Surface and Groundwater Availability

The "Surface and Groundwater Availability" fact sheet analyses water availability in mainland Portugal in a given year, making it possible to assess whether the year was wet, average or dry.


Water resources are used at national level for various purposes – public, domestic, industrial, agricultural and tourism supply. Therefore, it is important to know and monitor the evolution of water resources, this monitoring being more relevant in extreme periods.

Knowledge of water availability allows for a more sustainable management of resources, given that the climate variability that characterizes Portugal generates frequent droughts and floods. This fact sheet reveals the water availability in a given year, comparing it with average values, allowing us to evaluate whether it was a wet, average or dry year.

Surface water reserves are determined by the comparative analysis of the available water storage in 62 reservoirs, located in the main river basins of the country. In this analysis, water-fed reservoirs, those of private use and with limited regularisation capacity, are excluded.

The assessment of surface water availability is carried out by reservoir and river basin, monthly, during the hydrological year, comparing the observed storage volumes with the monthly average, which includes historical records since 1990/1991. In this way, the strict management of water availablility for different uses is ensured.

Groundwater resources have played an important role in meeting water needs due to their interannual regularization capacity. Most of the country (about 2/3) is occupied by undifferentiated bodies of water (hydrogeological unit of the Old Massif), with low water availability due to the small water storage capacity in the rocky substratum. These are formations with great annual water variability, highly dependent on precipitation. After the first rains, they start to store water, but at the end of the hydrological year, in the dry season, groundwater levels are very low. These are heterogeneous environments, with no spatial continuity, and with only local importance.

The analysis of groundwater reserves puts special emphasis on aquifer systems, as these are homogeneous environments, with significant groundwater storage capacity and inter-annual regularization. The main groundwater reserves, of regional importance, are located in these environments. 

However, due to the use of these resources, it is important to know the evolution of water availability in all water bodies, regardless of the hydrogeological environment.

This monitoring is carried out by the APA, over each hydrological year, based on a piezometric monitoring network. To this aim, the levels recorded throughout the hydrological year are compared with the mean monthly value or with percentile 20 (low water availability indicator) of the historical series, making it possible to assess the evolution of water availability in each water body.

It should be noted that water reserves, during a hydrological year with average rainfall, increase in the wet semester (October to March) and decrease during the dry semester (April to September). It should be added that the interannual regularisation capacity allows for minimising of the impact of meteorological droughts, however, the persistence of low precipitation in consecutive hydrological years can lead to water shortages.

The scarcity index makes it possible to relate the availabilities with the needs and thus measure demand in relation to supply in order to consider if there is scarcity in each river basin district.

The WEI+ scarcity index follows the WEI (Water Exploitation Index), which corresponds to the ratio between the average annual water demand and the available long-term average resources, thus allowing us to assess the water stress to which a given territory is subjected. The WEI+ aims to complement the WEI, incorporating into the calculation of the vulnerability to scarcity situations, the returns of water to the environment, as well as the environmental flows. The WEI+ is therefore defined as the ratio between the total volume of water abstracted and renewable water availability.

Water needs include not only environmental flows, but also the volumes that must be available in order to meet other requirements such as shipping or international treaties in cross-border Rivers. These volumes, calculated under the WEI+, correspond to 10% of the flow of each river basin district. “Return” means the volume of water that is returned to the environment after use by the sectors and that is available for reuse.

The scarcity assessment through the WEI calculation is based on the share of resources consumed and is divided into four categories:

  • No scarcity - countries that consume less than 10% of their renewable resources;
  • Reduced scarcity - countries that consume between 10% and 20% of their renewable resources;
  • Moderate scarcity - countries that consume between 20% and 40% of their renewable resources;
  • Severe scarcity - countries that consume more than 40% of their renewable resources.

This fact sheet concerns mainland Portugal and shall be updated on an annual basis.

Objectives and targets: 
  • To be aware of water availability by water body;
  • To compare annual water availability with average values for year characterisation (wet, average or dry);
  • To analyse the scarcity index by water body, based on the knowledge of water availabilities and needs.
Progress analysis:
Last update: 
Tuesday, 7 December, 2021