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.

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 indicator 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.

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.

Groundwater availability is understood as the volume of water that a body of groundwater can provide annually under natural conditions. This volume is intrinsically associated with direct recharge by precipitation. However, other sources of recharge may contribute to groundwater bodies, such as water exchange with other bodies of water and drainage processes.

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 ecological 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.


Groundwater resources are used at national level for various purposes – public, domestic, industrial, agricultural and tourism (golf courses) supply. Therefore, it is important to know and monitor the evolution of underground water resources, this monitoring being more relevant in extreme periods, where they 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 annually 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.

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 characterization (wet, average or dry);
  • To analyse the scarcity index by water body, based on the knowledge of water availabilities and needs.
Progress analysis:

For the characterization of the surface runoff, a long data series was used to calculate the mean annual runoff in a wet year (percentile 80), an average year (percentile 50) and a dry year (percentile 20).


Last update: 
Friday, 1 June, 2018