-to-date theories that attempt to formulate hydrological processes. Building a numerical model of hydrological processes is accomplished in four steps:
i. first is the conceptualisation of the dominant processes that are observed in nature including physical processes that dictate groundwater movement and the interrelationship between the soil, vegetation and atmosphere; chemical processes that are dependent on the physical processes (e.g. solute transport); and biological processes that are dependent on both the physical and chemical processes for survival. This knowledge base makes up the “perceptual model” of the hydrological system;
ii. secondly, an attempt is made to formulate these perceptual processes as mathematical expressions or “formal models”;
iii. third is the combination of this set of formal models into an algorithm or “procedural model” that distributes the processes and their interactions accordingly in space and time;
iv. lastly, the procedural model is built into computer code and thus creates a “numerical model” to be run on high-performance machines that allows the practitioner to make projections as to how the hydrological system is operating, and how the system can potentially be affected when stressed by changing conditions. Such conditions may include the effects of drought on water supply and ecological resilience, the extent of impact of a toxic spill, or the change that can be anticipated when land is developed or mined for resource extraction, etc.