Crop evapotranspiration and crop coefficients of Viburnum Odoratissimum (Ker.-Gawl)

Determination of irrigation requirements of nursery plants is becoming more important as the nursery growers are being challenged to conserve water and minimize runoff while meeting plant water needs. Historically, not enough attention has been given to the measurement of crop evapotranspiration (ET_c) and crop coefficients (K_c) of ornamental plants for irrigation scheduling, water management, and allocation. Since measurement of K_c is a difficult and time consuming task, especially for the container-grown ornamentals, there is a need to develop methods for estimating K_c from more easily obtainable variables. The ET_c and K_c values of Viburnum odoratissimum (Ker.-gawl) grown in white and black Multi-Pot Box System (MPBS) were measured in two growing seasons (summer and fall). The relationship between the K_c and week after transplanting (WAT), growth index (GI), cumulative reference evapotranspiration (ET_o), and fraction of thermal units (FTU) were developed and analyzed to find out whether these parameters could be used as the base scales to estimate K_c values. The measured ET_c ranged from 308 mm for the black MPBS plants in summer to 351 mm for the white MPBS plants in fall. Good relationships between K_c versus WAT, GI, cumulative ET_o, and FTU were observed in both seasons (r² ≥ 0.93). These relationships were explained by third order polynomial equations in both growing seasons. Specifically, GI and FTU values were found to be very useful, practical, and effective base scales in estimating K_c from transplanting to physiological maturity (marketable size) of V. odoratissimum. The relationships between the K_c versus base scale variables were significantly affected by the season. Thus, the equations developed are season-specific and should be used accordingly. The season-specific characteristic of the equations was attributed to the differences in growth rates (exponential in summer and linear in fall) of the plants grown in the white and black MPBS in the summer and fall seasons. Equations developed to estimate K_c would have advantages over the traditional method of estimating K_c from measured ET_c and ET_o for nursery crops since, in practice, measuring and/or estimating WAT, GI, ET_o, and FTU are easier than measuring ET_c and K_c. Experimentally derived K_c values and relationships developed in this study add useful information to the nursery growers, researchers and water management districts in terms of collection of ET_c and K_c data that can be utilized for irrigation scheduling and water resources planning and management, and are among the first ET_c and K_c data for the V. odoratissimum.