(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.

Crop evapotranspiration : guidelines for computing crop water requirements

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.

Large Area Hydrologic Modeling and Assessment Part i: Model Development

Boundary Layer Climates.

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.

/pdf/primary-production-of-the-biosphere-integrating-terrestrial-4t7yz6focw.pdf

Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components

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.

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

SUMMARY The irradiance of the solar beam was measured on cloudless days at Sutton Bonington in the English Midlands and at sites in north-west Scotland. Total and diffuse fluxes were also measured on some days. An attenuation coefficient for aerosol T. was defined by S(T~) = S(o) exp ( - Ta m) relating the measured flux at normal incidence S(T.) to the Rux calculated for adust-free atmosphere when the air mass number is m. Changes of T. from day to day were related to changes of air mass origin; local sources of aerosol were relatively unimportant. In maritime air, ra ranged from 0.05 to 0.15, and in continental air, from 0.1 to 0.5. In a tropical maritime air mass, T~ decreased from 0.13 at sea level to 0.07 at 1,340 m. The fraction of (ultra-violet f visible) to total radiation was (0.54 - 0.28 7.) and the ratio of diffuse to total radiation (m < 2) was (0.1 + 0.7 7.). The ratio of total scattering to absorption by aerosol decreased from 4 at m = 1.1 to 0.5 at m = 2. Mean monthly values of T. at four Meteorological Office stations were calculated from records of solar radiation and hours of sunshine and corresponding values of total and diffuse flux were tabulated for ‘ isolated ’, ‘ rural ’ and ‘ urban ’ sites. The presence of solid particles in the Earth’s atmosphere has important consequences for the transmission of solar radiation and for the nature of the radiation rkgime at the ground. The absorption of solar energy by a layer of aerosol increases the radiative heating of the atmosphere and decreases the amount of energy available at the surface. Scattering by aerosol increases the amount of radiation which is reflected by the atmosphere into space and increases the downward flux of diffuse radiation at the Earth’s surface. Attenuation also produces changes in the spectral composition of solar radiation which are significant biologically. To estimate the amount of radiant flux which is absorbed and scattered by aerosol as distinct from other atmospheric constituents, measurements of direct and diffuse radiation at, the ground may be compared with the fluxes predicted below a model atmosphere containing appropriate amounts of ozone, water vapour, and carbon dioxide (G. D. Robinson 1962, 1966). The height distribution of aerosol can be inferred by measuring solar radiation from aircraft (Roach 1961), and the presence of particles as high as 50 km has been demonstrated by measuring the scattering of light from searchlight beams (Elterman, Wexler and Chang 1969). Concern about possible changes in global climate has stimulated new interest in the radiative effects of aerosol. Recent calculations by Rasool andSchneider (1971) imply that any future increases of aerosol content will decrease the mean surface temperature of the Earth and that the heat balance of the atmosphere may become increasingly sensitive to changes of aerosol content. To be able to detect the radiative effects of changing aerosol content, it is essential to establish baselines for the income of solar radiation in different parts of the world and to show how this income is related to aerosol load. This kind of exercise has been attempted at relatively few sites. Valko (1963) analysed turbidity measurements at LocarnoMonti in Switzerland and demonstrated marked daily and annual changes in the strength of the direct solar beam which he ascribed to differences in the composition of aerosol in different air masses. Flowers, McCormick and Kurfis (1969) reported measurements of turbidity from a network of stations in the USA equipped with sun photometers. These instruments record the irradiance of the direct beam at a wavelength of 0.55 pm but they

https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/18562/Unsworth_and_Monteith_Q_J_R_Met_Soc_1972.pdf;sequence=1

Aerosol and solar radiation in Britain

A silicon cell and a radiometer were used to measure the transmission of short wave (0.4-1 $\mu $m) and long wave (3-100 $\mu $m) radiation through the coats of sheep and cattle. The reflectivity of the same coats was measured in a spectrophotometer. Measurements of transmission and reflexion were interpreted in terms of a theory of radiation scattering developed (for vegetation) by Cowan (1971), assuming that radiation striking a single hair could be absorbed or scattered either towards or away from the skin. One of the parameters used in the theory is the fraction of radiation intercepted by a ray in unit depth of coat (p). For diffuse radiation the appropriate mean value of the interception function ($\overline{p}$) is approximately twice the value of p for a ray at normal incidence. The value of $\overline{p}$ ranged from about 9 cm$^{-1}$ for sheep's fleece to 36 cm$^{-1}$ for calf and deer coats. In the short wave spectrum, mean reflexion coefficients for the whole coat ranged from 0.30 for Welsh Mountain Sheep (black fleece) to 0.79 for Dorset Down Sheep (white fleece); corresponding values of the absorption coefficient for individual hairs were 0.02 and 0.002. On the basis of these and related figures, the absorption of solar radiation by the skin surface was evaluated for different combinations of fleece and skin colour. The combination of a light fleece and a dark skin is a trap for solar radiation because the scattering by hair is predominantly forwards.

Transfer processes in animal coats I. Radiative transfer

Tube Solarimeter to Measure Radiation Among Plants

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.

The measurement and control of stomatal resistance in the field

Interplant competition for capture of the essential resources for plant growth i.e. light, water and nutrients, strongly affects the performance of natural, semi-natural and agricultural ecosystems. Ecologists have studied competition to understand the diversity and stability of plant communities, succession patterns of vegetation, and to help define management strategies for semi-natural ecosystems. Agroecologists have studied competitive phenomena to optimize plant densities of crops, to optimize intercropping systems and to quantify crop-weed interactions to improve weed management systems with minimum herbicide inputs. Similar approaches have been used to study interplant competition in natural and disturbed systems. However, because of the complex nature of interplant competition, it has taken a long time to develop generalized concepts and theories. The first systematic approaches for studying competitive phenomena were developed in the 1960s. For monocultures, much of our current understanding is based on the work of a Japanese group (e.g. Shinozaki and Kira, 1956) whereas de Wit (1960) developed the first systematic approach to study competition in mixtures. He introduced an experimental design (the replacement series in which the mixing ratio varies, but total density remains constant) with a model to analyse the results. These approaches were based on a hyperbolic relationship between plant density and yield, and have been used extensively in agricultural and ecological sciences to study competition between plants, plant population dynamics, and component contributions of intercropping systems (see reviews by Trenbath, 1976; Harper, 1977; Radosevich and Holt, 1984; Grace and Tilman, 1990). Recently, several papers discussed the disadvantages and pitfalls of the replacement series approach, such as its total density dependence ( cf. Connolly, 1986; Taylor and Aarssen, 1989). Only in the early 1980s, approaches were developed to describe competition over a range of population densities with varying mixing ratios and at a range of total densities, generally also based on the hyperbolic yield-density relationship (Suehiro and Ogawa, 1980; Wright, 1981; Spitters, 1983a, b; Cousens, 1985; Spitters, Kropff and de Groot, 1989). Similar approaches have been developed using the neighbourhood approach, in which the number of neighbours of an individual plant in a predefined area is related to the weight of the central plant (Silander and Pacala, 1985; Firbank and Watkinson,

John L. Monteith

Papers

Aerosol and solar radiation in Britain

Transfer processes in animal coats I. Radiative transfer

Tube Solarimeter to Measure Radiation Among Plants

The measurement and control of stomatal resistance in the field

Resource capture by crops.