This paper evaluates the possible consequences of decreasing in the amount of water provided by the irrigation board to the farms, and increasing in the unitary water charges that the irrigation board applies to the farms. The study has been conducted in a Mediterranean region of southern Italy where farmers can use water delivered by an irrigation board and by privately owned wells. The results of the analysis, performed by means of a mathematical programming model, show that in both cases farmers substitute the water supplied by the irrigation board with that extracted from farm wells. This generates an over-extraction of resources from the aquifer leading to likely negative environmental consequences. This also endangers the economic sustainability of the irrigation boards, which are institutions whose activity can be monitored and influenced by public administration bodies.