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Fig. 1. Generalized sensitivities of plant processes to water deficits (modified with permission from Hsiao, 1973
).

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Fig. 2. (A) Illustration of the calculation of Idso's Crop Water Stress Index: CWSI=(Tcanopy–Tnws)/(Tdry–Tnws), showing the dependence of Tnws (– – –) and Tdry (- - -) on air vapour pressure deficit (vpd, kPa). (B) Illustration of the effect of a given experimental ‘noise’ (for example resulting from measurement errors and variations in irradiance), indicated by the double-headed arrow, showing that the signal-to-noise ratio decreases markedly as the vpd decreases from levels found in hot and arid/semi-arid climates to values typical in humid or maritime climates.

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Source: Journal of Experimental Botany 2004 55(407):2427-2436