| الملخص: | This study was designed to evaluate different threshold values of crop water stress index (CWSI) to schedule irrigation for watermelon (Citrullus vulgaris) grown with drip irrigation Irrigations were started when CWSI values reached to 0.2, 0.4, 0.6, 0.8 and 1.0 (non-irrigation). The CWSI values were computed from measurements of canopy temperature, air temperature and vapor pressure defi cit. The total irrigations amount of 342, 280, 248 and 193 mm were applied to the 0.2, 0.4, 0.6 and 0.8 CWSI treatments, respectively. The maximum seasonal evapotranspiration (ET) as 412 mm was measured from 0.2 CWSI treatment. Irrigation levels signifi cantly affected fruit yield. Although the highest fruit yield (76.3 t ha-1) was obtained from the 0.2 CWSI treatment, the 0.4 and 0.6 of CWSI treatments were statistically in the same letter group with this treatment. Also, maximum water use effi ciency (WUE) and irrigation water use efficiency (IWUE) were obtained from 0.6 of CWSI treatment as 22.1 and 13.3 kg m-3, respectively. Therefore, based on these results, 0.6 of CWSI value should be used for irrigation time of watermelon under Tekirdag, Turkey conditions.
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