The paper explores a concept of irrigation control, where the supply of nutrient solution is controlled without the use of predictive uptake models but rather by the use of a direct feedback of a drain flow measurement. This concept proves to be a viable approach. Results are presented, showing the compensation of changes in water up-take due to variations in global radiation, with a very tight control of the drain flow, keeping it constant over long periods of time. A tipping-bucket type flow sensor and its use in controlled water supply to a to-mato crop is described. A calibration procedure for the tipping-bucket instrument is presented

Gieling, Th.H.

Janssen, H.J.J.

Loef, P.

Vries, H.C., de

English

Wageningen Yield 

 31
Feedback Control of Water Supply in an NFT Growing System 
 
Th.H. Gieling, H.J.J. Janssen, H.C.de Vree and P. Loef 
Institute of Environmental and Agricultural Engineering, Wageningen 
The Netherlands 
 
Keywords : constant drain return, tipping bucket flow sensor 
 
Abstract 
 The paper explores a concept of irrigation control, where the supply of nutrient 
solution is controlled without the use of predictive uptake models but rather by the use 
of a direct feedback of a drain flow measurement. This concept proves to be a viable 
approach. Results are presented, showing the compensation of changes in water up-
take due to variations in global radiation, with a very tight control of the drain flow, 
keeping it constant over long periods of time.  
 A tipping-bucket type flow sensor and its use in controlled water supply to a to-
mato crop is described. A calibration procedure for the tipping-bucket instrument is 
presented. 
 
INTRODUCTION 
 Commercially available control equipment for water and nutrient supply use classic ana-
logue or digital P, PI and PID controllers. In so called A/B type of systems, A or B type of sol-
ute mixtures are subsequently added into a mixing tank or injected into the main stream of the 
supply water (Gieling et al., 1996).  
 The growing system in use has a large influence on the dynamic properties of the con-
trolled process. In case of NFT (Nutrient Film Technique), the root mat will gradually fill up the 
growing channel and thus alter the delay time and dead time of the overall system. In growing 
systems with an artificial growing medium (i.e. rockwool, perlite), the growing medium may 
cause a time delay (comparable to a stirred tank). However, the physical characteristics of the 
growing medium will change over time, e.g. due to ageing effects. As a consequence, most 
commercially available systems show problems in compensating relatively fast changes of the 
nutrient uptake or - in case of re-circulating systems - changes in the composition of solution in 
the return tank due to addition of clean water (Kupers et al., 1992). Especially control of nutri-
ent application will suffer from these problems, as system identification of this sub-process 
shows (Gieling et al., 1997). To solve it, solutions from modern control theory have been sug-
gested (Chotai et al., 1991; Gieling et al., 1997).  
 The control of water supply is more direct, since most of the disturbance is caused by 
fluctuations in water uptake by the plant and is less dependent on changing properties of the 
growing system. The principle of a “controlled constant return of drain water”, with its intrinsic 
ability to compensate for changes in water uptake, will give opportunities to the control of water 
supply. This principle is discussed in section 3.1. 
 However, the good functioning of a drain flow sensor is of importance if constant drain 
return is chosen as control strategy. During the research project experience with propeller type 
of instruments showed that they easily get polluted and stop functioning. Other types of instru-
ments (eddy-current, heat-balance, etc.) were too expensive for application in horticultural prac-
tice. The tipping-bucket instrument (Fig. 1), is a cheap and simple alternative. When the spoon 
or bucket is full, it will tip over and cause a pulse on the output of the instrument. However, the 
instrument shows problems with linearity. During the time that the spoon is in the down position, 
water will leave the reservoir without touching the spoon. Hence it will not be counted. This ef-
fect is larger when the water flow from the reservoir is large. A calibration procedure - as dis-
cussed further on – handles this problem. 
Proc. Sensors in Hort. III  
Eds. I. Shmulevich et al. 
Acta Hort. 562, ISHS 2001 
 32
MATERIALS AND METHODS  
 In a 150 m2 greenhouse, a feedback controlled watering system supplies water to a 
NFT closed growing system with eight rows of 37 tomato plants each. As an actuator a stan-
dard commercial A/B water and nutrient supply system (PRIVA) is applied (Fig. 2). This actua-
tor system switches on for one minute. The off time till the next one minute on-time, is modulated 
according to the output of the controller, within the limits of a minimum of 1 min. and a maximum 
of 14 min. pause time.  
 Problems may arise as a result of the time delays and dead times in the return gully of the 
closed growing system, especially since in this test the concept of constant drain return is ap-
plied. This concept tries to keep the drain flow from the plants at a constant value, thus compen-
sating for the uptake of the plants. In order to minimise the values of the dead times, a so-called 
Tichelmann layout of the water supply pipes is used (Gieling et al., 1995). The gullies with root-
mats and small rockwool-seedling blocks and the return pit will add extra time delays. As a flow 
sensor for drain water, a tipping-bucket sensor (Fig. 1) is applied in the drain-pipe of a short 
starting gully with 5 plants. The starting gully is placed in-between the normal canopy of plants. 
The return flow from the short gully is kept constant by means of the applied control algorithm. 
The water supplied to the whole greenhouse, is according to the ratio between the total amount 
of plants in the greenhouse and the plants in the short gully. 
 A PI controller for water supply is designed according to the classic design method, us-
ing a unit step response (Fig. 3) and Bode plot design. A unit step function on the input brings 
the flow from 7.5 to 30 drain sensor pulses, as is shown by Fig. 3 curve 1. The response of the 
drain flow in supply system is shown by curve 2. Curve 3 shows the smoothed original response 
data. A second order response with dead time is fitted through the response data, resulting in 
curve 4. With the aid of the data of curve 4, a process transfers function Hp was constructed as 
in Equation 1. A classic tuning procedure results in the P and I factors of the control algorithms, 
which keep the Gain Margin and the Phase Margin just within the prescribed limits. It results in a 
transfer Hc (Eq. 2). 
 
