Definition of drought vulnerability for forecasting

Contributed by Luis Garrote

introForecasting droughts is an important part of the HEPEX activities. Like other natural hazards, the effects of drought have both a natural and social component.  Defining vulnerability to drought is complex and involves some measure of susceptibility and coping capacity. Vulnerability to drought varies spatially and is determined by natural factors, like the intensity and magnitude of the drought hazard that lead to its susceptibility, and by social factors that lead to exposure, coping capacity and adaptive capacity. The social factors that define vulnerability to drought – for example number of people exposed, per capita water availability, water use trends, technology, policies, etc – change over time, therefore vulnerability also changes. As result, subsequent droughts in the same region will have different effects, even if they are identical in intensity, duration, and spatial coverage, because societal characteristics evolve through time.


Figure 1 Proposed definition of vulnerability levels

There is a large number of indicators which may be used to characterise drought vulnerability that characterise the natural and social aspects. Ideally, the sets of indicator values may define vulnerability thresholds. Defining critical thresholds is very complex. A threshold is the value at which action is initiated – and not necessarily that at which problems occur. In some literature this leads to two types of threshold – the one is called an action or operational threshold, the other a result threshold. Figure 1 outlines a simple proposal to define vulnerability levels based in the quantification of hazard and adaptive capacity indicators.


Figure 2 Flow chart for evaluating vulnerability

In practice, the steps that that may be considered in the analysis include:

  1. Define hazard (meteorological or hydrological),
  2. Define the impacts in agriculture, water, ecosystems, health, etc.,
  3. Define exposure (number of people affected) and
  4. Define coping capacity or adaptive capacity (Figure 2).

Definition of hazard 

For practical applications, the selection of drought indicators may include the following criteria:

  • Require only existing and readily available data;
  • Be easy and cheap to apply;
  • Be appropriate to represent the particular rainfall and streamflow conditions in the area under consideration;
  • Discriminate to a reasonable degree between different levels of intensity; and
  • Be valid, the results being reasonable predictors of the results of more detailed studies.

Based on the characteristics of the indices and their relative strengths and weaknesses, the general data availability and the characteristics of the agricultural and water supply systems, we suggest the use of three basic indices: Precipitation deciles, Standard Precipitation Index (SPI), and Surface Water Supply Index (SWSI). These three indices will then be used to characterise drought and for attribution of the drought effects to specific vulnerable systems. If possible, the indicators should be calculated over a time period to evaluate the dynamics of drought.

Definition of impacts

Statistical databases may be used to evaluate impacts. In case the information is not available, consultation with stakeholders may provide qualitative results.

Definition of exposure

The definition of the number of people affected by drought in each case is extremely valuable for estimating overall vulnerability. The sources of data vary in each case study.

Definition of Adaptive capacity

The ACI evaluates the intrinsic characteristics of a certain system that define its response to drought. The ACI includes with five major components that characterize the social capacity, economic capacity, technological eco-efficiency and natural capital. There are two key challenges in the design of a ACI that is an adequate representation of the cause-effects relationship between drought and its impacts. First, the selection of the variables to be included in the ACI and second the weighting of each variable- These two questions are ideally answered in the context of decision-oriented stakeholder and expert dialogues. Here we present a ACI that includes a very large set of important variables from the theoretical point of view. The stakeholders in the case studies should select the most adequate variables and their weighting in each case.

For more details on the methodology, please read:

Concept report describing the outline of a framework for drought warning and mitigation in Africa. [pdf]

Framework for drought warning and mitigation from national to local scale. [pdf]

For more related publications (part of the DEWFORA project), please click here.



  1. First of all, thanks for the post. I only have a comment on indices used to describe the hazard. If a developed system should be operated in the future it makes a sense to be managed by nations involved. Most probably the National meteorological and hydrological services will be involved. Therefore it would be of a great advantage to develop something compatible with WMO standards and methods. As far as I know, WMO Commission for Climatology promotes the use of Standardized Precipitatio-evapotranspiration index (SPEI). There is a new Integrated Drought management programme (IDMP) of WMO and the GFCS – all of course with a focus on Africa. I advice you to try contact the WMO community to communicate with them.

  2. Dear Jan

    Thanks you for your input. Proposing the Standardized Precipitatio-evapotranspiration index (SPEI) is a good idea. I will certainly contact WMO on this.

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