It must be possible to get a indication of control irrigation using VPD and airmovent in the greenhouse. This will give an indication about evaporation.

For VPD you need to measure RH, planttemperature and greenhouse temperature. For the airmovement you can use windspeed outside.

But drain percentages are more easy to control irrigation.

There are several irrigation controller manufactured that use weather stations or sell their own weather station to give environmental feedback. I would look at Hunter Irrigation for a less expensive option. Otherwise both Toro Irrigation and Rainbird have ET feedback based systems but the weather stations are usually expensive such as Campbell Scientific.

Using a hydroponic system, the water can be circulated from a reservoir tank, through the growing beds and back to the reservoir. The water in the system can be topped up (to replace the amount transpired) automatically using a float switch or ball valve, without the need to measure climatic parameters. The process can be carried out manually on a daily basis, refilling to a set level.

If you do need to use climatic data, as suggested in the other posts, you must also take account of the leaf area

You can use ET using Penman & Monteith. In greenhouse you can use the information about wind according to FAO paper 56. Using PM ET, you can integrate many climatic parameters.

I agree with Martin. We don't need to go by climatic parameters in hydroponic system. Simply monitor the uptake / loss as proposed by Martin.

I would agree with all that is said above but I would argue that, depending on what you are trying to accomplish, there still might be a use for monitoring ET inside the house. Using a float valve you would be operating with stomates wide open and at maximum ET. This would be somewhat different if, lets say, you want to impose something less than that for some reason.

I agree with Robert: you should consider the purpose of your set-up. Are you just growing plants to sell the roses (or imitate the conditions of a commercial grower)? By the way, by hydroponic you mean totally soil-less, just nutrient solution? Or do you use a somewhat inert substrate? I guess is the second option, or you would not talk about irrigation but about nutrient solution turnover. Anyway, and I am sure you are also considering this, in the absence of a nutrient-providing substrate, nutrient replacement rate is probably more relevant than watering (in the sense that insufficient watering effects are more evident), or at least both parameters must be considered together.

Hi Mario

Your question was addressed to me by the editor.

Sincerely, I don't realy understand the question.

Wikipedia: "Hydrophonics is a subset of Hydo culture and is a method of growing plants using minerals nutrient solution in water, without soil".

So, what is the question if the routs are constantly dip in the solution. Meaning: the plant get water, and food, continueously. The plant is irrigated.

So, where the irrigation (in your question) is coming into the picture, if it is constant?

Please clear up the real situation and maybe that from there I can understand the problem and look for a solution.

In friendship

Amir

I do agree with Xavier and Amir. This question is erroneous. By definition a hydroponic system should have a continuous supply of water whether it is a soil-less system or using a substrate. The real design issue is the mineral turn-over with regard to nutrient consumption and water recharge which can be easily manually or automatically regulated as explained by Robert. In such a situation measuring and controlling irrigation' using climatic parameters is pointless if not economically unwarranted.

I am not in agreement. I think the questiion is fine. It depends on what is meant by the word "soil". Perhaps the word choice was incorrect. As an English speaker, can we change the word to "substrate" and continue the discussion? If we change the word to substrate, we have several types of hydroponic systems such as ebb and flow that rely on wetting the substrate and allowing it to dry between irrigations.There are other forms such as floating systems and NFT which do rely on a continuous supply of water. In systems that allow intermittent irrigations, the degree of drying is up to the manager of the system. The question then might become, "Why would we want limit water in a hydroponic system?" I can think of a couple of reasons. First, we might want to conserve water or only have water available during certain periods of time. Secondly, there is research to support that in some vegetables and fruits we see an improvement in taste and eating quality (sensory evaluations) that can translate into higher prices for the producer if the marketing channel exists or can be created (such as in branding a product). Having worked in high end markets for fruits and vegetables, this has been a major problem for hydroponic producers... they have not, at least to date, been able to create the qualtiy (sensory evaluations) that are done in the field under traditional growing techniques. Instead, the markets that hydroponic growers have had to go after are when prices are more competitive and local high quality products are not available.

Yes, I agree with Robert. In fact in my answer I did not challenge Mario's question (so, sorry Romulus, I think you misquoted me). I was just aware that by "hydroponics" people mean quite a range of things, from just a nutrient solution in which plants float, and that is constantly renewed , recirculated or just replenished from time to time (kind of equivalent to an irrigation event), to a complete substrate that is mostly inert, so nutrients are also delivered through the irrigation system, although usually not in every irrigation event. In fact, I was favoring this last option in my answer.

