Numerical Groundwater Flow Modeling, Part 3
Model Properties and the Groundwater Flow Equation
The previous blog on numerical groundwater modeling described the process of selecting the model boundary or bounding the model domain. This week’s LWS blog describes what hydraulic properties are necessary for solving the groundwater flow equation.
The groundwater flow solution requires values for properties such as hydraulic conductivity (“K”), storage (“Ss”), length (“x”), width (“y”), height (“z”), and time (“t”). Hydraulic conductivity describes how rapidly water can move through the model; storage describes the capacity for water to build up or drain away; the length, width, and height describe the size of the model area which yield the quantity or volume of water; and time is the limiting factor (i.e., shorter time periods yield smaller changes in groundwater flow).
Table of Acronyms
Table of Acronyms
Consider the governing equation (Anderson and Woessner, 1992) that is the basis for groundwater flow in numerical models (Figure 1). It says that there can be a different K value in the x, y, and z directions and that the flows in those directions (left-hand side of the equation) are equal to the change in the storage, inflows and outflows (R) on the right-hand side of the equation.
Figure 1: Groundwater Flow Equation (Anderson and Woessner, 1992)
With some serious mathematical techniques and a lot of rearranging, this equation is solved for the “holy grail” variable h, which is pressure head or water level. Simply put, if the model has the correct K and S values, it will compute a representative water level. If the input is incorrect, the solution will be incorrect (i.e., garbage-in, garbage-out). Therefore, reliable models require supporting data that are collected from aquifer testing of properly constructed wells. To provide a representative model of a groundwater system, typically it is necessary to put a great deal of emphasis, effort, and cost into collecting and analyzing data correctly.
The next LWS blog of this series, we will discuss hydraulic properties and model grids.
If you have any groundwater modeling needs or even general water resources issues, LWS can help! Please contact us at 303-350-4090 or by email.
Maura Metheny, Ph.D., P.G.: maura@lytlewater.com
Bruce Lytle, P.E.: bruce@lytlewater.com
Anna Elgqvist, EI: anna@lytlewater.com
Marlena McConville: marlena@lytlewater.com
