Showing papers by "John L. Monteith published in 1966"

The photosynthesis and transpiration of crops

Abstract: When a leaf absorbs radiant energy, only a small fraction is stored chemically in photosynthesis. In sunlight, this fraction is at most one-fifth of the energy in the visible spectrum, decreasing with increasing light intensity because of the finite resistance to the diffusion of carbon dioxide through the leaf to the chloroplasts. Energy absorbed but not stored in photosynthesis is dissipated by transpiration and convection. The potential or maximum photosynthesis of a crop canopy can be estimated from a set of six parameters describing the photosynthesis-light curve of single leaves, the arrangement of leaves in the canopy, and radiation climate. Comparing estimates of potential photosynthesis with measurements of carbon dioxide exchange over a field of sugar beet, the estimated rate of respiration was 2 gm carbohydrate per m2 leaf area per day, equivalent to 44 per cent of gross photosynthesis over the whole growing season. Over the season, the foliage lost 34 per cent of incident radiation by transmission to the soil. The potential rate of transpiration can be found from Penman's formula assuming values of external (aerodynamic) and internal (mainly stomatal) resistance for the canopy as a whole. In south-east England, the energy for potential transpiration is almost equal to net heat H in summer and is therefore about half the energy of incoming solar radiation. For a real crop of grass subject to moisture stress, transpiration was less than the potential rate at about 0·8 H on average and 0·3 H in very dry weather. During the summer, cumulative photosynthesis increases linearly with cumulative transpiration to give a production ratio (gross photosynthesis/transpiration) of 1/100 in the Thames Valley and 1/200 in the Sacramento Valley. The production ratio is expected to change with crop type as well as with climate.

Local differences in the attenuation of solar radiation over Britain

Abstract: In relation to other British stations, radiation totals at Aberporth on the Welsh coast are anomalously high. On the basis of recorded sunshine, the annual mean intensity of direct radiation at Aberporth is 31 mw cm−2 compared with 25 mw cm−2 elsewhere. Allowing for the change of extra-terrestrial radiation with latitude, the effective transmission of the atmosphere is greatest at Aberporth and Lerwick, less at Eskdale-muir, and least at Kew, consistent with differences of air pollution. Measurements at a station installed near Aberystwyth suggest that the relatively clean air over Aberporth may be confined to a very narrow coastal strip. At Kingsway, the direct radiation per hour of sunshine increased from 21 mw cm−2 in 1957 to 26 mw cm−2 in 1963 consistent with the decrease of smoke in central London. Kingsway and Kew now record about the same intensity of radiation attenuated at roughly 1 per cent per 10 μg m−3 of smoke. Applying this analysis to the radiation balance of Britain, previous estimates of potential evaporation in north Scotland were raised from 13 to 17 inches per year, removing most of the difference in estimated evaporation over Britain.