This was just my point, and I completely agree with Robert, as I stated above, that the objective of the set-up is crucial to answer the question.

My recommendation is to base it on accumulated light. In our greenhouses here (granted it is tomatoes and other vegetables) we have a pyronometer attached to a weather station on top of the greenhouse. Our irrigation frequency is based upon accumulated radiation (W/m2). So, higher solar radiation, more frequent irrigation and then less with less. That seems to be a good way. But, we still use a lysimeter to measure the input / output which allows us to make the minor adjustments needed to the crop. And to chime in, we frequently restrict and control our irrigation with tomatoes to steer the plants growth and affect fruit quality (we have a rockwool slab as substrate fed by drippers).

What I understand is that Mario is interested in the amounts of solution (what he has termed as irrigation) need to be estimated based on climatic parameters under hydroponics. It is an interesting question. There are many types of hydroponics systems and under most systems, the soil or substrate is minimal which reduces the evaporation from the field / system. Under such a situation the evaporation component might be negligible. The main contribution to the evapotranspiration could be by the transpiration. The transpiration within the greenhouse can be estimated based on air temperature and humidity.

Please refer

H. Fatnassi, T. Boulard, J. Lagier. 2004. Simple Indirect Estimation of Ventilation and Crop Transpiration Rates in a Greenhouse, Biosystems Engineering, 88(4), 467–478.

I.L. López-Cruz, M. Olivera-López, G. Herrera-Ruiz, SIMULATION OF GREENHOUSE TOMATO CROP TRANSPIRATION BY TWO THEORETICAL MODELS , ISHS Acta Horticulturae 797: International Workshop on Greenhouse Environmental Control and Crop Production in Semi-Arid Regions

Hi all friends participating in this matter.

Thank you, all, for your clever and knowable answers. But, I am afraid that we missed the target. This forum was established for every body's concern. Unfortunately, the initiator of this interesting question never clarified what he ment in "irrigation". So, all of us are walking in the darkness and guess what was the real interest of Mario, when he gave publicity to his hesitations.

I feel that now, after having that much answers, all of us know much more about Hydrophonic growing methods, but, maybe we still missed the target.

So, and let all of us learn from this experience: to ask questions, and to answer them, as accurate as possible to the point that we are looking for. We can, always, broadening the question if more misunderstood points were raised up during the discussion.

We, as scientists must be accurate as it may confuse, the people, to get so many and different answers to same, simple question.

I am, still, waiting Mario's clarification, in order to make sure that we helped him.

Thank you all

In friendship

Amir

Dear Robert and Xavier, thanks for the clarifications and corrections. I see your point as well as my misinterpretation of the whole context. But as we dissect and analyze this problem I think we should take a step back and confirm if the question we are answering is what was asked and I still believe a more succinct clarification of the boundaries of Mario's question would have gone a long way to help.

In substance though I agree that your answers are right on the money. Thanks.

It is perhaps time for the originator of the question to step in and make some clarifications?

Many thanks for all your answer to my question.

We have a coconut coir dust as substrate fed by drippers and nutrient solution is recaptured and reused. The Roses are cultivated with density of 4.5 plants/m2, following the ‘arching’ (or ‘bending’) technique.

Dear Myles Lewis,

We also have a solaritmeter placed on top of the greenhouse that is connected with a logger. We also have a temperature and relative humidity sensors inside the greenhouse. The problem is how to find the appropriated levels of these climatic variables to control our irrigation

Hi all, please let me join this interesting discussion. I am not experienced in hydroponic, but have some experiences with measurements and automation in irrigation. Basically, I always started from the water balance,the water that goes in should balance the water that goes up, so I would use Evapotranspiration to determine the amount of irrigation. It is not so difficult to find the relation between climatic parameters : the in-house radiation, air temperature and humidity, and the ET, and then to irrigation amount. I'd never used climatic parameters yet, but I've used the moisture of the medium as control input for automation which is much easier.

Dear all, it is interesting discussion:

From practice. At a flower farm in hydroponic media, we had been irrigating by detecting the drainage. It was like thumb rule, the drainage is tried to be maintained around 15 to 20% for each trough. The over- and under- irrigation had been controlled if the drainage is exceed or less than the set drainage percentages(15 to 20%) respectively.

