The nature of the seasonal water market is examined using a theoretical model and empirical evidence from the Victorian market. Drivers of the seasonal opportunity cost of water include the underlying nature of investment in the industry made in the context of risky entitlement yields; and the timing and nature of information regarding seasonal water availability and rainfall. Seasonal water markets facilitate the re-allocation of water availability according to this short-run opportunity cost. Evidence from the market suggests that transactions costs are low and most of the existing constraints to trade in seasonal allocations are the result of hydrological conditions. Analysis of market data suggests that the price response of the market to water availability is much more pronounced in years of low rainfall. The implications of the paper for wider policy reform are that attention should be paid to improving property rights for the management of intertemporal risk before other reforms, such as broadening of permanent water markets and institutionalising environmental flows, are implemented. This is because these other reforms will change the spatial and temporal pattern of water use and thus affect reliability, which underpins the value of water in irrigated agriculture.