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Canopy temperatures, when measured remotely, offer a method of estimating evapotranspiration with surface energy balance models. Equations which have been developed by others have been evaluated only at a limited number of locations and with a few crops. Our study was conducted at several locations with weighing lysimeters with a variety of crops around the United States: Brawley, CA; Temple, TX; Lincoln, NE; St. Paul, MN; Fargo, ND; Kimberly, ID; and Davis, CA, to evaluate evapotranspiration utilizing canopy temperature as an input into the surface energy balance. The results show that evapotranspiration calculated from the aerodynamic resistance form of the surface energy balance was well correlated with lysimeter measurements at all locations. The errors using the surface energy balance were less than 10% in all cases for full ground cover. The Bartholic—Namken—Wiegand method was more closely coupled to net radiation than canopy temperature.Under partial canopy cover, differences between the two models were apparent. The Bartholic—Namken—Wiegand model overpredicted when the actual evapotranspiration was above 200 W m?2 because of its insensitivity to surface temperature. However, the surface energy balance model exhibited only a slight overprediction above 200 W m?2 when a weighed composite surface temperature (representative of bare soil and crop temperature) was used. This small overprediction could be overcome by considering the soil heat flux term. There was no location bias in the surface energy balance model, which shows that it should work well at other locations. 相似文献
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Using the acetyl bromide assay to determine lignin concentrations in herbaceous plants: some cautionary notes 总被引：7，自引：0，他引：7
The acetyl bromide assay was developed to provide a rapid and sensitive method for quantifying lignin in woody plant species. The original procedure cautioned against prolonged reaction times and advised keeping the reaction temperature at 70 degrees C to prevent excessive carbohydrate degradation that would skew the absorption spectra. Characterization of the reaction conditions revealed that the acetyl bromide reagent readily degrades xylans, a prominent polysaccharide group within all lignified plants. This degradation results in increased absorbance in the 270-280 nm region that is used to quantify lignin. The degradation of xylans is temperature dependent and is exacerbated by the addition of perchloric acid. Lowering the reaction temperature to 50 degrees C and increasing the reaction time from 2 to 4 h allows complete lignin solubilization but minimizes degradation of the xylans. 相似文献
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Lignin extracted with acidic dioxane was investigated as a possible standard for quantitatively determining lignin content in plant samples using the spectrophotometric method employing acetyl bromide. Acidic dioxane lignins were analyzed for carbohydrate, total protein, nitrobenzene oxidation products, and UV spectral characteristics. Total carbohydrate content of isolated lignins ranged from 2.21 to 5.70%, while protein ranged from 0.95 to 6.06% depending upon the plant source of the original cell wall sample. Nitrobenzene analysis indicated differences in the amount of guaiacyl and syringyl units making up the lignins, but this did not alter the UV spectrum of lignin solubilized in acetyl bromide. Regression equations developed for the acetyl bromide method using the isolated lignins for all the plant samples were similar to each other. Lignin values obtained by the acetyl bromide method were similar to the lignin values obtained as acid insoluble residues following a Klason lignin procedure. 相似文献
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J. L. Hatfield 《Water, air, and soil pollution》1979,12(1):73-81
The area of land which man manipulates through cultivation is 3.2×109 ha; however, the impact from this system has been large. The soil-plant-atmosphere complex is very dynamic with the plant continually growing throughout a constantly changing atmospheric environment, i.e., radiation, temperature, and precipitation. However, a basis for all exchanges is the energy balance and all changes are made through this balance. Two impacts of this system are realized in water and wind erosion. Both are detrimental to agricultural land and proper management of the system alleviates the impact of these two forces. Other changes which occur are reduction in evaporation and soil temperature under crop cover. There is needed, however, a more thorough understanding of the energy exchanges throughout the growing season, especially as it applies to feed-backs between plant response and the environment, soil derived aerosols, and biogenic ice nuclei. These will provide a better understanding of how we manipulate this complex for maintenance of our world's food supply. 相似文献
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