Canopy reflectance can be useful indicator of crop health status. Canopy stress index (CSI) is usually expressed as canopy temperature minus air temperature, and this value is higher and a positive number in a well irrigated wheat field. Three main environmental variables constructing CSI are: plant canopy temperature (Tc), air temperature (Ta) and atmospheric vapor pressure deficiency (VPD). CSI is effected by biological and environmental factors such as soil water status, wind speed, evapotranspiration, conduction systems, plant metabolism, air temperature, relative humidity, etc. which all influence on final yield. This paper aims to investigate the relation of CSI calculated by Landsat images and wheat yield. So, eighteen wheat fields were selected for two years (2009 and 2010) and 5 Landsat images (TM and ETM+) from April to Jun were used to monitor field status in each year. Tc was calculated by applying single-channel method and VPD was computed from Tc, air temperature and humidity. Each single Landsat bands and CSI were defined as the descriptor variables. Relation between wheat yield and the descriptors was assessed by means of linear correlation. The results of stepwise correlation depicted that band 1 (blue) and 3 (red) had the most correlations to yield until grain filling stage. This reflects the importance of photosynthesis rate which absorb blue and red wavelength during mentioned period. This two bands also could capture yield changes (r2=0.77). However, during grain filling period CSI was the only descriptor determining yield volatility (r2=0.85). Low temperature is one of the key factors which increase remobilization of carbohydrate to grain. Therefore, grain yield in the canopy which has less temperature in compared to air temperature would be higher than others.

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