Response Strategies A Early Identification of Droughts

In Spain and other northern Mediterranean countries, drought forecasting based on climatic indicators is not operational. One of the most suitable indicators is the North Atlantic Oscillation (NAO), which shows the pressure gradient in the region. In the winter, when the NAO is low, westerlies are weaker and do not penetrate as far into Europe, so temperatures are influenced by cold high pressure located over Eurasia and precipitation is reduced (EEA, 2001

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Figure 6 Trends in water demand in Spain. (From EEA, 1999.)

Forecasting the evolution of NAO is not yet possible. Some researchers (Iglesias, 2000) believe that reliable predictions in the Mediterranean area are far from being available for operational purposes.

Experience from recent severe droughts in Spain has shown that droughts are identified too late, complicating the implementation of remediation measures. The "White Paper on Water in Spain" (Ministerio de Medio Ambiente, 2000) notes:

Taking into account the experience of recent droughts, it is advisable to implement an early warning system that can activate emergency plans. This early warning system should use indicators based on information easily available (precipitation, water stocks in reservoirs, or aquifer water table levels, for example) to alert people of the possible start date of a drought or to identify its intensity.

This approach has been adopted in the national hydrological plan of Spain. Title II (Complementary Rules for Planning) and article 27 (Drought Management) of the plan note:

The Environment Department and river basins affecting two or more Autonomous Communities, looking for the reduction of environmental, economic, and social impacts produced by drought situations, will implement a global system of hydrological indicators. These indicators should allow the River Basin Authorities to declare different warning situations.

To develop this system, river basin authorities have started to select a group of representative control points. The process has five main steps:

1. In each major river basin, elaboration of a zone classification, considering its importance in water resources generation

2. Selection of the most representative indicators to monitor water resources in each zone, and for each indicator, selection of representative control points

3. Collection of temporal data series for each control point

4. Elaboration of criteria to be used in early identification of droughts, according values to each indicator

5. Updating of the temporal series associated with each indicator (monthly data)

Preliminary classification of zones and selection of control points has been completed. The selection of control points includes:

• Rain gauge stations

• Streamflow gauging stations

• Reservoir stocks

• Water table indicators

Maps comparing the situation at the control points for the current month with historical data—for instance, by obtaining percentiles and indicating them by a color code—are the main output of the system. These maps, like the one shown in Figure 7, provide a general overview of the hydrological situation and can be used for defining drought severity or triggering emergency measures.

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