Drought Indicators and Indices
One aspect of the substantial progress in drought monitoring in recent decades has been the ability to measure drought severity through a variety of drought indicators and indices. As highlighted in Chapter 8 and the World Meteorological Organization/Global Water Partnership (2016) Indicator Handbook, approaches to drought monitoring can consist of (1) a single indicator or index, (2) an approach incorporating multiple indicators or indices, or (3) an approach that uses a composite of indicators or indices. A drought indicator is a variable or parameter used to measure and track changes in various components of the hydrological cycle (e.g., precipitation, temperature, streamflow, and soil moisture), derived primarily from point- based, in situ observations. A drought index, however, is a calculated representation of a condition, with the Palmer Drought Severity Index and the Standardized Precipitation Index being two more commonly used drought indices. A composite indicator combines multiple indicators and indices, and the USDM is a great example of a composite indicator.
Indicators and indices are often reflective of a particular disciplinary perspective such as agricultural, hydrological, or ecological conditions. Composite indicators, however, often cover multiple disciplinary perspectives. As described in Chapter 8, no one indicator or index is going to describe everything related to drought. Therefore, decision makers might have to look for the appropriate option or options available to provide the most relevant information. Thus, the WMO and GWP (2016) guide is a great starting point for decision makers establishing and maintaining early warning systems. One of the advantages of a composite drought indicator is that it potentially simplifies the options for decision makers, who can often be confused by the variety of indicators and indices—and their corresponding characteristics—that are available (Hayes et al. 2012; Mizzell 2008).