(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.

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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

The papers presented at this meeting have given us a comprehensive account of the state of the art in what one is tempted to call ‘forest meteorology’ though ‘meteorological forestry’ is probably nearer the mark. Older members of the audience like myself who can recall the descriptive and anecdotal nature of the subject in the early 1950s have enjoyed hearing how it has come of age through the painstaking collection of measurements in the field and through the emergence of principles that have guided the development of mathematical models. Forest meteorologists have been fortunate that much of the foundation of their subject has been laid by the pioneers of agricultural meteorology who have had an easier task experimentally and are therefore a little ahead, but not much! A psychologist might have felt quite at home at this meeting because there have been so many references to the way systems ‘behave’. It seemed that systems were ‘well behaved’ when processes being observed were consistent with theoretical predictions where they existed, or with intuition where they did not. Bad behaviour (by a forested catchment in Rob Roy’s territory for example) meant a discrepancy between performance and expectation. We should remember, however, that the recognition of so-called bad behaviour is the first step towards new developments in most branches of science; and that progress in forest meteorology will depend on the skill and patience of people who feel challenged to tackle the anomalies, the uncertainties and the loose ends that we have heard about yesterday and today.

Reflection and Review

This chapter reviews properties of electromagnetic radiation and principles of radiation absorption and emission by surfaces. The concept of “black body” or “full radiation” is introduced, with discussion of the laws of Wien, Kirchhoff, Planck, and Stefan-Boltzmann. Radiative exchange between surfaces or atmospheric layers is discussed to derive the concept of “radiative resistance.” Spatial relations for emission from point sources and surface elements are explained, along with the Cosine Law. Spectral reflectivity and absorptivity, and relationships between irradiance and radiance are introduced. The chapter concludes with discussion of the attenuation of radiation passing through a medium, leading to Beer’s Law and the Kubelka-Munk equations.

John L. Monteith

Papers

Reflection and Review

Transport of Radiant Energy

Refraction and the spider

Perspectives of Biophysical Ecology

Analysis of microclimate in cereals and grasses