The following are a few of the most common drought indices.

• The Standardized Precipitation Index (SPI) is a short-term meteorological drought index that uses a deviation in precipitation from a selected probability distribution as a simplified starting point for drought monitoring.

• The Palmer Drought Severity Index (PDSI) is a long-term meteorological drought index that uses temperature, precipitation, and a physical water-balance model to evaluate relative dryness and forecast meteorological droughts.

• The Crop Moisture Index (CMI) is a short-term agricultural drought index that incorporates precipitation, temperature, potential evapotranspiration, soil moisture, and the previous week’s CMI.

• The Streamflow Drought Index (SDI) is a hydrological drought index that uses an algorithm similar to that used to calculate SPI but adjusted to evaluate levels of streamflow. SDI may be biased if streamflow is affected by streamflow management decisions. Periods of no flow can skew the results.