- That portion of total precipitation used to satisfy vegetation needs.
- The actual availability of precipitation used in plant development.
Availability is affected by such factors as precipitation intensity, season, temperature, ground cover, sod type, etc. The dependence of precipitation effectiveness on temperature and/or evaporation has been expressed in many ways: Köppen's formulas for defining desert climate, Lang's moisture factor, De Martonne's index of aridity, Gorczyński's aridity coefficient, Angström's humidity coefficient, Transeau's precipitation–evaporation quotient, and Thornthwaite's precipitation-effectiveness index.
Transeau, E. N. 1905. Forest Center of Eastern America. American Naturalist. 39. 875–899.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geogr. Rev.. 38. 55–94.
Köppen, W. 1931. Grundriss der Klimakunde. Walter die Gruyter Co., Berlin.
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