Drought

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.
Description: 

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 crop productivity.
  • Hydrological drought - Associated with the storage status of reservoirs, lagoons, aquifers and watercourses in general. Hydrological drought is, therefore, related to a reduction in the average levels of surface and underground water 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 its impact 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 economical 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 and demand of water resources. Specifically, it is the imbalance between natural precipitation, the climate system and socio-economic activities. Socio-economic drought occurs when the reduction 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 should be adopted, as instrumental variables:

  • In the case of an agro-meteorological drought, precipitation, temperature, soil moisture conditions, Crop Conditions and Crop Forecasting, the Agricultural Markets Information System (SIMA) and the Farm Accountancy Information Network (RICA).
  • 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) and the SPI (Standardised Precipitation Index).

The PDSI is based on the concept of water balance, taking into account data on precipitation amount, air temperature and available water capacity. The application of this index allows enables the detection of occurring 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 scarcity reflected in the runoff and in 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 for two consecutive months and weak to moderate SPI;
  • Level A.2 - "Alert": PDSI in a severe drought for two consecutive months and moderate to severe SPI;
  • Level A.3 - "Emergency": PDSI in extreme drought and moderate to severe SPI.

The hydrological drought is evaluated by the Portuguese Environment Agency (APA) through the monitoring of the water bodies. The monthly evolution of storage levels of reservoirs, precipitation and outflow and their comparison with the historical series allows verifying whether there is a hydrological drought. On the other hand, due to significant hydrological differences in Portugal, the identification of hydrological drought in a given region does not mean that this drought exists in the entire national territory, or that its severity is of equal magnitude.

This analysis of the data is done automatically by APA, based on data from the hydrometeorological networks of the National Water Resources Information System (SNIRH) when preparing the monthly bulletins. This monitoring is the first measure of the control of existing water resources and serves to assess the effectiveness of planning measures and the efficiency of management measures and is also a means of making information directly available to interested parties.

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 analysis), end of May (confirmation of possible drought) and end of September (statistical analysis of the drought).

River basin

Storage (%) – Level H.2

31/jan

31/mar

31/may

30/sep

LIMA

50 to 55

55 to 60

50 to 60

45 to 50

CÁVADO

60 to 65

65 to 70

50 to 55

45 to 50

AVE

55 to 60

60 to 65

55 to 60

40 to 45

DOURO

60 to 65

65 to 70

55 to 60

45 to 50

MONDEGO

55 to 60

60 to 65

60 to 65

45 to 50

RIBEIRAS OESTE

60 to 65

65 to 70

50 to 55

35 to 40

TEJO

55 to 60

55 to 60

60 to 65

50 to 55

SADO

40 to 50

40 to 50

50 to 55

30 to 35

MIRA

50 to 55

55 to 60

60 to 65

50 to 55

GUADIANA

60 to 65

65 to 70

55 to 60

55 to 60

ARADE

25 to 30

20 to 25

15 to 20

10 to 15

BARLAVENTO

50 to 55

50 to 55

55 to 60

40 to 45

 

 

Warning levels have the following descriptions:

  • 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.

Groundwater 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.

Based on groundwater resources, monitored at four moments of each hydrological year - October, January, April and July -, the pertinence of adopting measures in terms of groundwater management is verified. The bodies of water in a critical situation and the bodies of water in surveillance situation are therefore identified, taking into account the water availability in the four mentioned moments. The critical situations relate to water bodies where, during the current hydrological year, levels below the 20th percentile persist, calling for the urgent application of the measures recommended within the scope of the drought. The situations under surveillance refer to water bodies where there are significant declines in the groundwater level and, therefore, deserve special attention.

Further information on surface and groundwater availability can be found in the fact sheet Surface and groundwater availability.

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

Objectives and targets: 

To have 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, 1 July, 2019