"Drought triggers, which are threshold values of indicators, determine the timing and level of drought responses associated with drought categories. Drought categories or levels typically use nomenclature such as “mild, moderate, severe, extreme drought,” or “level 1, level 2, level 3 drought.” Drought responses include both strategic longer-term actions, usually implemented before a drought (such as water pricing policies), and tactical shorter-term actions, usually implemented during a drought (such as water use restrictions). Thus, together, indicators and triggers form the linchpin of a drought plan, linking drought conditions with drought responses."
-- From Steinemann, A.C. and L.F.N. Cavalcanti (2006). Developing multiple indicators and triggers for drought plans, Journal of Water Resources Planning and Management 132(3): 167-174. A version of this paper is online at http://water.washington.edu/Research/Reports/multipleindicatorsandtriggers.pdf
See also Steinemann, A.C., Hayes, M.J., and L.F.N. Cavalcanti (2005). Drought indicators and triggers. In: D. A. Wilhite (ed.), Drought and water crises: Science, technology, and management issues (pp 71-92). CRC Press, Boca Raton: FL.
Examples of drought plans utilizing triggers:
Georgia Drought Management Plan
Jefferson River Watershed Council Drought Management Plan
Denver Water Drought Response Plan