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The vertical energy balance yields reliable estimates of evapotranspiration on an hourly (or halfhourly) basis under large variations of thermal stratification provided the measurements are made close to a reasonably homo geneous surface and provided that suitable spacial and timesampling procedures are followed. The energy bal ance method is relatively insensitive to incorrect assump tions concerning the eddy coefficients and to estimates of the Bowen ratio (ratio of sensible to latent heat) except during the unusual, and most often uninteresting, condi tion where the evaporation approaches the sensible heat flux to the surface (E = -A). The energy balance meth od, which measures the radiation exchange at the surface, places reasonable limits on evaporation estimates and thus is a promising method for daily estimates provided either periods of positive and negative net radiation are con sidered separately or that a reasonable estimate of the 24hour Bowen ratio can be developed. In humid regions there is little vertical transfer of sensible heat to the sur face, so that when "potential evapotranspiration" condi tions are obtained, the evapotranspiration will approximate the daily net radiation. In arid regions the evapotranspira tion from a wellwatered field may be, under extreme con ditions, almost twice the net radiation.Though the vertical energy balance is not adequate for estimating evapotranspiration from small plots, considera tion of the complete energy balance indicates serious limitations of smallplot experiments for evaluation of field practice. Energy balance measurements on corn, including the measurement of the radiation transmitted to the soil surface, indicates that the heat exchange at the soil surface is an appreciable fraction of the total heat exchange even in mature corn at high populations. Because of the rela tively high heat exchange at the soil surface, an under 'Contribution from the Department of Soils, University of Wisconsin, Madison. Published with the permission of the

Abstract. Modifications of the design and calibration procedure of a diffusion porometer permit determinations of stomatal resistance which agree well with results obtained bv leaf energy balance. The energy balance and the diffusion porometer measurements indicate that the boundary layer resistances of leaves in the field are substantially less than those predicted from hcat transport formulas based on wind flow and leaf size.

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Spectral distributions of shade light between 400 and 740 mm were measured under corn, sugar maple, oaks, pines, and spruce with a portable recording spectrophotometer. Differences were found between hardwoods and softwoods and between clear cloudy days. An energy maximum at 550 nm, a minimum at 670 to 680 nm, and a very high maximum in the near infrared occurred under all species. Four components of light within a plant canopy can be distinguished: both beam solar radiation and diffused sky radiation are transmitted both directly and indirectly (reflected and scattered). Separate consideration of each of these components leads to great understanding of similarities and differences between light regimes in different stands.