Typical Problems with Indicators and Triggers

Because drought can be characterized in many different ways, and because single indicators often prove inadequate for decision makers, multiple indicators and triggers can be useful. Challenges arise in trying to combine multiple variables and values in a drought management plan. Indicator scales may be incomparable, and trigger values may be statistically inconsistent.

Comparison of the three index scales above illustrates common problems with indicators and triggers in drought plans. These problems exist not only for values of indices (e.g., SPI, PDSI/PHDI, SWSI), but also for values of indicators (e.g., total monthly precipitation, average monthly streamflow, average monthly reservoir levels), for several reasons:

First, drought categories (levels) are inconsistent in terms of cumulative frequency. For instance, "severe drought" occurs 4.4% of the time for the SPI, 5% for the PDSI/PHDI, and 12% for the SWSI. Second, index values are difficult to interpret directly (What does a -1.5 index value mean?) and imply different probabilities of occurrence for different indicators. A value of -1.5 represents a cumulative probability of 6.7% for the SPI, approximately 27% for the PDSI/PHDI, and 32% for the SWSI. Third, as we saw earlier, the values of the indicator can vary, in terms of frequencies, depending on time and location (with the exception of the SPI). Finally, because of these inconsistencies, trying to use more than one indicator in operational drought management can cause confusion and impede effective and timely drought response.

An evaluation of state drought plans in the United States reveals wide variation in quality concerning indicators and triggers. The four plan types characterized represent incremental degrees of detail:

1. The plan mentions indicators, but without details on how these indicators are measured or used. For instance, an indicator of "precipitation" is mentioned, but not whether precipitation is measured by the SPI, deciles, or another approach. Also lacking are triggers and drought plan levels.

2. The plan provides some guidance on indicators, but without information on trigger values and corresponding drought levels. For instance, a plan may say that "streamflows" are monitored by the "monthly mean values," but the values associated with drought levels and responses are not specified.

3. The plan provides indicators and triggers, typically raw values of the indicators, which often lack statistical consistency. For instance, the plan may use the SPI-6, PHDI, and streamflows, but these indicator values have different probabilities of triggering each drought plan level. Thus, some indicators influence triggering more than others. Even if the plan specifies how the triggers may be combined, that combination method may also be statistically inconsistent.

4. The plan contains details on indicators and trigger values, plus triggers and associated drought levels are statistically comparable. One way to accomplish this is through a percentile-based approach, which is described in the next section.

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