Global Variables

ModelMuse can create Global Variables (Data|Edit Global Variables...). Global variables can be used in formulas in ModelMuse in the same manner as data sets are used. Global variables play a role in ModelMuse similar to parameters in MODFLOW; they allow numerous model inputs to be changed at a single location.

One way that global variables could be used is as parameters in parameter-estimation. The "gpt" files created by ModelMuse are text files and the section containing the global variables is clearly marked. Thus, it is possible to create a UCODE template file that the parameter-estimation program UCODE-2005 (Poeter and others, 2005) could use to vary the values of global variables. UCODE could then execute ModelMuse to create the model input files and execute the model. For modeling programs that do not have a built-in way of defining parameters, this could be useful.

There is a potential pitfall with using global variables as parameters for parameter estimation; there may be a nonlinear relationship between global variables and model inputs. For example, the user could define a global variable named "A" and then define hydraulic conductivity zones with inconsistent usages of A (in one zone the formula could be "A" and in another the formula could be "Sqr(A)"). In MODFLOW, there is always a linear relationship between parameters and model inputs. If additional nonlinearity is introduced, parameter-estimation may be difficult. The meaning of the parameters may also be problematic. In the example above, the physical meaning of A is unclear. This contrasts with MODFLOW in which, for example, HK parameters can be understood as representing hydraulic conductivity. Users would need to be careful how they use global variables if they wish to use them as parameters in parameter-estimation.