This paper examines present and future demand for water in the Zayandeh Rud basin, and determines the extent to which water will be available for agricultural use by the year 2020. Current demand and supply were more or less in balance in 2000 but by 2010 there will be an increase in supply of some 28% due to the completion of a third transbasin diversion and development of other local water sources. If growth in all sectors is assumed to be 2% p.a. then demand will exceed supply by 2010 and the basin will go into severe deficit by 2020. The same is true if demand only grows at 1% a year. As it is unlikely that urban, industrial and environmental demands will decrease, the only way to keep supply and demand in balance is to reduce allocations to agriculture. By 2020 agriculture will only have 5% more water than at present despite the transbasin diversions, and will shrink from 2025 onwards. However, in years when water supply is only 90% of normal then agriculture will have to shrink in order to keep other sectors supplied.