Definitions: Indicators versus Indices
It is important to define what is meant by drought indicators and indices.
Indicators are variables or parameters used to describe drought conditions. Examples include precipitation, temperature, streamflow, groundwater and reservoir levels, soil moisture, and snowpack.
Indices are typically computed numerical representations of drought severity, assessed using climatic or hydrometeorological inputs including the indicators listed above. They aim to measure the qualitative state of droughts on the landscape for a given time period. Indices are technically indicators as well. Monitoring the climate at various timescales allows identification of short-term wet periods within long-term droughts or short-term dry spells within long-term wet periods. Indices can simplify complex relationships and provide useful communication tools for diverse audiences and users, including the public. Indices are used to provide quantitative assessment of the severity, location, timing, and duration of drought events. Severity refers to the departure from normal of an index. A threshold for severity may be set to determine when a drought has begun, when it ends, and the geographic area affected. Location refers to the geographic area experiencing drought conditions. The timing and duration are determined by the approximate dates of onset and cessation. The interaction of the hazard event and the exposed elements (people, agricultural areas, reservoirs, and water supplies), and the vulnerabilities of these elements to droughts, determines the impacts. Vulnerabilities may have been exacerbated by previous droughts, which, for example, might have triggered the sale of productive assets to meet immediate needs. The timing of droughts may be as significant as their severity in determining impacts and outcomes. A short, relatively low severity, intraseason drought, if it occurs during the moisture sensitive period of a stable crop, can have a more devastating impact on crop yield than a longer, more severe drought occurring at a less critical time during the agricultural cycle. Thus, drought indices—in combination with additional information on exposed assets and their vulnerability characteristics—are essential for tracking and anticipating drought-related impacts and outcomes. Indices may also play another critical role, depending on the index, in that they can provide a historical reference for planners or decision makers. This provides users with a probability of occurrence, or recurrence, of droughts of varying severities. Importantly, however, climate change will begin to alter historical patterns.
Information derived from indicators and indices is useful in planning and designing applications (such as risk assessment, DEWSs and decision support tools for managing risks in drought- affected sectors), provided that the climate regime and drought climatology is known for the location. In addition, various indictors and indices can be used to validate modeled, assimilated, or remotely sensed indicators of drought.