Drought indices

Abstract

Why is it important to monitor droughts? Droughts are a normal part of the climate, and they can occur in any climate regime around the world, even deserts and rainforests. Droughts are one of the more costly natural hazards on a year-to-year basis; their impacts are significant and widespread, affecting many economic sectors and people at any one time. The hazard footprints of (areas affected by) droughts are typically larger than those for other hazards, which are usually constrained to floodplains, coastal regions, storm tracks or fault zones. Perhaps no other hazard lends itself quite so well to monitoring, because the slow onset of droughts allows time to observe changes in precipitation, temperature and the overall status of surface water and groundwater supplies in a region. Drought indicators or indices are often used to help track droughts, and these tools vary depending on the region and the season. Like other hazards, droughts can be characterized in terms of their severity, location, duration and timing. Droughts can arise from a range of hydrometeorological processes that supress precipitation and/or limit surface water or groundwater availability, creating conditions that are significantly drier than normal or otherwise limiting moisture availability to a potentially damaging extent. The indicators and indices discussed in this Handbook of Drought Indicators and Indices provide options for identifying the severity, location, duration onset and cessation of such conditions. It is important to note that the impacts of droughts can be as varied as the causes of droughts. Droughts can adversely affect agriculture and food security, hydropower generation and industry, human and animal health, livelihood security, personal security (for example, women walking long distances to fetch water) and access to education (for example, girls not attending school because of increased time spent on fetching water). Such impacts depend on the socioeconomic contexts in which droughts occur, in terms of who or what are exposed to the droughts and the specific vulnerabilities of the exposed entities. Therefore, the type of impacts relevant in a particular drought monitoring and early warning context is often a crucial consideration in determining the selection of drought indicators. A drought impact is an observable loss or change at a specific time because of drought. Drought risk management involves hazards, exposure, vulnerability and impact assessment, a drought early warning system (DEWS) (monitoring and forecasting, see Box below), and preparedness and mitigation (WMO, UNCCD and FAO, 2013). It is important that drought indicators or indices accurately reflect and represent the impacts being experienced during droughts. As droughts evolve, the impacts can vary by region and by season. Monitoring different aspects of the hydrologic cycle may require a variety of indicators and indices. It is desirable to align these and their depiction with the impacts of emerging conditions on the ground and management decisions being taken by different individuals, groups and organizations. Although a DEWS is ultimately concerned with impacts, drought impact assessment is a large gap in many DEWSs used around the globe at this time. Assessment of impacts is complicated, as socioeconomic factors other than the physical nature of droughts influence the levels and types of impacts related to drought exposure and vulnerability. Understanding how droughts affect people, communities, businesses or economic sectors is key to taking steps towards mitigating the impacts of future droughts.