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Abstract:Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely sensed vegetation indices (VI) that measured the actual status of the crop on a field-by-field basis. VIs measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance-based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI-based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under optimal conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time-series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible-NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. The primary advantage of VI methods for estimating ET is that transpiration is closely related to radiation absorbed by the plant canopy, which is closely related to VIs. The primary disadvantage is that they cannot capture stress effects or soil evaporation.

Summary• The likely consequences of future high levels of atmospheric CO 2 concentration on wheat ( Triticum aestivum L.) grain nutritional and baking quality were determined.• Two free-air CO 2 enrichment (FACE; 550 mmol mol − 1 ) experiments were conducted at ample (Wet) and limiting (Dry) levels of irrigation, and a further two experiments at ample (High-N) and limiting (Low-N) nitrogen concentrations. Harvested grain samples were subjected to a battery of nutritional and bread-making quality tests.• The Dry treatment improved grain quality slightly (protein +2%; bread loaf volume +3%). By contrast, Low-N decreased quality drastically (protein − 36%; loaf volume − 26%). At ample water and N, FACE decreased quality slightly (protein − 5%; loaf volume − 2%) in the irrigation experiments and there was no change in the nitrogen experiments. At Low-N, FACE tended to make the deleterious effects of Low-N worse (protein − 33% and − 39%, at ambient CO 2 and FACE, respectively; loaf volume − 22% and − 29% at ambient CO 2 and FACE, respectively).• The data suggest that future elevated CO 2 concentrations will exacerbate the deleterious effects of low soil nitrogen on grain quality, but with ample nitrogen fertilizer, the effects will be minor.

Abstract. In order to determine the likely effects of the increasing atmospheric GO 2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550/xmol/mol CO2 (or 200/xmol/mol above current ambient levels of about 360/xmol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, R n; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime T s about 0.6 ø and 1.1øC at high and low N, respectively. Daily total R n was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively. IntroductionThe CO 2 concentration of the atmosphere is increasing, and climate modelers have predicted a consequent global warming as well as changes in precipitation patterns. The report of the IPCC [Intergovernmental Panel on Climate Change, 1996] projects CO2 increasing from present day concentrations of about 360/xmol/mol to about 500/xmol/mol by the end of the next century if emissions are maintained at 1994 levels. They further project that the increase in CO2 plus that of other radiatively active "greenhouse" gases (methane, nitrous oxide, chlorofluorocarbons (CFCs), ozone) will cause an increase in global mean temperature of 0.9 ø to 3.5øC depending on future emission rates. Some regions might receive increases in precipitation, while others might receive less. However, these projected changes in climate are very uncertain.Increasing CO2 concentration has been shown to cause partial closure of plant leaf stomata, which reduces the conductance of water vapor from inside the leaf stomatal cavities to the outside air [Morison, 1987] [1997] exposed grassland to elevated CO2 using open-top chambers and attempted to measure ET with smaller gas-exchange chambers. They found reductions of ET of 12-39% due to elevated CO2 on sandstone-derived soil, but on serpentinederived soil, ET actually increased from -1% to + 14%. They believe that the latter increase is because the serpentine canopy was sparse, so there was more E (evaporation from soil) than T, and E would not be affected by CO2. Thus the prior experimental work using chambers has been somewhat variable but explainable based on leaf area growth, canopy, and stomatal effects. Except for the results of Dugas et al. [1997], however, generally the effects of CO2 on ET have been small. 1179

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This study reviews the abundant research on FAO56 crop coefficients, published following introduction of the FAO56 paper in 1998. The primary goal was to evaluate, update, and consolidate the mid-season and end-season single (K c ) and basal (K cb ) crop coefficients, tabulated for many field crops in FAO56. The review found that the prevalent approach for estimating crop evapotranspiration (ET c ) is the FAO56 K c -ET o approach, i.e., the product of the K c and reference evapotranspiration (ET o ). The FAO56 K c -ET o approach requires use of the FAO56 PM-ET o grass reference equation with appropriate crop-specific K c and/or K cb . Reviewed research provided various approaches to determine K c and K cb and used a variety of actual crop ET (ET c act ) measurements. Significant attention was placed on accessing the accuracy of the field measurements and models used in these studies. Accuracy requirements, upper limits for K c values, and related causal errors are discussed. Conceptual approaches relative to K c transferability requirements are provided with focus on standard crop conditions and use of the FAO56 segmented K c curve. Papers selected to update K c ∕K cb used the FAO56 PM-ET o , provided accurate measurements to determine and partition ET c act , and satisfied transferability requirements. Selected observed K c and K cb values were converted to standard, sub-humid climate as adopted in FAO56. Observed values, with respect to tabulated FAO56 K c and K cb , were used in consolidating updated values for crops within general categories of grain legumes, fiber crops, oil crops, sugar crops, small grain cereals, maize and sorghum, and rice. Ancillary data, e.g., maximum root depth and crop height, were also collected from selected literature and tabulated. Results showed good agreement between updated and original tabulated FAO56 K c and K cb , confirming the reliability of the FAO56 values. This indicates change in the K c (ET c /ET o ratio) of crops has not occurred due to climate change during the past ≈sixty years. New K c ∕K cb data for crops, not included in FAO56, are also now presented for several oil crops and pseudocereals. The approach adopted for rice differs from FAO56 because consideration was given to the numerous rice water management practices currently used and, thus, K c ∕K cb values for the initial season of rice were also presented. The review also observed that many research papers did not satisfy the adopted requirements in terms of ET o method and/or the accuracy of ET c act determinations and, therefore, could not be used. Thus, emphasis is placed on adopting improved accuracy and quality control in future research aimed at determining K c data comparable to presented values. The transferability of standard K c and K cb has been assured for the values tabulated herein. Improved future applications of the FAO56 K c -ET o method should consider remote sensing observations when available, particularly in defining crop growth stages at given locations.

Nitrogen fertilizer use efficiency (NUE) is low in surface-irrigated cotton (Gossypium hirsutum L.), especially when adding N to irrigation water. A NO 3 soil-test algorithm was compared with canopy reflectance-based N management with surface-overhead sprinkler-irrigation in central Arizona. The surface irrigation studies also compared fertigation of N fertilizer with knifing-in of N and the addition of a urease and nitrification inhibitor (Agrotain Plus, Koch Agronomic Services, Wichita, KS) to urea ammonium nitrate (UAN). Cotton lint and seed yields responded positively to N fertilizer in all four site-years. Recovery efficiency (RE) of N at low N fertilizer rates (60 to 76 kg N ha-1) ranged from 21 to 61% with surface irrigation and from 81 to 97% with overhead sprinkler irrigation. Deep percolation below 1.8 m was 4 to 11% of applied surface irrigations and rain, but was undetectable in the overhead sprinkler. Leaching of NO 3 was apparently the largest N loss pathway in the surface-irrigated system. Fertigating UAN into surface irrigation resulted in similar lint yields and RE as knifing UAN. Use of Agrotain Plus with UAN gave similar yields and RE as using UAN alone. Reflectance-based N management using normalized difference vegetation index-amber (NDVIA) saved 50% of N fertilizer of the full soil-test based dose without a yield reduction in three of four site-years. Nitrogen fertilizer was over-prescribed with the soil-test-based treatment. This may have been due to not accounting for N mineralization, which the reflectance method indirectly measures.