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The time between waterings needs to be adjusted to match the transpiration rate of the crop. The more the crop is transpiring, the shorter the interval between waterings. In a greenhouse, the transpiration of the crop closely matches the amount of solar energy that the crop is receiving. Other factors that have some influence on transpiration include air temperature, humidity and air movement. For crops grown outside, wind is a significant drying influence and should be taken into account but inside a greenhouse solar energy alone is accurate enough to use for predicting when the next watering should be applied. To use the suns energy to trigger the irrigation controller a solar integrator (sometimes called a calorie counter) is used. This has a solar sensor to measure the suns intensity and this is used to drive a counter (integrator) such that the brighter the sun the faster the count rate. When this count reaches the trigger setting and irrigation occurs and the counter is reset to zero (together with any timer override) Solar sensor measures the rate at which the energy is received and is often measured in Joules per square metre per second (J/m2/S) or, for photosynthetic active radiation, micromols per square metre per second (umol/m2/S) The integrator accumulates total energy and so it measures in either MJ/m2, J/cm2 or mol/m2. For a crop that intercepts all of the light entering the greenhouse (ie there is little direct sunlight reaching the floor) the transpiration will be approximately Transpiration = E x 0.2 / 1000 mL/m2 where E is measured in J/m2 and is measured outside OR Transpiration = EPAR x 0.2 / 2000 mL/m2 where E is measured in umol/m2 outside These calculations assume that the light level is measured outside and that the crop cover reduces this to 70%. If the light level is measured inside then the transpiration calculated above should be multiplied by 1.4 An example: Lets assume that you are growing tomatoes in coir or rockwool slabs. These are typically irrigated using drippers that deliver 50mL/min to each plant for 2 minutes. So, for every irrigation, each plant gets 100mL. Next let us assume that we have 2.5 plants to every square metre. That means that for every irrigation each sqare metre is going to get 250mL of nutrient enriched water. What we need to determine is how much energy is required to cause the crop to transpire 250mL. That will give us the setting to dial into the solar integrator irrigation trigger. To find this we need to rearrange the equations above so that we find the Enery required to transpire 250mL of water E = Transpiration/ (0.2/1000) = 250 * 1000 / 0.2 = 1250000 J/m2 or 1.25 MJ/m2 In order to achieve a 20% to 30% run off this should be reduced to about 0.9 MJ/m2 or 90J/cm2 For PAR sensors this becomes approximately E = Transpiration/ (0.2/2000) = 250 * 2000 / 0.2 = 2.5 mol/m2 and for a 20% to 30% run-off it should be set at 1.8 mol/m2 It is interesting that this will work for all crop types including shrubby plants, large vine plants or, in fact, any plant providing that the floor is pretty much shaded by the crop. If the crop does not shade the whole floor then tanspiration will be reduced pro-rata. ie if only 3/4 of the floor is shaded then transpiration will be 3/4 of the above calculated value. The above calculations provide a starting point for setting up your controller. Following initial setting, the grower should closely monitor the run-off quantity and its EC, the moisture level in the media and the state of the crop. This is best done automatically using a NutriMinder which will display the on-line logged data graphically which can clearly show how these parameters vary from early morning, through the heat of the day and into the evening. This information is absolutely essential if you wish to set your irrigation system to peak performance. Minder
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