(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 heat balance of a leaf is determined mainly by its environment but the heat balance of warm-blooded animals (homeotherms) may also be modified by the animal’s ability to adjust its rate of internal metabolic heat production and its rate of heat dissipation by sweating. Cold-blooded animals (poikilotherms) have less physiological control of their heat balance but can seek shade, sun, or shelter from the wind to alter energy gains and losses. In this chapter each term of the animal heat balance equation is reviewed. The basal metabolic rate of homeotherms can be related to body mass or surface area by an empirical power law. Maximum metabolic rates are about one order of magnitude larger. Latent heat is lost during breathing and by sweating. Desert animals conserve water by several mechanisms. The resistance to convective heat loss is proportional to the square root of size and to the inverse square root of windspeed, so that temperatures of small animals are closely coupled to air temperature. Heat loss by conduction to the ground is reviewed. Some homeotherms (e.g. camels) and all poikilotherms store heat by allowing their body temperature to increase, but most homeotherms keep their body temperature almost constant. The thermo-neutral diagram, representing the fundamental relation between metabolic heat production and environmental temperature, is discussed in detail. A diagrammatic method of analyzing animal heat balance terms is described and applied to several examples. Finally the heat balance of man, including activities involving sweating, is analyzed.

Steady-State Heat Balance: (ii) Animals

This is the first of several chapters concerned with the principles by which energy absorbed when plants and animals are exposed to radiative fluxes is transferred to the atmosphere by convection and by the evaporation of water (latent heat transfer). These transfers take place through a boundary layer with properties dependent on the viscosity of air and the transport of momentum from moving air to the plant or animal surface. This chapter discusses laminar and turbulent boundary layers and the frictional drag that is a consequence of momentum transfer through the boundary layer (skin friction) and pressure gradients set up by flow patterns around the object (form drag). Measurements of drag on leaves, trees, natural surfaces, and particles are reviewed and analyzed using principles from fluid dynamics. Lodging of crops and windthrow of trees by strong winds are discussed using observations in the field and in wind tunnels.

Chapter 9 – Momentum Transfer

This chapter deals with the mechanisms of heat transfer between organisms and the atmosphere by convection and conduction. The principles of convective heat transfer are introduced using non-dimensional groups from engineering theory. The Nusselt number Nu provides the basis for comparing rates of convective heat transfer from similar-shaped bodies of different scales. Leaves and animals can often be approximated by flat plates, cylinders, and spheres, and this allows heat transfer to be estimated from theoretical and empirical relations. Methods of estimating Nu in forced and free convection and in laminar and turbulent flow are explained, and the concept of resistance to convective heat transfer is introduced. Measurements of convective heat loss from leaves and animals are then compared with estimates from engineering theory, demonstrating reasonable agreement. Conduction is discussed primarily in relation to heat transfer through animal tissue and coats and human clothing. Thermal resistances to heat transfer through peripheral tissue and animal coats are reviewed, and it is shown that heat transfer through animal coats usually involves multiple mechanisms: molecular conduction, radiation, and free and forced convection.

John L. Monteith

Papers

Steady-State Heat Balance: (ii) Animals

Chapter 9 – Momentum Transfer

Chapter 10 – Heat Transfer