Table 3.1 and Box 3.3 present examples of existing water restriction rules, depending on the initial water allocation system already in place for "normal" circumstances. The two mechanisms can be put in place together. Typically, priority rules generally concern different water users and uses, such as agriculture, industry, and tourism, while a proportional reduction of rights are applied within the agricultural or irrigation sector. Another common mechanism to manage temporary water shortages is to forbid irrigation for a certain time period, i.e. a time quota, which can be considered an imperfect proxy for a reduction of water rights based on quantities. Such administrative pricing formulas are seldom used in practice, for different reasons: difficulty of practical implementation, unintended redistributive effects, uncertainty in the outcome in terms of reduction of water demand due to unknown price elasticity of water demand, etc. Such block-rate tariffs, when based on individual water consumptions, cannot be considered as restriction mechanisms stricto sensu, since they are not based on aggregate water supply.