Also because of the capital lumpiness in water supply this provides an incentive to expand the capacity in surface water storage at a single point in time rather than spread out over time, which can mean that it may be a considerable duration before demand materialises to use this capacity. Policy interventions in agriculture regarding water commonly involve changing the quantity and/or quality of access, as usually farmers have some access to water. Hence, to measure the benefit from an increment in water supply for farming in the receiving areas it is necessary to estimate the marginal value of water (marginal net profit) in the agricultural uses that would go out of production without the new increment of water.