Much of the work relating canopy-air temperature differential to crop stress of maize (Zea mays, L) has been done in the arid or semi-arid regions of the Western U.S.A. In this paper, three indices developed for arid conditions were examined under humid conditions. The first index used only canopy-air temperature differentials. The second index used canopy-air temperature differentials and vapor pressure deficits. The third index included canopy-air temperature differential, vapor pressure deficit and net radiation. These three indices were tested at a differential soil moisture installation. This installation uses controlled rooting depths to ensure the occurrence of drought stress even under humid conditions. The soils in the controlled rooting plots are the topsoil of a Mexico silt loam (Udollic Ochraqualfs, fine, montmorillonitic, mesic). Nearby undisturbed plots are also Mexico silt loams. Two years of data, an arid year and a humid year, are included in this analysis.
When data from the arid year were examined alone, all indices related to yield equally well. With the addition of the humid year data, the second and third indices performed better than the first, indicating that the addition of vapor pressure deficit and possibly net radiation have some benefit. Similar comparisons of the three indices were made with the yield components, kernel weight and number of kernels. Kernel weights showed no difference due to drought stress in that, when plotted against each of the indices, all slopes were 0.0. Most of the effect of drought stress on yield is on the kernel number component. As with yield, all indices work equally well in the arid year. However, when the humid year is included, the third index seems to work the best. Thus, when the components of yield are examined, it appears possible that the marginal increase in fit due to the addition of net radiation may be real. This finding shows that vapor pressure deficits and possibly net radiation may be variable enough in a humid environment to make it necessary to include them in any canopy-air temperature differential based indicator of drought stress.