(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

Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth (cell division and enlargement) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: Th...

https://www.jstor.org/stable/pdfplus/2830650.pdf

The dilemma of plants: To grow or defend.

Contents

 Summary 1

I. What is FACE? 2
II. Materials and methods 2
III. Photosynthetic carbon uptake 3
IV. Acclimation of photosynthesis 6
V. Growth, above-ground production and yield 8
VI. So, what have we learned? 10

 Acknowledgements 11


 References 11


Appendix 1. References included in the database for meta-analyses  14


 Appendix 2. Results of the meta-analysis of FACE effects 18



Summary
Free-air CO2 enrichment (FACE) experiments allow study of the effects of elevated [CO2] on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer-reviewed articles describing physiology and production in the 12 large-scale FACE experiments (475–600 ppm) were collected and summarized using meta-analytic techniques. The results confirm some results from previous chamber experiments: light-saturated carbon uptake, diurnal C assimilation, growth and above-ground production increased, while specific leaf area and stomatal conductance decreased in elevated [CO2]. There were differences in FACE. Trees were more responsive than herbaceous species to elevated [CO2]. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated [CO2] may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric [CO2]; but even with FACE there are limitations, which are also discussed.

What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2.

A remote sensing surface energy balance algorithm for land (SEBAL)-1. Formulation

http://cursosihlla.bdh.org.ar/curso%20posgrado-tdi1/3_Bibliografia/1997%20-%20RSE_Pv%20Carlson.pdf

On the relation between NDVI, fractional vegetation cover, and leaf area index

Canopy temperatures, obtained by infrared thermometry, along with wet- and dry-bulb air temperatures and an estimate of net radiation were used in equations derived from energy balance considerations to calculate a crop water stress index (CWSI). Theoretical limits were developed for the canopy air temperature difference as related to the air vapor pressure deficit. The CWSI was shown to be equal to 1 - E/Ep, the ratio of actual to potential evapotranspiration obtained from the Penman-Monteith equation. Four experimental plots, planted to wheat, received postemergence irrigations at different times to create different degrees of water stress. Pertinent variables were measured between 1340 and 1400 each day (except some weekends). The CWSI, plotted as a function of time, closely paralleled a plot of the extractable soil water in the 0- to 1.1-m zone. The usefulness and limitations of the index are discussed.

Canopy temperature as a crop water stress indicator

Canopy temperatures were measured on durum wheat grown in six differentially irrigated plots. Soil water content was measured by using a neutron-scattering technique at two locations within each plot. Water contents, in 20-cm increments to 160 cm, were determined two to five times per week. Using a sliding cubic smoothing technique, we calculated daily water contents and thus water depletion rates for the entire growing season. Canopy temperatures were measured daily between 1330 and 1400 hours. Air temperatures measured at 150 cm above the soil surface were subtracted from the canopy temperatures to form the difference Tc – Ta. The summation of Tc – Ta over time yielded a factor termed the ‘stress degree day’ (SDD). The SDD concept shows promise as an indicator for determining the times and amounts of irrigations. An expression relating evapotranspiration (ET) to net radiation and Tc – Ta was simplified and tested by using ET measurements with a lysimeter. The expression was used to predict water use by wheat in the six plots. Predicted ET and measured water used agreed reasonably well. The expression may be useful in determining amounts of irrigation water to apply.

Wheat canopy temperature: A practical tool for evaluating water requirements

Relations between evaporation coefficients and vegetation indices studied by model simulations

Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research

Simple albedo measurement may prove useful for sensing surface soil water content and as a research tool in the study of evaporation of water from soil. Intensive concurrent measurements of the albedo and soil water content of a drying bare soil indicate that albedo, normalized for sun zenith angle effects, is a linear function of the soil water content of a very thin surface layer (less than 0.2 cm thick) over a sizeable volumetric water content range (0.00 to 0.18 for an Avondale loam). Albedo is also well correlated with the average soil water content of greater soil thicknesses. Measurements to a depth of 10 cm indicate that the relation is relatively independent of season.

Sherwood B. Idso

Papers

Canopy temperature as a crop water stress indicator

Wheat canopy temperature: A practical tool for evaluating water requirements

Relations between evaporation coefficients and vegetation indices studied by model simulations

Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research

The Dependence of Bare Soil Albedo on Soil Water Content.