Systems Operation

The indicators previously described are mainly based on analysis of precipitation and flows. These two types of indicators

Figure 7 Selection of rain gauges for triggering emergency measures. (Example is for 1991.)

take into account only the water supply, not the demand.* For practical purposes, an indicator for a drought early warning system should indicate if the system is close to a failure, given that the main goal of the system should be to simulate the effect of mitigation measures or to put emergency plans into practice.

The use of mathematical modeling for the management of complex water resources systems (Sánchez Quispe et al., 2001) is a real improvement. These models use data such as water stock in reservoirs, groundwater levels, and streamflow. Their application for quantifying failure probabilities of the system elements allows a risk assessment for different demand scenarios. Therefore, these models offer worthy tools not only to detect drought but also to evaluate measures to reduce its effects.

In Madrid, the water supply company Canal de Isabel II has defined four phases for water stress situations (Canal de Isabel II, 2003) based on the total water stocks in the reservoirs managed by the company (see Figure 8) and the impacts of the water yield reduction on the management of the system.

Phase 0 or drought alert. Preparatory measures are triggered during this stage. Its definition ensures a minimum period of 2 months before the subsequent phase, even in the event of the lowest yields ever recorded.

Phase 1 or severe water stress. This stage defines the actual starting point for the drought. The main objective of this phase will be to reach a total reduction of 9% in consumptive uses, based on information campaigns.

Phase 2 or serious water stress. In this phase, several restrictions are imposed to reach an average reduction of up to 26% in consumptive uses (on average; values depend on the different water uses).

* However, reservoir stocks and water table indicators integrate supply and demand.

Figure 8 Water stress situation phases for the water supply company in Madrid for 2003. The figures are based on statistical analysis, so they must be updated yearly.

Phase 3 or emergency situation. This theoretical scenario would be a catastrophic social situation that would force the national government to apply civil protection measures. Water consumption should be reduced to 80 liters per capita per day.

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