Simulating the irrigation requirements of a greenhouse crop

Most UK production of protected vegetable crops uses the rockwool run-to-waste system in which considerably more water is applied than required for plant transpiration and for drainage (to prevent the build-up of harmful salts). Direct measurements of irrigation requirements are difficult to obtain and therefore estimates are required which can be based on easy-to-measure environmental variables. A greenhouse simulation model is used to generate data for a long season tomato crop. Hourly values of transpiration are related to leaf area index, solar radiation and inside saturation deficit. Solar radiation accounts for most of the variation in transpiration with saturation deficit important at certain times of the year. Combinations of these two variables account for 98.5% of the variation for the period of the season when there is a complete crop canopy. When the canopy is incomplete, a simple model based on thermal time accurately predicts crop water use in the simulation. The use of the regression model to predict transpiration of other crops is discussed.