Showing papers by "John L. Monteith published in 1965"

Evaporation and environment.

Abstract: A turgid leaf exposed to bright sunshine can transpire an amount of water several times its own weight during a summer day. Rapid evaporation is sustained by a supply of heat from the atmosphere and by a movement of water within the plant preventing the desiccation of leaf tissue. In analysis, the need for energy and the need for water have often been disassociated. Meteorologists investigating the energetics of transpiration have assumed that leaves behave like pieces of wet, green blotting paper, and plant physiologists have demonstrated mechanisms for the conduction of water at arbitrary rates unrelated to the physics of the environment. This paper describes progress towards a reconciliation of parallel concepts in meteorology and physiology. The path for the diffusion of water vapour from leaf cells to the free atmosphere is divided into two parts, one determined primarily by the size and distribution of stomata, and the other by wind speed and the aerodynamic properties of the plant surface. Diffusive resistances for single leaves and for plant communities are established from measurements in the laboratory and in the field and are then used: (i) to predict relative rates of evaporation from leaves with wet and dry surfaces; (ii) to investigate the dependence of transpiration rate on wind speed and surface roughness; (iii) to demonstrate that the relation between transpiration rate and lead area is governed by stomatal closure in leaves well shaded from sunlight; (iv) to calculate maximum rates of transpiration for different crops and climates. A final section on the convection of dry air stresses the importance of physiological restraint on the rate of transpiration from an irrigated field surrounded by dry land.

The measurement and control of stomatal resistance in the field

Abstract: The rate of evaporation from a crop is expressed in terms of weather parameters and a quantity rs derived from profiles of temperature, humidity, and wind. In transfer equations, rs is formally similar to the diffusion resistance of the stomata in a single leaf, and measurements in a field of barley support the hypothesis that rs is the diffusion resistance of the complete crop canopy. The resistance is relatively small when the leaf area index is great, when soil is moist, and when sunlight is bright. It increases as the plants mature, but is independent of wind speed and is therefore unrelated to the rate of diffusion by turbulent mixing. More directly, rs was correlated with stomatal resistance rp measured on individual leaves with a porometer. From seventy-five sets of porometer readings taken when the leaf area index was between 6 and 10, the relation was rp = 1.11+0.87 rs Because rp was measured on upper, sunlit leaves, which were more porous than lower, shaded leaves, the ratio of rp to rs was expected, and was found, to be less than the leaf area index. Finally, stomata were closed and rp was increased by spraying the top of the canopy with NSA (the methyl ester of nonenyl succinic acid). The increase of rs calculated from the decrease of transpiration rate was consistent with the change of rp.

Radiation and Crops

Abstract: The analysis of radiation climate is a central problem of agricultural meteorology because rates of photosynthesis depend on the receipt of visible light and rates of transpiration depend on the net exchange of radiation by a crop canopy. Both short-wave (solar) and long-wave (terrestrial) radiation are correlated with cloud amount, and in south-east England the income of net radiation in summer is proportional to the income of solar radiation.In principle, the fraction of total radiation in the visible waveband depends on cloud cover and on the amount of absorption and scattering in the atmosphere, but in practice the fraction is often between 0·40 and 0·45. The calculation of photosynthetic efficiency needs a figure for the number of quanta (or Einsteins) per unit energy, and this figure can be calculated from the mean wavelength of the radiation weighted by energy, about 0·55μ for direct sunlight.The reflection of radiation by vegetation changes with solar elevation, and at angles between 40° and 60° it ranges between 0·15 for a rough crop (e.g. pineapple) to 0·26 for smoother crops (e.g. sugar beet, kale). The transmission of radiation through the canopy can be expressed as a function of the leaf area index and a parameter that depends on the distribution and orientation of leaves.