USING SENSITIVITY ANALYSIS TO ASSIST PARAMETER ZONATION IN-GROUND WATER-FLOW MODEL

For numerical modeling of ground water movement in a real aquifer syst em, the aquifer is usually divided into hydrogeologically defined zone s, each with its own parameter values. The responses of the system, su ch as head or drawdown, are often available only in some of the zones. The estimated parameters of all the zones are based on the measured r esponse in these limited zones. However, the estimates for some of the zones may be very uncertain, and these zones are therefore not justif ied by the data. In this paper, an approach is presented to understand which zone may produce uncertain parameter values and should be lumpe d with its neighbor. This approach is demonstrated using a regional nu merical model for pumping test analysis in the Nottinghamshire aquifer , UK. A step-by-step process is used in identifying the aquifer zones and estimating their parameters based on the principle of using the sm allest possible numbers of zones and parameters for adequate represent ation of the drawdown response. After the parameters of each zone are estimated, the sensitivity features of these parameters are examined. The results show that the parameters in one zone can be estimated prop erly by the drawdown in another zone only when there is significant se nsitivity. For transmissivity, sensitivity between zones occurs when t here is significant flow between them. For storativity, sufficient sen sitivity can occur without large flows between the zones, provided tha t one zone causes significant drawdown in the other. This idea can be extended to the flow model for a large aquifer system. If the aquifer is divided in such a way that aquifer responses are not sensitive to t he parameters in some of the zones, the parameters in those zones cann ot be estimated properly and should be lumped into their neighboring z ones. In this way, a simple but more reasonable model can be built.