Water availability in semiarid regions commonly exhibits patterns of extreme variability. Even in basins with large infrastructure development, some users are subject to low levels of water reliability, incurring economic losses during periods of scarcity. More flexible instruments, such as voluntary exchanges of water among users, may help users reduce their risk exposure. Recent changes in the Spanish water Law have given an initial impulse to allow for lease-out contracts of water use rights. This paper analyses, from theoretical and empirical standpoints, the effect that establishing water markets has on the economic risk caused by water availability variations. The empirical study is performed on an irrigation district of the Guadalquivir Valley (Spain) with fair levels of average water availability but a high probability of periods of extreme scarcity. A non-linear programming model is used to simulate irrigators' behaviour and derive water demand functions. Another spatial equilibrium model is used to compute market exchange and equilibrium. These programming models are combined with statistical simulation techniques. It is shown that the probability distribution of profits for a representative irrigator is modified if water exchanges are authorised, resulting in unambiguous risk reductions. Results also suggests that if the market would be extended to several irrigation districts and users, each characterised by different hydrological risk exposure, the occurrence of extremely low benefits events would become more unlikely. In sum, it is shown that exchanging water in annual spot markets allows for the reduction of farmers' economic vulnerability caused by the variability of water supply across irrigation seasons.