| The North China Plain (NCP) Located in the semi-humid and semi-arid regions of eastern China is one of the most important agricultural regions in china. In this area, the scarcity of water resources had become the important factor limiting the development of agricultural production potential and crop normal growth and reproduction. Accurate determination of irrigation water supply and irrigation time and developing Water-saving Agriculture were necessary for reducing water resources waste, sustainable development and environmentally sound water management. Monitoring crop and soil water condition by remote sensing technology is a promising tool to resolve the above-mentioned questions.Field studies were conducted at luancheng Agro-ecological Experimental Station located in the farmland of Taihang Piedmont, to investigate utility of remotely sensed canopy temperature to assess crop water stress and to quantify terminal water stress in winter wheat during the spring of 2001-2002. Canopy-air temperature differential were measured by thermal IR thermometer, and evapotranspiration were measured by eddy correlation system. Factors effecting on canopy temperature, diurnal and season pattern of water, heat and COa flux were analyzed at field scale. The statistical model for estimating daily evapotranspiration of winter wheat field in North China Plain was established based on the data of canopy-air temperature. The results showed that:The diurnal variation of evaporation fraction(EF) was different for the difference of soil water condition and crop growth potential, such as "U" shape, reciprocal "S" shape, or the pattern that EF increase quickly from 6:00 A.M to 8:00 A.M, slowly or not after 8:0~9:00A.m under good soil moisture condition. In diurnal variation, transient stress is manifested by a plateau in the evapotranspiration during the middle part of the day under clear day and lack of enough soil water content. In the diurnal pattern, there was positive linear correlation between daytime evaporation rate and available energy, but negative linear correlation between nocturnal values. Under water stress not existing, the diurnal variation of CO2 flux is "U" shape, CO2 flux was closely linked to net radiation flux, latent heat flux and canopy temperature. In daytime, there was negative linear correlation between CO2 flux and canopy temperature, CO2 flux, from atmosphere to canopy, increased gradually with canopy temperature increasing range from 18 ? to 24?; but, positive correlation between them in nocturnal stage, and CO2 flux, from canopy to atmosphere, increased with canopy temperature increasing.Surface temperature was determined by many factors, such as crop growth potential, radiation and soil water conditions et.al. The relationship between surface temperature and air temperature at field in diurnal variation was remarkable different for the difference of leaf area index, it's good linear correlation between them at 0.01 level during the beginning growth stage, but complex at reproduction development stage. Net radiation flux is the mainly important energy factor effecting on canopytemperature, and negative correlation between wind speed and canopy-air temperature was significant at 0.01 level. Under fractional vegetation cover, surface temperature at 13:40 time could detect soil water condition in the depth of Ocm~5cm; After reviving of winter wheat(LAI#3 ), canopy-air temperature difference at 13:30-14:00 time could better response to water stress, but it could not detect soil water condition as fact in clear day if soil water stress existed after the amount of precipitation is low 3mm.Under clear day , surface-air temperature difference closely link to soil water content in depth of Ocm~5cm, net radiation flux, sensible heat flux and saturation vapor pressure deficit et.al before over-wintering of winter wheat; Canopy-air temperature difference (Tc-Tamax) were significantly (0.01) related soil water content in depth of Ocm~ 100cm and evapotranspiration during reproductive ontogeny. Signifi...
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