Selecting the proper indices is only part of the process. Establishing appropriate and justifiable triggers for each index is critical for guiding and streamlining action during a drought event. Thresholds for action can be both qualitative and quantitative as both provide value, but the use of one without the other can lessen the effectiveness by creating a trigger point that is either hard to define and subjective, or so rigid it triggers a response when one may not yet be warranted.
For instance, a drought plan could state that when the 3-month SPI drops below -1.0, departmental public information officers will work together to establish coordinated media outreach and messaging. In the midst of a dry, hot summer when crops are likely suffering and fires may be burning, this would make a great deal of sense; but what if this occurs over the winter season when there may be fewer impacts and reservoir storage is still high? Implementing a messaging campaign could create alarm when there is not yet a need for it. By using the quantitative data hand-in-hand with qualitative information, response actions can be initiated at an appropriate time and scale.
Because a drought can develop slowly and last for years, or even decades, a multistage response is also important, as the response needs to reflect whether conditions are improving or deteriorating. Having multiple stages, and thus multiple trigger points for each index, enables mitigation and response actions to be phased in and out. This structure also provides a mechanism to guide decision makers during an event, enabling hard decisions to be made objectively by simply following the plan.