Documenting crop senescence rates is often difficult because of the need for frequent sampling during periods of rapid change and the subjective nature of human visual observations. The purpose of this study was to determine the feasibility of using images produced by a digital camera to measure the senescence rate of wheat and to compare the results with changes in greenness determined by two established methods. Measurements were made as part of an experiment to determine the effects of elevated C02 and limited soil nitrogen on spring wheat (Triticum aestivum L.) at the University of Arizona's Maricopa Agricultural Center, near Phoenix, AZ. “Greenness” measurements were made during senescence of the crop with a color digital camera, a hand‐held radiometer, and a SPAD chlorophyll meter. The green to red (GIR) for each pixel in an image was calculated and the average GIR computed for cropped images from a digital camera representing 1 m2 for each treatment and sample date. The normalized difference vegetation index (NDVI) was calculated from the red and near‐infrared canopy reflectances measured with a hand held radiometer. A SPAD reading was obtained from randomly selected flag leaves. All three methods of measuring plant greenness showed similar temporal trends. The relationships between GIR with NDVI and SPAD were linear over most of the range of G/R. However, NDVI was more sensitive at low values than G/R. GIR was more sensitive above GIR values of 1.2 than SPAD because the upper limits of SPAD measurements were constrained by the amount of chlorophyll in the leaf, while GIR responded to both chlorophyll concentration in the leaves as well as the number of leaves present. Color digital imaging appears useful for quantifying the senescence of crop canopies. The cost of color digital cameras is expected to decrease and the quality and convenience of use to improve.

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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The application of plant nutrients with irrigation water is an efficient and cost-effective method for fertilizer application to enhance crop production and reduce or eliminate potential environmental problems related to conventional application methods. In this study, a combined overland water flow and solute transport model for analysis and management of surface fertigation/chemigation is presented. Water flow is predicted with the well-known Saint-Venant's equations using a control volume of moving cells, while solute transport is modeled with the advection-dispersion equation. The 1D transport equation was solved using a Crank-Nicholson finitedifference scheme. Four, large-scale, field experiments were conducted on blocked-end and free draining furrows to calibrate and verify the proposed model. The results showed that application of solute during the entire irrigation event, or during the second half of the irrigation for blocked end conditions with appropriate inflow rates, produced higher solute uniformity than application of solute during the first half of the irrigation event. Measured fertilizer distribution uniformity of the low quarter ranged from 21 to 76% while fertilizer distribution uniformity of the low half values varied between 62 to 87%. The model was subsequently applied to the experimental data; results showed good agreement with all field data. Water balance errors for the different experiments varied from 0.004 to 1.8%, whereas fertilizer mass balance errors ranged from 1.2 to 3.6%. A sensitivity analysis was also performed to assess the effects of longitudinal dispersivity parameter on overland solute concentrations. A value of 10 cm for dispersivity provided a reasonable fit to the experimental data.