Sustainability of least cost policies for meeting Mexico City's future water demand

Meeting future water demand without degrading ecosystems is one important i ndicator of sustainable development. Using simulations, we showed that comp ared to existing policy, more sustainable water supply options are similar or cheaper in cost. We probabilistically forecasted the Mexico City metropo litan zone population for the year 2015 to be 23.5 million and total requir ed water supply to be 106 m(3) s(-1). We optimized existing and potential s upply sources from aquifers, surface water, treatment/reuse, and efficiency /demand management by cost to meet future supply needs; the applied source supply limits determined the degree of sustainability. In two scenarios to supply 106 m(3) s(-1) the business-as-usual scenario (zero sustainability) had an average relative unit cost of 1.133; while for the most sustainable scenario (it includes reducing potential supply basins' exploitation limits by 50%), the value was 1.121. One extreme scenario to supply the forecast' s 95% confidence value (124 m(3) s(-1)) showed little unit cost change (1.1 06). The simulation shows sustainable policies can be cost-effective.