John L. Monteith

Bio: John L. Monteith is an academic researcher from International Crops Research Institute for the Semi-Arid Tropics. The author has contributed to research in topics: Atmosphere & Transpiration. The author has an hindex of 58, co-authored 138 publications receiving 30024 citations. Previous affiliations of John L. Monteith include Goddard Space Flight Center & University of Nottingham.

Tree Lines [and Discussion]

Abstract: Trees do not generally grow in places where the mean temperature of the warmest month is less than about 10 $^\circ$ C. At their limit, trees are often short and crooked, the condition known as krummholz; and the transition from tall forest to dwarf shrubby vegetation is often abrupt, forming a distinct tree line. Tree lines fluctuate with climatic change. There is compelling evidence to suggest that they shift to higher elevations and higher latitudes in warmer periods. In northern Europe, they were about 200 m higher in the Boreal period when the temperature is believed to have been 2$^\circ$ C warmer than now. Controlled-environment studies and tree-ring evidence also point to considerable sensitivity of growth at the tree line to fluctuations in the summer temperature. Forest vegetation differs aerodynamically from dwarf vegetation in being aerodynamically rough. Consequently, the temperatures of above-ground tissues are closely coupled to temperatures of the air. In contrast, shorter vegetation experiences tissue temperatures and microclimates that depend substantially on other climatological variables, notably radiation and wind speed. Short vegetation is, on average, warmer than the air; this is the main reason why dwarf shrubs can succeed in cold climates where trees fail to grow and reproduce. Water stress commonly occurs in late winter and early spring when soil water is frozen. The foliage of trees at the tree line displays an inability to restrict water loss, either because the epidermis is damaged by abrasion or because the cuticle does not properly develop in the reduced growing season. Consequently, the longevity of leaves is reduced. Winter damage to trees is also likely as a result of gales and the deposition of ice in the canopy, both of which break branches and may contribute to the generally misshapen form of the crown.

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.

Response to Temperature in a Stand of Pearl Millet VI. LIGHT INTERCEPTION AND DRY MATTER PRODUCTION

Abstract: Stands of pearl millet Pennisetum americanum were grown in glasshouses in which the mean air temp. was controlled to 19, 22, 25, 28 and 31°C. During the main growth period, LAI increased at a constant rate which was proportional to mean temp above a base of 10°. The warmest stand therefore intercepted more radiation before anthesis but the transmission coeff. was independent of temp. The efficiency of conversion for intercepted radiation varied from 2.1 to 2.4 g/MJ. The data suggests growth rate should be max. at 25-27° and total dry wt. at 20-22°

Crop evapotranspiration : guidelines for computing crop water requirements

Abstract: (First edition: 1998, this reprint: 2004). This publication presents an updated procedure for calculating reference and crop evapotranspiration from meteorological data and crop coefficients. The procedure, first presented in FAO Irrigation and Drainage Paper No. 24, Crop water requirements, in 1977, allows estimation of the amount of water used by a crop, taking into account the effect of the climate and the crop characteristics. The publication incorporates advances in research and more accurate procedures for determining crop water use as recommended by a panel of high-level experts organised by FAO in May 1990. The first part of the guidelines includes procedures for determining reference crop evapotranspiration according to the FAO Penman-Monteith method. These are followed by updated procedures for estimating the evapotranspiration of different crops for different growth stages and ecological conditions.

Large Area Hydrologic Modeling and Assessment Part i: Model Development

Abstract: A conceptual, continuous time model called SWAT (Soil and Water Assessment Tool) was developed to assist water resource managers in assessing the impact of management on water supplies and nonpoint source pollution in watersheds and large river basins. The model is currently being utilized in several large area projects by EPA, NOAA, NRCS and others to estimate the off-site impacts of climate and management on water use, nonpoint source loadings, and pesticide contamination. Model development, operation, limitations, and assumptions are discussed and components of the model are described. In Part II, a GIS input/output interface is presented along with model validation on three basins within the Upper Trinity basin in Texas.

Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components

Abstract: Integrating conceptually similar models of the growth of marine and terrestrial primary producers yielded an estimated global net primary production (NPP) of 104.9 petagrams of carbon per year, with roughly equal contributions from land and oceans. Approaches based on satellite indices of absorbed solar radiation indicate marked heterogeneity in NPP for both land and oceans, reflecting the influence of physical and ecological processes. The spatial and temporal distributions of ocean NPP are consistent with primary limitation by light, nutrients, and temperature. On land, water limitation imposes additional constraints. On land and ocean, progressive changes in NPP can result in altered carbon storage, although contrasts in mechanisms of carbon storage and rates of organic matter turnover result in a range of relations between carbon storage and changes in NPP.

Correction of flux measurements for density effects due to heat and water vapour transfer

Abstract: When the atmospheric turbulent flux of a minor constituent such as CO2 (or of water vapour as a special case) is measured by either the eddy covariance or the mean gradient technique, account may need to be taken of variations of the constituent's density due to the presence of a flux of heat and/or water vapour. In this paper the basic relationships are discussed in the context of vertical transfer in the lower atmosphere, and the required corrections to the measured flux are derived. If the measurement involves sensing of the fluctuations or mean gradient of the constituent's mixing ratio relative to the dry air component, then no correction is required; while with sensing of the constituent's specific mass content relative to the total moist air, a correction arising from the water vapour flux only is required. Correspondingly, if in mean gradient measurements the constituent's density is measured in air from different heights which has been pre-dried and brought to a common temperature, then again no correction is required; while if the original (moist) air itself is brought to a common temperature, then only a correction arising from the water vapour flux is required. If the constituent's density fluctuations or mean gradients are measured directly in the air in situ, then corrections arising from both heat and water vapour fluxes are required. These corrections will often be very important. That due to the heat flux is about five times as great as that due to an equal latent heat (water vapour) flux. In CO2 flux measurements the magnitude of the correction will commonly exceed that of the flux itself. The correction to measurements of water vapour flux will often be only a few per cent but will sometimes exceed 10 per cent.