M. Owen writes: The area studied by the author is the location of the existing Stage 1 and proposed Stage 2 of the Thames Groundwater Scheme for augmenting river flow at times of severe drought. For an aquifer model to produce reliable forecasts of various management options for developing groundwater resources it must represent real conditions as closely as possible. This requirement is particularly difficult to achieve in an aquifer such as the Chalk which, except in conditions of restricted space and time, must be regarded as heterogeneous. The characteristics of the aquifer in the study area are well demonstrated by field evidence which should be the basis of the model. The representation of this evidence in the model is covered in the following discussion of this paper by Connorton.

In assessing the yield of a major groundwater resource development in relation to the aquifer and not the engineering works, the following criteria should be considered:

  • Resources—natural and artificial recharge and volume in storage.

  • Storage removal—regional drawdown in relation to effective saturated depth.

  • Net gain—the proportion of pumped water which is added to supply.

Connorton later explains how Morel's basic approach to modelling produces unrepresentative distributions of transmissivity and storativity in relation to the operational conditions of the scheme. The over-optimistic predictions that are likely to arise from Morel's model apply particularly to the estimates of the volume in storage and regional drawdown, and therefore lead, under criteria (i) and (ii), to misleadingly high estimates of the quantity

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