Crop water stress index for assessing irrigation scheduling of drip irrigated broccoli (Brassica oleracea L. var. italica)

This study was conducted to determine the possibility of a crop water stress index (CWSI) to schedule irrigation for broccoli (Brassica oleracea L. var. italica) grown with drip irrigation during the spring and autumn cultivation periods of 2007. The effects of five irrigation levels which were applied based on a ratio of Class A pan evaporation (kcp = 0, 0.50, 0.75, 1.00 and 1.25) with 7 days interval on broccoli yields and resulting CWSl were investigated. The highest yield and water use was obtained in the treatment of applied 50% of the evaporation measured in Class A pan for two seasons, while the effects of irrigation programs on yield was not statistically significant. The seasonal evapotranspiration in the treatments varied from 187 to 326 mm during the spring period and from 242 to 346 mm during the autumn period. The CWSI was calculated using the empirical approach from measurements of infrared canopy temperatures, ambient air temperatures and vapor pressure deficit values for five irrigation levels. An average threshold CWSI value of about 0.51 before irrigation produced the maximum yield. The yield was directly correlated with mean CWSI values and the linear equation “Y = −36.51(CWSI)2 + 32.958(CWSI) + 0.772” with a determination coefficient R2 = 0.99 and standard deviation Syx = 1.38 Mg ha−1 can be used for yield prediction. The soil water content, soil matric potential, above ground biomass and leaf area index values were also found to correlate better with CWSI as a key function for modeling the crop growth model. The CWSI value was useful for evaluating crop water stress in broccoli and should be useful for timing irrigation and predicting yield.