(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

In the field successive leaves of winter wheat appear at a rate which varies because it depends strongly upon temperature. When plotted against ‘thermal time’, however (temperature accumulated above a fixed base of 0°C), leaf appearance was a strictly linear function of temperature. The mean rate of leaf appearance in thermal time, R′, was faster for a spring sowing than for an autumn sowing. The variation in R′ between sowings was better correlated with the rate at which daylength was changing when the plants emerged than with the mean daylength while leaves were appearing.

Daylength change and leaf appearance in winter wheat

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Light Interception and Radiative Exchange in Crop Stands

Measurements of the heat balance under plastic mulches. Part I. Radiation balance and soil heat flux

The upward flux of carbon dioxide at the soil surface was calculated from the weight increase of soda lime granules exposed inside a glass tank covering 400 cm2 soil. Over bare soil, the flux varied annually with a summer maximum of about 7 g CO2 m-2 day-1, a winter minimum of 1 g m-2 day-1, and a Q10 of 3. The contribution of root respiration, calculated from the difference between the fluxes over fallow and cropped soil, was usually about 1-3 g m-2 day-1. From dry matter determinations, the amount of soil carbon assimilated by crops was about 6% of the net carbon uptake for rapidly growing grass in spring and about 20% for other crops during the summer. The top 46 cm of the soil profile contained 12 kg carbon m-2 and lost 0.4 kg m-2 annually by respiration. The corresponding half-life of soil organic matter is 22 years. The variation of carbon dioxide concentration and rate of photosynthesis with soil flux depends on wind speed and atmospheric stability. In most weather, atmospheric mixing is so vigorous that the concentrations in the canopy and in the free atmosphere are very similar (c. 300 ppm), and photosynthesis is independent of soil flux. In glass-houses, where mixing is less, the concentration may be much less than 300 ppm. In chosen conditions gross photosynthesis increased by 30% when the upward flux of carbon dioxide below the canopy increased by 10 g m-2 day-1.

Crop photosynthesis and the flux of carbon dioxide below the canopy

The apparent emissivity of the atmosphere ϵ, defined as the ratio of incoming long-wave radiation to black-body radiation at screen temperature Ta, was measured under clear skies in the English Midlands and in the Sudan. At a zenith angle Z the emissivity was given by ϵ(Z) = a + bIn(usec Z) where u is the reduced depth of precipitable water (cm). For a set of 46 scans in England, the mean values of a and b were 0·70±0·05 and 0·090 ± 0·002. Systematic deviations about these mean values could be ascribed to: (i) temperature gradients; (ii) aerosol. The Sudan measurements gave a = 0·67 ± 0·03 and b = 0·085 ± 0·002 consistent with the English results and observations already published. There is some evidence that minimum values of a have increased over the past 50 years.



Integration over the hemisphere gives the flux density (Wm−2) of atmospheric radiation as 1·06 σTa4 − 119 (T in K), where σ is Stefan's constant, or 5·5 Ta + 213 (T in °C).



Radiation records by Dines and Dines (1927) for overcast skies are analysed to show that the angular distribution is the same as for cloudless skies; that the mean temperature of cloud base at their site was UK below screen temperature; and that when the mean fraction of cloud cover is c, the apparent emissivity is ϵa(c) = (1 − 0·84c)ϵa(0) + 0·84c.

John L. Monteith

Papers

Daylength change and leaf appearance in winter wheat

Light Interception and Radiative Exchange in Crop Stands

Measurements of the heat balance under plastic mulches. Part I. Radiation balance and soil heat flux

Crop photosynthesis and the flux of carbon dioxide below the canopy

Long‐wave radiation at the ground I. Angular distribution of incoming radiation