AUTOMATION OF THE WATER SUPPLY OF GLASSHOUSE CROPS BY MEANS OF CALCULATION THE TRANSPIRATION AND MEASURING THE AMOUNT OF DRAINAGE WATER
There are a number of possibilities to come to automation and subsequently to a more accurate control of the water supply. Growers use water level regulators and in some cases balances (transpiration of one or more plants).
These methods are fairly suitable but they have disadvantages, mostly technical imperfections, breakdown, so that in most cases only a starting signal is given; consequently the water and leaching capacity have to be assessed empirically, and checking of the water supply and possible corrections are mostly not carried out. An additional disadvantage is that a measuring set up is not always representative for an entire glasshouse or part of a glasshouse.
Further possibilities are offered by a new method in which the water requirement is controlled automatically with the aid of a model. The crux of this method is that the water supply is controlled by means of a water supply model. The water supply model consists of calculation of the transpiration (transpiration model), a quantity of water for leaching (leaching model) and a small amount of water for crop growth (growth, water uptake model). The transpiration is calculated on the basis of glasshouse climate data (short wave radiation, glasshouse air temperature and pipe temperature), plant size and sometimes also the outdoor climate (wind direction and velocity). The leaching quantity is calculated on the basis of the quality of the irrigation water, the transpiration, and the EC of the rockwool slabs and the drainage water.
Checking whether the water supply is correct is essential and takes place by measuring the amount of drainage water. This is done automatically by means of level measurements in a reservoir in which also EC and pH are recorded. Possible deviations from the desired amount of drainage water (water supply) are corrected by the