In principle, irrigation should satisfy the evapotranspiration demand of the Rose within the green house environment (as it differs the environment outside the green house). The amount, frequency and supply duration of irrigation vary based on the ET within the green house and the water holding capacity of the substrate which is often minimal. If continuous irrigation is to be applied, make sure not to exceed ET plus loss? (If you have any). Otherwise, divide the water demand (ET plus losses?) of the daily requirement to the number of pulses that you want to irrigate within a day. Then, supply the required amount of each cycle (V) for the duration (t); where, t=V/Q, (Q is discharge of supply). But make sure that the amount of supply (V) at a time not to exceed the moisture holding capacity of the substrate.

Hi Mario,

Some other thoughts for what they are worth. I think you have the appropriate instruments and should perhaps use a mutli-step process to control if possible. I agree with Hans at the beginning that VPD would be a good indicator based on your interior conditions. Start by keeping the VPD within normal ranges for a greenhouse: 0.45 kPa to 1.25 kPa, which you probably are already, and adjust to a set point within your house that keeps the plants doing what you want them to do and note it (we might have different set points depending on the stage of growth). You will still need to have lysimeters to measure drainage output, ideally somewhere between 10 - 30%. I think using those two (VPD as the trigger, and lysimeters to verify and validate), combined with observing your accumulated solar radiation (as a trigger, validation, or back-up trigger) would be a good first step. It requires a little work on the front though. You of course know your crop, your crops irrigation needs, your greenhouse, and your climate better than us, so you will need to obviously modify based on your situation.

Alternatively, do you have access to a load cell to hook up to your logger to measure the weight of the slab or pot? You could perhaps identify how your crop reacts during various VPD / radiation points by measuring the use of water by the weight of the substrate at saturation, and as it consumes water throughout a day.

Good luck!

Dear Miles Lewis,

Thanks for your important reply. I will be very grateful if you could suggest some paper about this subject.

Many Thanks

Dear Cunha

Please refer

7. Policy alternatives of the management of minor and medium irrigation schemes to develop groundwater system in restricted catchments for the improvement in food productivity in the dry zone of Sri Lanka. Proceedings of National Conference on Water, Food Security and Climate Change in Sri Lanka Vol. 3, Page 73-88 (2009) IWMI Publication ISBN 978-92-9090-720-6 http://ageconsearch.umn.edu/bitstream/118415/2/SLWC_Vol_3_final-low.pdf

http://publications.iwmi.org/pdf/H042806.pdf

With regards

siva

Hi, I've come to this discussion late but one approach could be to use some control theory methods. To get a taste of what's possible take a look at:

Young, P. C. and Lees, M. J. and Chotai, A. and Tych, W. and Chalabi, Z. S. (1994) Modelling and PIP control of a glasshouse micro-climate. Control Engineering Practice, 2 (4). pp. 591-604.

The starting point is to identify a transfer function model relating your inputs to outputs then wrap this in a recursive control equation. The result is a, usually simple, 'control law' that specifies the control input for the next time step (sounds like that would be watering rate) based on a set of metrics measured from the system (which could be temperature, lysimeter value etc).

We tested successfully the newly developed leaf patch clamp, which shows online the water status of the leaves. You may contact U. ZImmermann. for these instruments.

Rüger,S., Ehrenberger, W., Arend, M., Geßner, P. Zimmermann, G., Zimmermann, D., Bentrup, F.-W., Nadler, A., Raveh, E., Sukhorukov, V.L., Zimmermann, U. (2010): Comparative monitoring of temporal and spatial changes in tree waterstatus using the non-invasive leaf patch clamp pressure probe and the pressure bomb. Agricultural Water Mangement 98/2, 283-290, doi:10.1016/j.agwat.2010.08.022

Ehrenberger, Wilhelm, Rüger, Simon, Fitzke, Ronald, Vollenweider, Pierre, Günthardt-Goerg, Madeleine, Kuster, Thomas, Zimmermann, Ulrich, Arend, Matthias (2012): Concomitant dendrometer and leaf patch pressure probe measurements reveal the effect of microclimate and soil moisture on diurnal trunk water and leaf turgor variations in young oak trees. Functional Plant Biology 39, 297-305. http://dx.doi.org/10.1071/FP11206

You can check this paper: https://www.researchgate.net/publication/251586863_Effects_of_nutrition_systems_and_irrigation_programs_on_tomato_in_soilless_culture

Article Effects of nutrition systems and irrigation programs on toma...