H s
K e
s s
e
s s
p
t s sd
( )
( ) ( )
.
( ) ( )
=
×
+ × +
=
×
+ × +
- × -
t t
1 2
75
1 1
12 9
90 1 114 1                  
(1)  
 
H s K
s
s
s
sc
i
i
( ) .= ×
× +
×
= ×
+t
t
1
0 07
120 1
120                                      
(2) 
 
RESULTS AND DISCUSSION 
 
The Controlled Process 
 Figure 4a shows the flow response of the drain output on the controlled water supply. 
Curve 4b depicts the actuator activity (ON- and OFF-time). The evenly divided pattern of 
pulses from 400 till 1200 in curve 4a shows the response of the drain of the short gully on a wa-
ter supply regime of 1 minute on-time and on average 5 minutes off-time. The first part of both 
curves depicts the overshoot of the start-up transient phenomena. In Fig. 5 the response is 
shown over a period of approx. 24 hours of the drain return. During this time, the drain of the 
short gully is kept at a constant level of 1.8 l×h-1. The data of Figure 5 starts at approx. 12:30 
hours. 
 The ripple of the flow signal from the tipping-bucket flow meter is due to the ON-OFF 
switching of water supply. The surface of each peak is the total drain return of one ON-period 
of water supply. Evaporation and water uptake causes a high ripple frequency, from the start 
until 19:00 hours. Then from 19:00 hours and onwards the ripple shows a much lower fre-
quency, indicating the lower water uptake and - as a consequence - less water supply due to the 
chosen strategy of constant drain returns.  
 The controller clearly tries to react on variations in water uptake and compensates for it. 
During the next period of high sun radiation the water supply - and hence the ripple frequency - 
 33
is higher again. Fig. 6 shows a drain flow of 1.2 l×h-1, where during each supply cycle the return 
flow from the start gully is averaged. 
 Figure 7 shows the data for radiation and flow of two consecutive days. Here, the radia-
tion as function of time (Fig. 7-1) is shown in relation to the water supply as function of time 
(Fig. 7-2a) during the same period of time. In the second diagram (Fig. 7-2b) also the constant 
drain return (averaged over each supply cycle) is shown.  
 By keeping the drain return at a constant level, the controller automatically compensates 
for the water uptake by the plants. It increases the supplied amount when plant water uptake 
increases due to higher global radiation (and vice versa). Now, the value of the drain water flow 
can be chosen freely. It can be lowered in such a way, that only a minimal amount is supplied, 
but still all the plants in the greenhouse are receiving water. In this way, the amount of return wa-
ter is minimised and thus the amount of recycled water, which has to be cleaned before being re-
used, is reduced. No harm to the plants is risked, since the controlled supply system will act al-
most instantaneously and will immediately compensate for any change in the returned amount of 
drain water, as is shown in Fig. 7 and 8.  
 The small delay between the radiation curve and the curve of water uptake by the plants 
(approx. 5-10 minutes) is not completely explained by the delayed reaction of the plants on 
changing radiation. Earlier research by Bruggink et al. (1988) and Van Ieperen (1997) confirms 
that radiation and water uptake only show negligible time delays. The time delays are better ex-
plained by the lingering time of the water in gullies, root mats and return pit. 
 The data of Fig. 7 were recorded on 4 - 5 of June 1997. The data of Fig. 8 were re-
corded in spring 1997 on 20 - 21 of May. Here, the average radiation is lower - and of much 
shorter duration at high level - than the radiation of June in Fig. 7. Nevertheless these lower ra-
diation energies, the relation between radiation and water uptake stays the same. 
 
