The radiation balance of bare soil and vegetation

Incoming short‐wave radiation S , reflected short‐wave radiation α S and net radiation R were measured over bare soil and crops from 1957 to 1959, and net long‐wave radiation ( L ) was deduced from\documentclass{article}\pagestyle{empty}\begin{document}$$R = (1 - \alpha)S + L$$\end{document}For grass, α increased from 0·23 at solar elevation 60° to 0·28 at 20° with daily mean 0·26. For bare soil, the corresponding increase was from 0·16 to 0·19 with mean 0·17. In mid‐June, L for bare soil decreased from – 0·1 cal cm −2 min −1 during the night to – 0·4 cal cm −2 min −1 in the early afternoon. For long grass, in August, the corresponding change was from – 0·05 to – 0·22 cal cm −2 min −1 . Under clear skies the incoming long‐wave component varied much less than the outgoing component, and net flux L was closely related to surface temperature. With a heating coefficient β = – dL/dR , the observed linear dependence of R on S in the absence of cloud may be expressed as\documentclass{article}\pagestyle{empty}\begin{document}$$R = (1 - \alpha)S + L$$\end{document}Where, formally, R = L 0 when S = 0. For grass, sugar beet and potatoes, β lay between 0·15 and 0·22 with a variation which may depend on wind speed rather than on crop. The value for dry bare soil was higher (0·41) because there was greater surface heating. Measurements under clear skies and over grass at Cambridge and Kew agree well with Rothamsted values (β = 0·22, L 0 = – 5·9 cal cm −2 hr −1 ). Over Nebraska prairie, β = 0·25, L 0 = – 4·5 cal cm −2 hr −1 from selected observations during Projects Great Plains and Prairie grass.