Description of GRM

Evaluation System
The Core of GRM is the developement of an evaluation strategy using a method called "value benefit analysis". First of all the modeller sets up evaluation points or/and evaluation areas. At these points the groundwater table is "optimised" by an attempt to optimise the so called fulfillment function at this specific point. A fulfillment function is a function which sets for every groundwater table value, or more exactly every depth to groundwater value, a fulfillment value as a percentage. E.g the depth to groundwater is for 5 meters 70% optimal, for 7 meters it is 100% optimal, and so on.

These evaluation points and/or areas are then categorized into Groups according to their membership (e.g. agriculture vs. ecology). Then every Group gets a weighting. This is the most difficult task in GRM modelling. When starting GRM, the groundwater table is calculated and then at every evaluation point/area a degree of performance is calculated. That is the percentage value at this specific point. So in the case of the above example, when the depth to groundwater is  6 m, the degree of performance is 85 %.

Example of an Evaluation Tree, defining Weights and the Structure for the Optimization

Optimisation System
The optimisation algorithm PEST developed by Dogherty is included in GRM. During the optimisation process the pumping rates of the wells are getting altered so that the sum of all degrees of performance of the evaluation points/areas multiplied by their weighting factors is maximised. In this way the best possible groundwater table is obtained according to the fulfillment functions and the weightings set up.

Predicting Groundwater Model
GRM allows two different ways to predict the piezometric heads in the investigation area:

In "internal" mode it is possible with GRM to set up a "simple" model. Internal means in this case that there is no other software necessary. Simple means a two dimensional, horizontal model. Boundary conditions can be set up as constant head or no flow. The different heights of the surface, bottom and top of aquifer, as well as the conductivity, recharge and the starting heads can be set up cell by cell on a non-aquidistant finite difference grid. The software used by GRM for the modelling task is Modflow 2000 by USGS.

In "external" mode it is possible to use an existing model for the calculations, no matter what kind or how complex this model is, and use GRM as an controlling tool for the optimisation. This works in the way, that the modeler has to code an interface between these two programs. Interface means in this case nothing more, as that the modeller has to tell GRM where (in which file) to find the model results, where (file) to change the pumping rates of the wells and the command to start the model. No modelling has to be done again, like defining the boundary conditions etc. "Just" the evaluation startegy (see below) has to be thought about.