Approaches for Monitoring Drought and Guiding Early Warning and Assessment
There are three main methods for monitoring drought and guiding early warning and assessment:
- 1. Using a single indicator or index
- 2. Using multiple indicators or indices
- 3. Using composite or hybrid indicators
In the past, decision makers and scientists employed one indicator or index because that was the only measurement available to them, or they had only limited time in which to acquire data and compute derivative indices or other deliverables. Over the past 20 years or so, there has been strong global interest and growth in the development of new indices based on various indicators that are suitable for different applications and scales, both spatial and temporal. These new tools have given decision makers and policymakers more choices, but, until recently, they have still lacked a clear-cut method to synthesize results into a simple message that can be relayed to the public. The advent of geographic information systems and increasing computing and display capabilities has increased the capacity to overlay, map, and compare various indicators or indices. For a more detailed discussion on mapping drought indices and indicators, see the Standardized Precipitation Index User Guide (WMO 2012).
Confusion can arise when trying to determine which indicators or indices to use, especially if they are linked to a comprehensive drought plan and used as a trigger for drought management actions. It takes time and a system of trial and error to determine the best fit for any given location, area, basin, or region. In the past decade or so, a new type of composite (sometimes referred to as hybrid) indicator has emerged as a means to merge different indicators and indices, either weighted or not, or in a modeled fashion. The idea is to use the strengths of a variety of inputs, yet maintain a single, simple source of information for decision makers, policymakers, or the public. Given that drought severity is best evaluated on the basis of multiple indicators associated with water availability for a given area or region, the composite or hybrid approach allows an increased number of elements to be incorporated into the assessment process.
While this handbook does not aim to state exactly which indicators or indices to integrate or apply in terms of drought management guidance, it is important to note the role of indices and indicators in a DEWS within an overall drought risk management strategy. They provide useful triggers to help direct decision makers and policymakers toward proactive risk management.
Triggers are specific values of an indicator or index that initiate and/or terminate each level of a drought plan and associated mitigation and emergency management responses. In other words, they trigger action and allow for accountability as to who is doing what and when they need to do it. This should ultimately tie in with a comprehensive drought management plan or policy (WMO and GWP 2014). It is essential to have a complete list of triggers for indicators or indices, which should also be aligned with an action plan to guide a coordinated set of actions by individual agencies or ministries. Without this alignment, there is likely to be considerable delay in action at the onset of drought in an area or region.  
- • Are the indicators/indices sensitive to climate, space, and time in order to determine drought onset and termination?
- • Are the indicators/indices and various severity levels responsive and reflective of the impacts occurring on the ground for a given location or region?
- • Are the chosen indicators, indices and triggers the same, or different, for going into and coming out of drought? It is critical to account for both situations.
- • Are composite (hybrid) indicators being used in order to take many factors and inputs into account?
- • Are the data and resultant indices/indicators available and stable?
In other words, is there a long period of record for the data source that can give planners and decision makers a strong historical and statistical marker?
• Are the indicators/indices easy to implement? Do the users have the resources (time and human) to dedicate to efforts and will they be maintained diligently when not in a drought situation? This can be better justified if such a system is set up for monitoring all aspects of the hydrologic or climatic cycles, not just droughts.
The simplest indicator/index to use is typically one that is already being produced operationally and freely available, but this does not necessarily mean that it is the best or most applicable.
Ultimately, the choice has to be determined by users at the regional, national, or local levels. The preferred and recommended approach is for users to take a multiple or composite/hybrid indicator/index approach as part of a DEWS within the context of a comprehensive drought mitigation plan. Ideally, this requires thorough analyses and a research approach to determine which indicators work best in particular climate regimes, regions, basins, and locations. Research is also required to determine which seasons the indicators are most relevant to, representing impacts occurring on the ground. Once identified, the indicators/indices can be recommended or implemented in a DEWS as potential triggers tied to emergency response or mitigation actions within a drought plan.
-  8.4 Selecting Indicators and Indices Just as there is no "one-size-fits-all" definition of drought, there is no singleindex or indicator that can account for and be applied to all types of droughts,climate regimes, and sectors affected by droughts. This handbook is notintended to be prescriptive by telling readers which indices and indicatorsare best to use and when; in fact, many factors feed in to determining whichindicator, index or trigger (or combination thereof) is the best to use for a particular need or application. The following questions may help users to decidewhich indicators and indices are most appropriate for their current situation:
-  Do the indicators/indices allow for timely detection of drought inorder to trigger appropriate communication and coordination ofdrought response or mitigation actions?