The "Drought" fact sheet assesses the occurrence of possible periods of reduced water availability, considering different definitions of drought: meteorological, agricultural, agro-meteorological and hydrological.


A drought is a temporary reduction of water availability due to insufficient precipitation, making it a natural disaster with very specific properties. In general, it is understood as a transient physical condition, associated with longer or shorter periods of reduced precipitation, with negative repercussions on ecosystems and socio-economic activities.

The duration of abnormally low precipitation, as well as the amplitude of its deviations from the climatological normal, determine the intensity of the drought and the extent of its effects in terms of hydrological reserves, economic activities in general (including agriculture), the environment and ecosystems.

In general, there is a distinction between meteorological, agricultural and hydrological drought, which cannot be dissociated from the socio-economic and environmental impacts each one entails:

  • Meteorological drought - Associated with the lack of precipitation, it is defined as the degree of the deviation in precipitation from the normal value (average 1971-2000) and characterised by a lack of water induced by an imbalance between precipitation and evaporation, on which all the other elements, such as wind speed, temperature, air humidity and insolation, depend. The definition of meteorological drought should be considered as region-specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable from region to region.
  • Agricultural drought - Associated with the lack of water caused by an imbalance between the water available in the soil, the needs of the crops and plant transpiration. This type of drought is related to the characteristics of the crops, of the natural vegetation, i.e., of agricultural systems in general.
  • Agro-meteorological drought - A combination of the concepts of Meteorological Drought and Agricultural Drought, since there is a causal relationship between the two. Therefore, the lack of water induced by an imbalance between precipitation and evaporation will have direct consequences on the availability of groundwater and, subsequently, on the crops productivity.
  • Hydrological drought - Associated with the storage status of reservoirs, lagoons, aquifers and watercourses in general. Hydrological drought is, therefore, related to a decrease in the average levels of water in surface and underground reservoirs and with the depletion of groundwater. This type of drought is usually out of step with meteorological drought, as a longer period of time is necessary for the lack of precipitation to show itsimpact on the various components of the hydrological system.

It is important to make a distinction between the concepts of drought and scarcity. Water scarcity is a shortage of the available water resources considering those which would be required to meet the water use needs of a given region. Water scarcity can be the result of two mechanisms: a physical mechanism or an economic mechanism. The former is the result of the lack of enough natural water resources to meet the demand of a given region. Economic scarcity is the result of an inefficient management of the available water resources, such as the existence of high values of loss in distribution networks, either in irrigation or in the public supply for human consumption, and the case of countries or regions where there is a natural availability of the water required to meet the different needs, but where there are no means to supply it in an accessible way.

The socio-economic and environmental impacts of drought are associated with the joint effect of the natural and social impacts that result in the lack of water, due to an imbalance between supply, demand and distribution of water resources. Specifically, it is the imbalance between natural precipitation, the climate system and socio-economic activities. Socio-economic drought occurs when the decrease in water availability is so sharp that it has negative consequences for people and economic activities, i.e., for society in general.

The management of a droughts prevention, monitoring and coordination system should turn to different methodologies according to the type of drought it is facing.

The following instrumental variables should be adopted:

  • In the case of an agro-meteorological drought, precipitation, temperature, soil moisture conditions, Crop Conditions and Crop Forecasting, the Agricultural Markets Information System and the Farm Accountancy Data Network.
  • In the case of a hydrological drought, runoffs in water courses, the volumes stored in surface reserves (reservoirs with water storage capacity, hence, excluding run-of-the-river schemes, reservoirs for private use or reservoirs with very low regularisation capacities) and the piezometric levels of aquifer systems, supported by the Drought Surveillance and Warning Programme (PVAS).

In Portugal, meteorological drought monitoring is performed by the Portuguese Institute for the Sea and Atmosphere (IPMA), using the Palmer Index or PDSI (Palmer Drought Severity Index), based on the concept of water balances, taking into account data on precipitation amount, air temperature and available water capacity. The application of this indexenables the detection the occurring of periods of drought, rating them according to the level of intensity (mild, moderate, severe and extreme).

The SPI index quantifies the deficit or the excess of precipitation in different time scales, which reflect the impact of the drought on the availability of water. The smallest scales, up to 6 months, refer to the meteorological and agricultural drought (precipitation and soil moisture deficit, respectively) and between 9 and 12 months to the hydrological drought with water scarcityreflected in the runoff and the artificial reservoirs.


