(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

This chapter describes how radiant energy reaching a surface is absorbed, reflected (scattered), and transmitted. These processes depend on wavelength and on the angle of incidence of direct radiation. Radiative properties of water, soil, leaves, vegetation canopies, and animal coats are reviewed. Still water is an effective reflector of the solar beam at low angles of elevation, but only reflects about 5% of solar radiation when the sun is high. Water absorbs least in the blue-green portion of the spectrum. In the infra-red region water absorbs strongly in several wavebands. Reflection of solar radiation by soil depends on soil structure and water content. Leaves and canopies absorb least (reflect most) around green wavelengths and transmit near-infra-red radiation very effectively.

Microclimatology of Radiation: (i) Radiative Properties of Natural Materials

In recent decades there has been considerable i nternational research into the meanings, uses, and consumption of the past among various peoples around the world. These memory studies have been fuelled not only by the nationally oriented work of Pierre Nora and David Lowenthal, among others, but also by the exploration of commemoration, trauma, and genocide as sites of history. Holocaust, and then postcolonial, studies precipitated an expansive research interest that was less nationally bound and more defi ned by ethnicity and experience. Both of these closely related fi elds have seen a wealth of theoretical speculation, semiotic analysis, and investigation of representations of the past and of those who produce them. With that burgeoning fi eld of research also began the important interpretive work of interrogating and making meaning from history sites, such as monuments, textbooks, remembrances, and exhibitions. The result of such scholarly attention meant these markers in eff ect became “works” of history, as laden with meaning as any text, and demanded critique accordingly: memorial architecture was analyzed and interrogated, syllabuses and textbooks were rigorously combed, commemorations and museum exhibits were re-read from critical perspectives, family histories were interpreted with a scholarly lens. Building on the scholarship into these public historical iterations, feminist and postcolonial scholars also revealed the embodied power of the past, as memories of slavery, colonization, Indigenous histories, the Holocaust, and motherhood registered corporeally. Archaeologists and

I. theory and practice

An alcohol-filled Gunn-Bellani radiation integrator, exposed at Rothamsted from June to December 1958, gave weekly distillation totals that were a linear function of short-wave radiation on a horizontal surface above a limit of 500 cal cm−2 week−1. Laboratory experiments with both alcohol- and water-filled models reveal a threshold radiation intensity which can be estimated from the properties of the distilling fluid and the partial pressure of air within the instruments. Distillation is negligible below the threshold, but above, it is proportional to the excess radiation. Because the relation between absorbed heat and radiation incident on a horizontal surface depends on solar altitude and on the proportions of direct and diffuse radiation, it is difficult to predict field response from laboratory data.

The performance of a Gunn‐Bellani radiation integrator

This chapter reviews some aspects of micrometeorology, the study of atmospheric physics on the scale of vegetation canopies. In Environmental Physics micrometeorology has been very effectively used to study hydrology, environmental physiology, and ecology. The turbulent boundary layer that develops over extensive soil and vegetation surfaces grows at a rate depending on surface roughness and atmospheric stability. Properties of turbulence are discussed, leading to a review of the principles of the eddy covariance method of studying rates of turbulent transfer of heat mass and momentum (fluxes) in the surface boundary layer. Instrumentation requirements for, and corrections necessary to eddy covariance measurements, are summarized. The less direct method of studying turbulent fluxes by relating analyzing mean vertical gradients (profiles) of windspeed, temperature, and mass concentration is also reviewed. Roughness parameters depending on vegetation height and structure influence profile properties. Aerodynamic resistance is derived as a quantity relating momentum flux to windspeed gradients. The influence of atmospheric stability on profiles and therefore on fluxes is reviewed, including issues encountered when measuring close to vegetation elements. Principles of the aerodynamic and Bowen ratio methods for estimating fluxes from gradients are reviewed, and the strengths and weaknesses of eddy covariance and flux-gradient methods are summarized. Finally, aspects of turbulent transfer in vegetation canopies are discussed.

Micrometeorology: (i) Turbulent Transfer, Profiles, and Fluxes

Building on the principles of radiation, momentum, heat, and mass transfer in previous chapters, we now address the steady-state heat balance of water bodies, soil, and vegetation by applying the First Law of Thermodynamics. We begin with the heat balance of dry-bulb and wet-bulb thermometers to establish basic principles and introduce the concept of resistances to heat and mass transfer. The heat balance of wet surfaces introduces adiabatic and diabatic processes. This leads to the Penman Equation, discussion of its application to estimating evaporation from natural surfaces, and analysis of the dependence of evaporation rate on the weather. Considering the heat balance of leaves, the Penman-Monteith (PM) Equation is developed, identifying the distinction between boundary layer resistances to heat and mass transfer and the stomatal resistance. Differences between factors influencing evaporation from wet surfaces and those influencing transpiration from leaves are discussed. The PM Equation is used to explore how transpiration and leaf temperature depend on radiation, humidity, windspeed, and stomatal resistance; rates of dew deposition are also discussed. Developments from the Penman and PM Equations conclude the chapter, including discussion of the “big-leaf model” for vegetation canopies, the equilibrium evaporation rate, and Priestley-Taylor coefficient, and the concept of “coupling” between vegetation and the atmosphere.

John L. Monteith

Papers

Microclimatology of Radiation: (i) Radiative Properties of Natural Materials

I. theory and practice

The performance of a Gunn‐Bellani radiation integrator

Micrometeorology: (i) Turbulent Transfer, Profiles, and Fluxes

Steady-State Heat Balance: (i) Water Surfaces, Soil, and Vegetation