When an irrigation canal is put on restriction, what is fair depends on the water each user receives, not on what was allocated: transport delay and the stabilising low-pass filter re-shape every order on the way to the offtake. We pose the problem as lexicographic max-min optimisation of the fraction of each user’s order delivered inside an agreed window, after the filter. The orders are the decision variables, re-shaped before the filter, leaving the controller and filter untouched. The delivered fractions are affine in the orders, so this is a finite sequence of linear programmes. Relaxing the control law alone yields a certified upper bound; an infeasibility certificate reports whose order cannot be met and what must give. On a published eight-pool test canal the criterion raises the worst-off delivered fraction by 0.074 to 0.180 over leaving orders unchanged; the bound is attained at nine of eleven operable supply levels, where the control layer loses nothing. A 50% larger volume budget leaves the lexicographic answer unchanged. The filter’s order decides whether a feasible schedule exists, its cut-off only what is delivered; neither moves the comparison between criteria much. On this canal filter design precedes the choice of fairness criterion.