A model is a simplification of a system. The term hydrologic model can be used to refer to everything from a physical model of a watershed to an integrated surface water groundwater computational model. In all cases a hydrologic model is a term used to describe a model or simplification of a complex system.
Under SGMA the term model most commonly refers to either 1) hydrogeologic conceptual models, which are described in the above section under basin setting], or 2) mathematical hydrologic models used to understand hydrologic systems. This section provides a brief overview of mathematical hydrologic models.
Mathematical hydrologic models
Hydrologic models use equations to represent surface water hydrology and the groundwater systems to which they are connected. They use model codes, which are then tailored to a specific site using a particular set of governing equations, parameters, and boundary conditions. For example, working with the USGS, Santa Clara Valley Water District used the MODFLOW-2000 model code to develop a hydrologic model for its district. This model is referred to as the Santa Clara Valley Regional Ground-Water/Surface-Water Flow Model.
While models are a simplification of reality, they can serve as powerful tools to develop a better understanding of hydrologic systems, reliable estimates of groundwater budgets, identify data gaps, forecast the outcome of future management actions on a groundwater basin, and explore alternative management strategies. Models provide the link between established management criteria and the management approaches necessary to achieve them.
That being said, developing useful, predictive models capable of simulating the potential changes resulting from changes in management actions take significant data, time, energy, technical expertise, and financial resources to develop. Agencies should work closely with model developers to understand what data are available, and to ensure that model development is consistent with the data in a basin. Finally, hydrologic models do not provide as a single “true” value, rather they provide potential outcomes. Agencies using hydrologic models for management decisions should work with the model developer to understand model uncertainty, its sources, and the potential range of possible outcomes.