From AMS Glossary
- That portion of total precipitation used to satisfy plant (vegetation) needs.
- As used by C. W. Thornthwaite in his 1948 climatic classification: an overall measure of precipitation effectiveness for plant growth that takes into consideration the weighted influence of water surplus and water deficiency as related to water need and as they vary according to season. For a given station, it is calculated by the formulawhich becomeswhere Im is the moisture index, s the water surplus, d the water deficiency, and n the water need. The calculation of s and d is made on a normal month-to-month basis, with s being the total surplus from all months having a water surplus, and d the total of all monthly deficiencies; each is represented by the difference between monthly precipitation and monthly potential evapotranspiration (in centimeters or inches). Here n is the annual potential evapotranspiration.
The moisture index replaced Thornthwaite's previously used (1931) precipitation-effectiveness index.
Compare Palmer Drought Severity Index.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geographical Review. 38. 55–94.
Thornthwaite, C. W. 1931. Climates of North America according to a new classification. Geographical Review. 21. 633–655.