SPATIAL VARIABILITY OF PHOTOSYNTHETICALLY ACTIVE RADIATION IN A GREENHOUSE

Photosynthetically active radiation (PAR) was measured at two times of day (8:00 AM and noon Central Standard Time) in a 915 x 915-cm area o f a 1006 x 915-cm gable roof greenhouse, PAR measurements were taken a cross a grid at 40-cm intervals, a total of 529 data points, Spatial v ariation of PAR in the greenhouse was evaluated through contour plots and the geostatistical technique of semivariogram construction, Semiva riograms provide a visual guide to the degree of spatial correlation o f a variable, allowing a quantification of the distance at which varia bles cease to be spatially correlated (the range) Measured PAR contain ed distinct zones of lowered values, a function of overhead greenhouse structures, wall-hung electrical boxes, and tall plants in adjacent g reenhouses, Although the amount of PAR changed over time, zones of hig h and low PAR remained relatively constant, except at the sides of the greenhouse, Constructed semivariograms revealed that PAR contained st rong spatial correlation (up to a 350-cm separation) as measured in th e north-south direction, moving parallel to greenhouse bench placement , When PAR measurements perpendicular to benches (east-west) were used in directional semivariograms PAR was found to be completely random, plotting as a horizontal Line called a nugget effect, Thus, plants pla ced perpendicular to the greenhouse benches (east-west) would not be a ffected by the spatial correlation of PAR.