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Crop evapotranspiration : guidelines for computing crop water requirements
01 Jan 1998-Iss: 1
TL;DR: In this paper, an updated procedure for calculating reference and crop evapotranspiration from meteorological data and crop coefficients is presented, based on the FAO Penman-Monteith method.
Abstract: (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.
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TL;DR: In this paper, an updated gridded climate dataset (referred to as CRU TS3.10) from monthly observations at meteorological stations across the world's land areas is presented.
Abstract: This paper describes the construction of an updated gridded climate dataset (referred to as CRU TS3.10) from monthly observations at meteorological stations across the world's land areas. Station anomalies (from 1961 to 1990 means) were interpolated into 0.5° latitude/longitude grid cells covering the global land surface (excluding Antarctica), and combined with an existing climatology to obtain absolute monthly values. The dataset includes six mostly independent climate variables (mean temperature, diurnal temperature range, precipitation, wet-day frequency, vapour pressure and cloud cover). Maximum and minimum temperatures have been arithmetically derived from these. Secondary variables (frost day frequency and potential evapotranspiration) have been estimated from the six primary variables using well-known formulae. Time series for hemispheric averages and 20 large sub-continental scale regions were calculated (for mean, maximum and minimum temperature and precipitation totals) and compared to a number of similar gridded products. The new dataset compares very favourably, with the major deviations mostly in regions and/or time periods with sparser observational data. CRU TS3.10 includes diagnostics associated with each interpolated value that indicates the number of stations used in the interpolation, allowing determination of the reliability of values in an objective way. This gridded product will be publicly available, including the input station series (http://www.cru.uea.ac.uk/ and http://badc.nerc.ac.uk/data/cru/). © 2013 Royal Meteorological Society
5,552 citations
TL;DR: In this article, a new climatic drought index, the standardized precipitation evapotranspiration index (SPEI), is proposed, which combines multiscalar character with the capacity to include the effects of temperature variability on drought assessment.
Abstract: The authors propose a new climatic drought index: the standardized precipitation evapotranspiration index (SPEI). The SPEI is based on precipitation and temperature data, and it has the advantage of combining multiscalar character with the capacity to include the effects of temperature variability on drought assessment. The procedure to calculate the index is detailed and involves a climatic water balance, the accumulation of deficit/surplus at different time scales, and adjustment to a log-logistic probability distribution. Mathematically, the SPEI is similar to the standardized precipitation index (SPI), but it includes the role of temperature. Because the SPEI is based on a water balance, it can be compared to the self-calibrated Palmer drought severity index (sc-PDSI). Time series of the three indices were compared for a set of observatories with different climate characteristics, located in different parts of the world. Under global warming conditions, only the sc-PDSI and SPEI identified an...
5,088 citations
TL;DR: The benefits of the new, re-designed DSSAT-CSM will provide considerable opportunities to its developers and others in the scientific community for greater cooperation in interdisciplinary research and in the application of knowledge to solve problems at field, farm, and higher levels.
3,339 citations
TL;DR: There are two categories of environmental changes with altitude: those physically tied to meters above sea level, such as atmospheric pressure, temperature and clear-sky turbidity; and those that are not generally altitude specific, suchAs moisture, hours of sunshine, wind, season length, geology and even human land use.
Abstract: Altitudinal gradients are among the most powerful 'natural experiments' for testing ecological and evolutionary responses of biota to geophysical influences, such as low temperature. However, there are two categories of environmental changes with altitude: those physically tied to meters above sea level, such as atmospheric pressure, temperature and clear-sky turbidity; and those that are not generally altitude specific, such as moisture, hours of sunshine, wind, season length, geology and even human land use. The confounding of the first category by the latter has introduced confusion in the scientific literature on altitude phenomena.
2,130 citations
Paul Sabatier University1, National Autonomous University of Mexico2, Carnegie Institution for Science3, University of São Paulo4, National University of Colombia5, Norwegian University of Life Sciences6, Amazon.com7, University of Leeds8, United Nations9, Smithsonian Tropical Research Institute10, University of Edinburgh11, Costa Rica Institute of Technology12
TL;DR: This work analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types, and found a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height.
Abstract: Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees ≥ 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development.
1,750 citations
Cites methods from "Crop evapotranspiration : guideline..."
...…local allometric equations, we also extracted monthly values of reference evapotranspiration (ET), as computed by the FAO Penman–Monteith equation (Allen et al., 1998) at a 10 arc min resolution from a mean © 2014 John Wiley & Sons Ltd, Global Change Biology, 20, 3177–3190 monthly climatology…...
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TL;DR: In this article, improved crop coefficients for various Pacific Northwest irrigated crops were developed for estimating crop evapotranspiration (ET) from estimates or measurements of reference ET, based on that for well watered, actively growing alfalfa with sufficient growth for near maximum ET in arid, irrigated regions.
