1. Suitability for drought types of concern. An indicator needs to reflect the type of drought of concern, including aspects of water demands, water supplies, drought vulnerabilities, and potential impacts. Because drought depends on numerous factors, no single indicator is likely to cover all types of drought. In choosing indicators, a first consideration is that they should make sense for the context. For instance, the Palmer indices may not be appropriate as sole indicators for managed water systems because they do not incorporate reservoir storage. Reservoir storage, on the other hand, may not be appropriate as a sole indicator for agricultural areas that use only groundwater for irrigation.
2. Data availability and consistency. The performance of an indicator depends on the availability and quality of the data. Many indicators may be conceptually attractive, but are difficult, costly, unreliable, or impractical to generate, so they may not be appropriate for use. When choosing an indicator, consider the following questions: Are the data readily available? Is the indicator straightforward to calculate? Are the data trustworthy? Will the analytic expense justify the decision-making value? Does the value of the indicator vary, depending on the source of data or method of calculation? Is there a consistent long-term record, and will the data be consistently generated in the future? Many drought plans use indicators based on data that are already collected, subjected to quality control, and consistently reported, such as by a government agency.
3. Clarity and validity. Indicators and triggers need to be readily understood and scientifically sound, so that drought decisions can be made and defended on the basis of them. In addition, they should be tested before a drought and evaluated after a drought to see how well they performed. A pre-drought assessment could involve generating historic sequences of triggers and comparing them to human assessments of the drought triggers that should have been invoked during that time. Another approach is to conduct virtual drought exercises with stakeholders and decision makers, using different sets of triggers and comparing management responses. A post-drought evaluation could involve a similar process of comparing actual triggers invoked to triggers that would have provided the greatest decision-making value.
4. Temporal and spatial sensitivity. Indicators and triggers need to consider both temporal and spatial variability, because indicator levels that imply drought conditions for one time period or one region could imply wet or normal conditions for another time period or region. For instance, "monthly total precipitation of 3 inches" could imply dry conditions in early spring but wet conditions in late summer, or imply dry conditions for a mountainous area of a state but wet conditions for a desert area of the same state.
5. Temporal and spatial specificity. Indicators and triggers should specify the temporal and spatial scale of analysis. Indicators need to be associated with a specific period of calculation. For instance, the SPI-6 calculates the precipitation anomaly for a prior 6-month period relative to that same 6-month period historically. Triggers also need to be associated with a time period for determining drought levels and responses. For example, "SPI-6 below -1.5 for two consecutive months would invoke Level 2 drought." In this case, the indicator's time period would be the six prior months and the trigger's time period would be two consecutive months. Indicators also should define the scale of analysis, such as a climate division or hydrological basin for precipitation, soil moisture, and snowpack. For other indicators, such as ground-water, reservoir levels and storage, and streamflows, the spatial scale may be implicitly defined by the selection of a specific well, reservoir, or gauging station. Indicators should, nevertheless, specify the spatial scale of drought that they seek to represent, such as a set of streamflows to represent drought within a certain river basin. Triggers need to define the spatial scale of implementation of drought responses, such as the use of three groundwater wells to trigger drought responses within an entire climate division or county. Even a trigger such as a reservoir level does not necessarily imply that the spatial scale of response is that reservoir, but instead could trigger responses, such as water use restrictions, for an entire state.
6. Drought progression and recession. At each level of a drought plan, indicators and triggers should be defined for getting into and out of a drought,. Even though many drought plans assume that the progressing triggers can be reversed to function as the receding triggers, this approach may not be desirable from a drought management perspective. Different management goals may exist for going into a drought versus coming out of a drought. For instance, it may be important to implement water use restrictions as soon as drought conditions start developing, but to be more conservative and wait to lift restrictions when drought conditions appear to be recovering. Trigger examples would be to invoke a drought level after two consecutive months in a certain or more severe level, but to wait to revoke drought restrictions until after four consecutive months in a certain or less severe level.
7. Statistical consistency. Triggers need to be statistically consistent with drought levels and other triggers in a drought plan. As we saw earlier, the probabilities of occurrence of the Palmer index were not consistent among drought levels and varied according to time and location. Moreover, the index scales of the PDSI/PHDI, SPI, and SWSI were not consistent with each other. For instance, the value of -1.5 had different cumulative frequencies for each index. From the perspective of a decision maker, choosing drought indicators may be difficult, but that difficulty will be compounded if indicator scales and trigger values cannot be validly compared and combined in drought decision making.
8. Linked with drought management goals. Indicators need to be linked with drought management and impact reduction goals, and trigger levels should be set to invoke responses at times and stages that are consistent with these goals. For instance, triggers can be set so that a certain percentile will invoke responses that will produce a desired percentage reduction in water use. One should also consider drought indicator performance; for instance, the degree of responsiveness or persistence desired in an indicator. Some water managers may prefer an indicator that responds quickly to short-term anomalies, such as the SPI-3, so they can take early action to reduce drought impacts, whereas other water managers may prefer an indicator with greater stability and persistence, such as the SPI-12, to avoid frequent invoking and revoking of drought responses. Intermediate indicators, such as the SPI-6, can provide elements of both.
9. Explicit combination methods. Drought plans often rely on multiple indicators. But a question arises: How are multiple indicators considered or combined to determine a final drought level? Multiple triggers may suggest different drought levels, so methods need to be specified for combining triggers and deter mining the final level. These can include quantitative methods and criteria such as: the most severe of the indicators, at least one of the indicators, or a majority of indicators. These can also include qualitative methods, such as convening a drought committee to determine when to implement responses. Whether quantitative or qualitative, the methods for calculating indicators and the process for combining opinions or weighting individual data for an overall indicator should be specified. 10. Quantitative and qualitative indicators. Indicators can be based on quantitative data and qualitative assessments, or both. Although many drought plans center on quantitative indicators, the importance of qualitative expertise should not be overlooked. A human expert is able to consider and synthesize numerous indicators, applying years of experience and expertise to assess drought conditions. Perhaps even more important is the recognition that indicators and triggers are meant to help decision makers, not replace them. A drought plan is only one source of information, and other considerations will likely be important for decision making.
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