Continual monitoring of selected hydroclimatic variables such as precipitation, snowpack, streamflow, reservoir levels, and evaporation rates, even during nondrought periods, provides baseline data to help detect emerging drought conditions long before impacts are felt. Using indices to determine triggers or thresholds at which actions should be taken provides guidance to decision makers during the onset of an event. These should be viewed as experience-based guidelines rather than strict rules, since droughts differ so much from one event to the next. Some are prolonged and persistent but not initially intense, while others may be short-lived but extremely severe with broad and costly impacts. An appropriate response or triggered action during one event may not be applicable during the next.
Selection of Indices
There are many drought-related indices, each targeting specific types of information. Some provide information on one discrete variable, such as reservoir storage levels or snowpack on a certain date. Other indices blend multiple variables into one composite index to give the user additional information, such as the first index used in Colorado drought response, the Palmer Drought Severity Index (PDSI), which blends information about the temperature and precipitation of a region into one index. Which indices (and how many) are best for monitoring depends entirely on the climate, user, vulnerabilities, and desired outcome (see Chapters 7 and 8).
The selection of the most appropriate indicator(s) is context-specific: it depends partly on the characteristics of a region's climate, but also on the particular societal and ecological vulnerabilities identified in the drought planning process, and the impacts that are desired to be reduced. Ideally, multiple drought indicators will be used (e.g., standardized precipitation index [SPI] and PDSI), since the unique indicators will represent different dimensions of the same drought event. An exception to this rule is observed with water managers, who will rely primarily on reservoir storage levels in key reservoirs to trigger drought response actions, as is the case with Colorado's largest municipal water provider, Denver Water (Denver Water 2016).