(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

Simple thermopiles formed by copper-plating a constantan wire were used to build a solarimeter and a net radiometer. Output was 5–10 mV per cal·cm−2·min−1, adequate to operate a small portable galvanometer for spot readings, a continuously recording potentiometer, or, for integration, a Siemens electrolytic meter. The net radiometer was unventilated, but when oscillated manually, output was independent of wind speed. With two solarimeters in a null-balance circuit, reflection and transmission coefficients of a field crop were read directly on a dial. A circuit was designed to integrate solarimeter output and the standard error of weekly readings was only ±40 cal·cm−2.

Simple devices for radiation measurement and integration

Vegetation and the Atmosphere, Volume 2: Case Studies

In monsoon climates, the mean daytime saturation vapour pressure deficit (D), measured at screen height, decreases from a maximum of 3–4 kPa attained several months before rain arrives to a minimum of 0.5–1 kPa in the month of highest rainfall. Climatic records from India and West Africa were analysed to give the relation between D and precipitation (P mm month −1 ) as: where n ranged from 0.5 to 0.9 between stations. The dependence of dry matter production on D and therefore on P is evaluated when growth is restricted by rain and when this restriction is removed by irrigation. In the first case, the decrease in demand for water associated with a decrease in D is comparable in importance with the increase of water supply (P). The analysis has implications for the marginal response of crops to supplemental irrigation and for the interpretation of experiments with a line-source or rain-shelter.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0014479700014575

Significance of the coupling between saturation vapour pressure deficit and rainfall in monsoon climates.

The paper discusses the main sources of error in the determination of vapour pressure with a thermocouple psychrometer. The predictions of theory and the results of experiment lead to no general agreement on the value of the constant in the classical psychrometer equation. A new derivation of the equation suggests a reduction of the commonly accepted value by a power of the ratio of diffusion coefficients for heat and water vapour. An expression is derived for the radiation error due to the difference of temperature between the wet bulb and its surroundings. Conduction of heat along the thermocouple wires and extraneous radiation produce errors for which an expression is derived for a particular thermocouple model. Finally, wet-bulb temperature errors are related to the corresponding vapour pressure errors.

https://iopscience.iop.org/article/10.1088/0370-1301/67/3/306/pdf

Error and Accuracy in Thermocouple Psychrometry

During rainless weather following a monsoon, sorghum (Sorghum bicolor cv. SPH-280) was grown on a Vertisol at ICRISAT Centre either unirrigated throughout growth or irrigated for 7 weeks after emergence and rainfed thereafter. Before sowing, ammonium sulphate was applied at six rates from 0 to 150 kg/ha N. Roots were sampled every 2 weeks to determine biomass and root length density as a function of depth. Every week, soil water content in all treatments was measured gravimetrically to a depth of 0.23 m and with a neutron probe from 0.3 to 1.5 m. Below 0.45 m, volumetric water content was a negative exponential function of time after roots arrived and the maximum depth of extraction moved downwards at 2-5 cm per day. In the dry treatment,the extraction ''front'' lagged behind the deepest roots by c. 12 days initially but the two fronts eventually converged. Irrigation delayed the descent of the extraction front by c. 20 days but thereafter it appeared to descend faster than without irrigation. Averaged over N rates, the time constant of the exponential function was inversely related to the root length density, l sub(v), decreasing with depth from about 20 to 10 days as l sub(v) increased from 2.5 to 4.0 km/m sup(3). The biomass:water ratio was almost independent of N but increased from a mean of 5.3 g dry matter per kg water in the dry treatments to 6.9 g/kg with irrigation. When normalized by the seasonal mean difference in vapour pressure deficit within irrigated and unirrigated plots, the ratios were 13.1 and 13.3 kPa g per kg water, respectively.

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John L. Monteith

Papers

Simple devices for radiation measurement and integration

Vegetation and the Atmosphere, Volume 2: Case Studies

Significance of the coupling between saturation vapour pressure deficit and rainfall in monsoon climates.

Error and Accuracy in Thermocouple Psychrometry

Response to fertilizer nitrogen and water of post-rainy season sorghum on a Vertisol. 2. Biomass and water extraction