PDSI Drought classes SPI
4.00 or higher extremely wet 2.00 or higher
3.00 to 3.99 very wet 1.50 to 1.99
2.00 to 2.99 moderately wet 1.00 to 1.49
0.50 to 1.99 slightly wet 0.99 to 0.50
0.49 to -0.49 normal 0.49 to -0.49
-0.50 to -1.99 mild drought -0.50 to -0.99
-2.00 to -2.99 moderate drought - 1.00 to -1.49
-3.00 to -3.99 severe drought - 1.50 to -1.99
-4.00 or lower extreme drought - 2.00 or lower


Alert levels for agrometeorological drought correspond to the following descriptions of PDSI and SPI indexes:

  • Level A.1 - "Pre-Alert": PDSI in moderate drought and weak to moderate SPI;
  • Level A.2 - "Alert": PDSI in severe drought and moderate to severe SPI;
  • Level A.3 - "Emergency": PDSI in extreme drought and moderate to severe SPI.

On the other hand, hydrological drought is assessed by the Portuguese Environmental Agency (APA) through monitoring of the different water bodies. This monitoring is the first measuring element for the control of the existing water availabilities. It is used to assess the effectiveness of the planning measures and the efficiency of the management measures, being also a means of making the collected information directly available to the interested parties.

The monthly evolution in the reservoir storage levels is taken into account together with the monthly precipitation and drainage values; these parameters are also monitored in the stations managed by the National Water Resources Information System (SNIRH), enabling an assessment of the existing water availabilities. A comparison with historical storage series provides information that might contribute to determining the possible existence of a hydrological drought event. On the other hand, considering the significant hydrological differences that exist in Portugal, the fact that a hydrological drought is identified in a certain region does not mean that it affects the entire national territory, or that its severity has a similar magnitude across the entire country.

So, throughout the hydrological year (period of 12 months spanning between the beginning of two consecutive rain seasons, to allow a more significant comparison of data - in Portugal it starts in October each year and ends in September of the following year) there is an overall assessment in four different moments: end of January and March (intermediate analyses), end of May (confirmation of possible drought) and end of September (statistical analysis of the drought).

Warning levels have the following definitions:

  • Level H.1 - "Pre-Warning:" Below-normal precipitation, causing a slight deviation from the average water reserve level;
  • Level H.2 - "Warning:" Escalation of the forewarning signs of drought, affecting the normal water reserve levels;
  • Level H.3 - "Emergency:" Persistence and escalation of the drought situation.

Underground water availability plays a particularly important role in drought periods due to its capacity for inter-annual regularization, allowing aquifer systems to continue to supply the water needs of various sectors of activity against the water they have stored. However, if the low precipitation lasts longer, underground reserves will continue to decline, as there is no replenishment of groundwater through recharge.

It should also be noted that geological formations have different behaviours, both in terms of response to rainfall events (there are more permeable formations where recharge occurs faster) and in terms of water storage capacity (aquifer systems have higher capacity for water storage, while the formations of the old massif located in the interior of the country from north to south have small storage capacity). These situations become more evident during periods of drought and have to be taken into account with regard to the analysis of water availability as well as the measures to be applied.

In this context, the monthly evolution of the groundwater level during the hydrological year is analysed, with two determining stages corresponding to the months of April and September. The month of April is an indicator of the groundwater level situation since it corresponds to the end of the period where significant recharge of the groundwater bodies resulting from the pluvial events occurred can take place. This month enables gauging of the evolution of groundwater levels and works as an alert during periods of drought, namely for the need to apply measures to minimize its effects.

In the month of September, which corresponds to the end of the hydrological year, underground water reserves are expected to reach the lowest levels, since there were no significant rainfall events leading to the recharge of the aquifers. The end of the hydrological year is an aid to the situation of the groundwater levels, giving an idea of the groundwater availability at the beginning of the new hydrological year.

With regard to the analysis of the surface availability, the analysis of the data is carried out by means of the comparison with accumulated average values associated with the monitoring network. It is done automatically by APA when the monthly bulletins are elaborated.

After monitoring and analysing the data, the system has enough information to carry out an assessment that leads to the definition of alert levels, from the crossing of precipitation values with water storage values by river basin.

This fact sheet concerns mainland Portugal and will be updated annually.

Objectives and targets: 

Having a Drought Forecasting and Management System capable of timely addressing, with the cooperation of the parties involved, the potential impacts of droughts by implementing the measures assigned to each warning level.


Progress analysis:

The application of the PDSI index allows the detection of the occurrence of periods of meteorological drought, rating them in terms of intensity as per the following examples.

Last update: 
Monday, 6 June, 2016