Interest in reducing irrigation volumes during production of landscape ornamental plants has increased worldwide as population pressures on freshwater supplies have increased (Bacci et al., 2008; Caceres et al., 2008; Jury and Vaux, 2005). Early work by Burger et al. (1987) and Regan (1997) calculated crop coefficients (Kc) to relate woody plant actual ETA to Penman-Montieth-based reference evapotranspiration. However, these Kcs were based on a fixed value of container surface area; thus, they are accurate only for plants of the same canopy and container size. Regan (1997) attempted to remedy this by periodically calculating Kcs per species and cultivar as they grew from liners to market size plants in 3.8-L containers. Later, Schuch and Burger (1997) used Fourier curve transformations to calculate Kc, again based on upper container diameters for several species of woody ornamental plants to adjust for the quiescent period during the winter for plants that required two growing seasons to reach marketable size. Each species required a unique equation.
In 2004, a new model to calculate a crop coefficient, termed water needs index (WNI), was proposed (Beeson, 2004). The term was invented because nursery production and landscapes violate the conditions of a large uniform fetch required for calculating crop coefficients as defined previously (Doorenbos and Pruitt, 1977). WNI is calculated as a function of canopy closure of groups of plants, relating individual plant ETA to plant size and canopy ventilation and radiation (Beeson, 2005; Eq. 1):
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BeesonR.C.Jr2010bModeling actual evapotranspiration of Viburnum odoratissimum during production from rooted cuttings to market size plants in 11.4 L containersHortScience4512601264
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