Frequently there is a large gap between historic and future costs because of the lumpiness and longevity of surface water supply systems. As demand grows and the capacity is more fully utilised it is optimal to switch to a charging system based on long-run (i.e. replacement) marginal cost, but often public water agencies get locked ‘politically’ into only recovering historic costs. There are also equity considerations in recovering financial costs in that farmers might be expected by society to repay the benefits they receive where public investment has been involved.