INVERSE PROBLEM IN HYDROGEOLOGY

The state of the groundwater inverse problem is synthesized. Emphasis is placed on aquifer characterization, where modellers have to deal with conceptual model uncertainty (notably spatial and temporal variability), scale dependence, many types of unknown parameters (transmissivity, recharge, boundary conditions, etc), and often low sensitivity of state variables (typically heads and concentrations) to aquifer properties. Because of these difficulties calibration cannot be separated from the modelling process, as it is done in other fields. Instead, it should be viewed as one step in the process of understanding aquifer behaviour. In fact, actual parameter estimation methods do not differ essentially from each other. It is argued that there is ample room for improvement in groundwater inversion: development of user-friendly codes, accommodation of variability through geostatistics, incorporation of geological information and different types of data (temperature, isotopes, age, etc.), proper accounting of uncertainty, etc. Despite of this, even with existing codes, automatic calibration facilitates enormously the task of modeling. Therefore, it is contended that its use should become standard practice.