Hydrologic Modeling
What is a Groundwater Model?
A Groundwater Model is the tool used to represent or approximate the natural groundwater system. A mathematical or numerical model simulates groundwater flow of the natural system by using mathematical equations and procedures that describe the heads or flows along the boundaries of the model. Using the basic laws of physics that govern groundwater flow, we instruct the computer to consider the physical boundaries of the aquifer, recharge, pumping, interaction with rivers, or other phenomenon to model the behavior of the aquifer over time.
A boundary can be defined as the conditions which establish the hydraulic characteristics of the system at its boundaries. Correctly identifying the boundary conditions in a model means that the boundary conditions in the model correspond adequately to those in the natural system to ensure that the response of model and natural system will match reasonably well. No model is perfect. However, with sufficient information, the assumptions made in the model can be fairly accurate. A model is primarily about quantifying the external influences that affect the flow of water in the aquifer and surface drainage system.
Representation of a model in a ground-water system -- Illustration from The System Concept: Ground-Water Systems and Models, Buxton and others
A groundwater model can help us better understand how an aquifer responds to various hydrologic conditions. In addition, we can gain insight into the various factors that affect site-specific areas in the model as a framework for gathering and organizing data which will help formulate ideas about the natural system. The results of the model assist in making reliable decisions to manage the water resources better.
An accurate groundwater model requires a tremendous amount of information about the aquifer. The general steps in developing a groundwater model include: 1. developing the conceptual model, 2. defining the model architecture, 3. calibrating and verifying the model, and 4. making predictions. The conceptual model represents our best idea of how the aquifer works. Developing a good conceptual model requires compiling detailed information on the geology, water quality, recharge, rivers, water levels, hydraulic parameters and pumping. The model architecture refers to which computer program we use, and how many and the dimensions of the layers and cells that make up a model. Calibrating and verifying involve showing that the model can reproduce water levels measured in the past. A good calibration and verification gives us confidence that the model produces reasonable predictions of water levels in the future. Following calibrations the model is ready to run future scenarios. Selecting a representative period for recharge and water use is required.