TRANSPIRATIONAL COOLING OF A GREENHOUSE CROP WITH PARTIAL GROUND COVER

The well-being of sparsely planted seedlings in greenhouses under high energy loads is a matter of concern, since transpirational (cooling) flux of the small plants is rather limited. A simplified analysis of t he problem suggests the following conclusions. (1) If water supply to the stomata is not limiting, then, for a given ventilation rate, the c anopy temperature of a sparse (young) crop should normally not be high er than that of a dense (mature) crop. (2) For a given canopy temperat ure, sparse plants transpire more per unit leaf area, due to micro-adv ection of energy 'surplus' from the surrounding bare and dry ground (l ocal oasis effect). (3) High foliar potential transpiration may lead t o water stress. Relieving this stress may justify control measures, su ch as artificial evaporative cooling or increased ground albedo. (4) A rtificial evaporative cooling (e.g. wet pad), is only effective when s tomatal resistance is considerable and outside air is dry. (5) Tempora ry storage of heat in passive systems has a cooling effect similar to increased ground albedo.