Abstract: Improved crop coefficients for various Pacific Northwest irrigated crops were developed for estimating crop evapotranspiration (ET) from estimates or measurements of reference ET. Reference ET was based on that for well watered, actively growing alfalfa with sufficient growth for near maximum ET in arid, irrigated regions. ET for the alfalfa reference crop and other crops was measured with sensitive weighing lysimeters at the field site near Kimberly, Idaho. The new crop coefficients are basal or minimal coefficients for conditions when soil evaporation is minimal but root-zone soil moisture is adequate. When combined with improved estimates of evaporation from wet soils, they should permit more accurate estimates of daily crop ET, more accurate irrigation scheduling, and more reliable estimates of crop water requirements. Curves were developed for alfalfa, potatoes, snap beans, sugarbeets, peas, sweet and field corn and winter and spring cereals.
614 citations
"Crop evapotranspiration : guideline..." refers background in this paper
...Wright (1993) found that ETc averaged 1 mm/day over winter periods at Kimberly, Idaho, the United States, that were six months long (1 October to 30 March)....
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...FIGURE 47 Mean evapotranspiration measured during non-growing, winter periods at Kimberly, Idaho, United States by Wright (1993)...
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TL;DR: In this paper, measurements were made in a wind-tunnel of the drag on elements of a simply-structured artificial crop, and of the wind profiles above and within the crop.
Abstract: Measurements were made in a wind-tunnel of the drag on elements of a simply-structured artificial crop, and of the wind profiles above and within the crop. Analysis demonstrates
(i)that the drag force on an element of such an array can be calculated from the profile of the turbulent shear flow within the array, using the known (and unmodified), wind-tunnel value of the drag coefficient of the individual element;
(ii)that the zero-plane displacement (d) of an aerodynamically rough surface can be identified with the level of action of the drag on its elements; and
(iii)that von Karman's constant = 0.41 ± 0.03.
The relation z0 = 0.36 (h – d) is suggested for the roughness parameter of vegetation of height h.
Calculated values of the drag force, f, on unit column of a real stand of beans in the field, using individual-element drag coefficients (Cd) and measured wind speeds, give f = 3.5 τ0 where τ0 is the downward momentum flux derived from the shape of the wind profile above. On the evidence of conclusion (i) and the dense and complex nature of the bean canopy, the factor 3.5 is attributed to mutual sheltering of neighbouring canopy elements rather than as evidence that the Cd – values are modified by turbulent shear flow.
For the artificial crop, and for the real crop, recognition of the wind-speed dependence of the individual-element drag coefficients gives values of eddy viscosity, KM, almost constant in the height range h/3 < z ⩽ h and significantly larger than those found when constant drag coefficients are assumed. Constant KM within a crop canopy is consistent with the wind profile u(z)/u(h) = {1 + α(1 – z/h)}−2: an explicit expression is given for the parameter α.
573 citations
TL;DR: In this paper, a general expression for B−1 in terms of the exchange characteristics of the individual elements of a vegetative canopy is derived, which does not contain the surface roughness parameter Z0.
Abstract: Vegetation is treated as a complex surface roughness to which the transfer of mass or heat encounters greater aerodynamic resistance, γP, than the transfer of momentum, γD. The excess resistance (γP – γD) is equated to B/u*, where B−1 is the non-dimensional bulk parameter introduced by Owen and Thomson (1963) and used by Chamberlain (1966, 1968). A general expression is obtained for B−1 in terms of the exchange characteristics of the individual elements of a vegetative canopy: this expression does not contain the surface roughness parameter Z0.
Using exchange coefficients of individual bean leaves (Thom 1968) and the bulk momentum absorption properties of a particular bean crop (Thom 1971) the relation B−1 = (constant) u*1/3 is derived. With u* in cm s−1, the constant is 1.35 for heat exchange and transpiration, 2.18 for CO2 exchange, and 1.13 for evaporation from the crop when wet. It is suggested, partly on the basis of the lack of dependence of B−1 on z0, that the same set of equations may provide a first approximation to B−1 for many types of vegetation.
Demonstrated are (i) that Monteith's (1963) method of extrapolating to zero wind speed to determine representative surface values of vapour pressure and of temperature (es and Ts) is much more rigorous if extrapolation is made to u = −B−1u* rather than to u = 0; and (ii) that the surface resistance γS, proportional to (ew(Ts) − es) (Monteith 1965) exceeds the bulk physiological, or stomatal, resistance γST of vegetation by an amount {1 − (Δ/γ).β}.B−1/u*, significant only when the Bowen ratio β is less than about 3/4(γ/Δ). (γ = 0.66 mb °C−1; Δ = dew/dT.) In particular, for B−1 = 4 and β = 0: (i) γST = 1/3 to 1/2 of γS; and (ii) use of γS with γD in the Penman equation (instead of γS, with which γD is compatible) overestimates λE by about 15 per cent.
513 citations
01 Jan 1991
471 citations