ABSTRACT
I. Overview of Indicators and Triggers .......................... 72 A. Precipitation........................................................... 74 B. Palmer Drought Severity Index (PDSI) and
Palmer Hydrologic Drought Index (PHDI).......... 76 C. Surface Water Supply Index................................. 78
II. Multiple Indicators and Triggers: Challenges and Solutions ................................................................ 79 A. Typical Problems with Indicators and
Triggers .................................................................. 79 B. Percentiles for Drought Indicators and
Triggers .................................................................. 80 C. Example: The U.S. Drought Monitor.................... 82
III. Developing and Evaluating Indicators and Triggers.. 83 A. Considerations for Drought Indicators and
Triggers .................................................................. 83 B. Checklist for Indicators and Triggers in a
Drought Plan ......................................................... 87
IV. Conclusion..................................................................... 89 Acknowledgments.................................................................. 89 References.............................................................................. 90
I. OVERVIEW OF INDICATORS AND TRIGGERS
Drought indicators are variables that describe the magnitude, duration, severity, and spatial extent of drought. Typical indicators are based on meteorological and hydrological variables, such as precipitation, streamflows, soil moisture, reservoir storage, and groundwater levels. Several indicators can be synthesized into a single indicator on a quantitative scale, often called a drought index. Although drought indices can provide ease of implementation, the scientific and operational meaning of an index value may raise questions, such as how each indicator is combined and weighted in the index and how an arbitrary index value relates to geophysical and statistical characteristics of drought.
