Chance constrained mathematical programming was used to compare the effects of two
different crop yield relationships on the economic cost to irrigation farmers while maintaining
an instream flow requirement. Results showed that the Stewart-Hagan programming model
underestimated the economic cost and overestimated potential return flows. Water policies
based on this model will lead to the implementation of socially unacceptable water allocation
policies that may even apply more pressure on stream flows. Meaningful water policies could
only be advanced by a thorough understanding of the economic and hydrological
consequences of alternative water allocation policies through the integration of economic and
hydrological models at catchment level.
