Within FLAG ground water contributing area is assumed to consist of the set of points that are continuously and monotonically uphill of a point. This may include points outside the catchment area as long as they are connected through an uphill path. Discharge will not occur where lowness or concavity are sufficient but contributing area is insufficient. This filter is named upness . This creates a fuzzy discharge index where values nearer to unity have a greater probability of being discharge.
Fuzzy logic carries the uncertainty through the modelling process in a reliable and simple manner.
This is because it allows the modeller to treat data in a continuous manner. However, a fuzzy discharge map is difficult for end users to work with because it contains a continuous range of values. To do this the threshold value at which discharge occurs must be found. In FLAG alpha is currently determined using a ground truthed sub-set within the area being modelled to use as a training set, effectively calibrating the model. 1. Accuracy .