The objective of this study was to obtain the weekly crop coefficient (Kc) for tomato seedlings in substratum with waterproofing polymer and irrigated by automatic system of localized irrigation. The experiment was conducted in an agricultural greenhouse located in the irrigation experimental area of the Department of Agricultural Engineering, Viçosa-MG, Brazil. The crop coefficient (Kc) for tomato seedlings was obtained for the conditions of the experiment (in plastic trays of 128 cells, in

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... enhouse, in Viçosa-MG, for the spring/summer season), using the substrates with and without hydrogel. The irrigation system was micro sprinkler, consisting of four emitters per tray of seedlings, so that each tray constitutes an individualized experimental unit. The method for estimation of reference evapotranspiration was Penman-Monteith FAO56. The weighing lysimetry was used to obtain the variation of the tray mass from the evapotranspiration of the culture. Two consecutive daily weights, spaced at intervals of 60 min (during the morning shift), were performed for five sample units (SU) arranged in a completely randomized design in the greenhouse. Each SU was composed of a tray of 128 cells. The values of Kc obtained were used to calibrate the crop evapotranspiration (ETc) estimates used in the later stages. Weekly Kc values were generated. Irrigation was managed so that the crop did not undergo water stress during the moulting phase. For the conditions tested it was possible to obtain Kc values between 0.72 and 2.86 for the substrate without hydrogel and 0.81 to 3.46 for the substrate with hydrogel. It should be considered that the production stage of seedlings is very demanding in water and in protected environment conditions it is possible to obtain high Kc values.