Given the importance of greenhouse cultivation, accurate and precise estimates of water requirement of greenhouse crops in order to have a good and effective management of irrigation in controlled conditions is inevitable. This study has also done with the purpose to identify the impact of deficit irrigation tension and salinity on water requirement and crop coefficient of greenhouse bell pepper during the winter and spring of 2013-2014 in the experimental greenhouse located in Ferdowsi University of Mashhad. Soil moisture has been measured at a depth of 10 cm of the pot by using REC-P55 sensors. The results of the present study has shown that evapotranspiration rate, soil moisture and irrigation water consumption have been reduced by increasing salinity and reducing irrigation water. Potential evapotranspiration amounts were almost constant during growth period. The dual crop coefficient values were calculated at different periods of growth for applied treatments in this study by examining the changes process of crop coefficient. Results show that crop coefficient value is decreased by increasing salinity and shortage of irrigation water. Results evaluation has shown that it is needed the coefficients related to the plant be estimated and planning irrigation carried on based on the situation in order to determine the water requirement. If applying tension (deficit irrigation / salinity), it is better the dual crop coefficient used instead of using a single crop coefficient to increase planning accuracy to determine the water requirement.

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