The Tipping-Bucket Flow Sensor 
 When the spoon of the tipping-bucket sensor is in the down position, problems may 
arise, since water will pass by the spoon without being counted. To overcome this problem, an 
interpolation table is constructed, where apparent spoon volumes connect to a value in a set of 
time intervals. When the time between two tipping-over pulses is in one of these intervals, the 
apparent spoon volume connected to this interval is used. Two methods of calibration for the 
instrument have been applied, using a measurement set up as in Fig. 9. 
· The reservoir above the tipping spoon was filled with 1 litre of water. The pulse count 
was logged, whilst measuring the weight of the water leaving the instrument. Advantage: 
many calibration points. Disadvantage: needs a accurate weighing instrument (< 0.01 
gram) and logging of weight and pulse count at the moment as the spoon is tipping over. 
· Apply a constant known flow of water to the instrument and measure the time between 
pulses of the spoon. Repeat it for a number of known flows within the normal range of 
expected flow values. Advantage: can be simply done with available equipment and 
gives a lot of readings for one calibration point. Disadvantage: limited number of calibra-
tion points, which results in an approximation of the characteristics in a small part of the 
working range. The accuracy is never better than the accuracy of the known calibration 
flow.  
 The calibration procedures lead to an interpolation table that can be used in interpreting 
measuring data in-line with the data acquisition process (Table 1). The table provides an appar-
ent spoon volume as function of the time between pulses from the tipping bucket, which corrects 
for the water that passes by the spoon. After installation of the tipping-bucket in the greenhouse 
and application of the calibration procedure in the data acquisition process, it showed that pre-
cise horizontal alignment of the instrument is hard to achieve. Applying a multiplication factor in 
addition to the interpolation table can compensate for the resulting error. After testing, this set up 
needed a multiplication factor of 0.984. 
 
CONCLUSIONS 
 Control of water supply by means of the method of constant drain return showed to be 
adequate to compensate the water uptake of ten rows of 37 plants each during several consecu-
 34
tive days. The drain flow can be chosen freely at a low value, giving rise to a reduction in the 
amount of water that has to be cleaned before being re-used, hence to a cost reduction. After 
calibration, the tipping-bucket sensor showed to be a reliable sensor for measuring the drain wa-
ter flow. 
 
ACKNOWLEDGEMENTS 
 The authors wish to thank the EU Air III Research Program (CT93-1603), The Dutch 
Ministry of Agriculture, Nature Management and Fisheries and the IMAG-DLO institute for the 
sponsoring of this research. 
 
Literature Cited 
Bruggink, G.T., Schouwink, H.E. and Gieling, Th.H. 1988. Modelling of water potential and 
water uptake rate of tomato plants in the greenhouse: preliminary results. Acta Hort. 
229:77-85. 
Chotai, A. and Young, P.C. 1991. Self-adaptive and self-tuning control of a Nutrient Film 
(NFT) System. In: Y. Hashimoto and W. Day (Eds.). Mathematical and Control applica-
tions in agriculture and horticulture. IFAC Workshop series 1991 nr.1, 33-40. 
Gieling Th.H., Bontsema, J. and van Os, E.A.1996. Monitoring and control of water and nutri-
ent supply in closed growing systems. In: E. Goto et al. (Eds) Plant Production in Closed 
Ecosystems, 103-121, Kluwer Academic publishers. 
Gieling, Th.H., Bontsema, J., van Antwerpen, A.W.J. and Lucasse, L.J.S. 1995. Monitoring 
and control of water and fertilizer distribution in greenhouses. Acta Hort. 401, 365-372. 
Gieling, Th.H., Bontsema, J., Bouwmans, T.W.B.M. and Steeghs, R.H. 1997. Modelling and 
simulation for control of nutrient application in closed growing systems. Neth. Journal of Ag-
ricultural Science 45, p. 127-142. 
Kupers, G., van Gaalen, J., Gieling Th.H. and van Os, E.A. 1992. Diurnal changes in the ion 
concentration of the supply and return water of a tomato crop grown on rockwool. Acta 
Hort. 304:291-300. 
Van Ieperen, W. 1996. Consequences of diurnal variation in salinity on water relations and yield 
of tomato. Thesis Wageningen Agricultural University. 
 35
Tables 
 
Table 1. Time between pulses and apparent spoon volume as function of 5 actual flow values 
Actual flow [ml×min-1] 91.305 68.475 54.709 45.474 38.828 33.800 
Time between pulses [s] 2.597 5.021 5.195 6.630 8.108 13.480 
App. spoon vol. [ml] 4.565 4.565 4.559 4.547 4.530 4.507 
 
Figures 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2. Layout of the controlled process. 
 
 
 
 
 
Tomato plant
Gully
Reservoir
tipping bucket
 
Fig. 1. Tipping-bucket sensor. 
 
 36
 
 
0 5 10 15 20
7,5
12,5
17,5
22,5
27,5
32,5
37,5
Time [min]
Pulse/min
1
4
3
2
 
Fig. 3. Response of the drain flow on a unit step excitation on the water supply. 
 
 
0 200 400 600 800 1000 1200
0
1
2
0 200 400 600 800 1000 1200
0
1
2
3
4
Time [5 sec intervals]
Flow
[l×h-1]
Time [5 sec intervals]
Start-up  overshoot Stable control
 Set  Value
 1.5 [l×h
-1
]
On
 a
 b
Off
 
 
Fig. 4. Response of the controlled system during the first 100 min. 
 
 
04:000
2
04:00 12:00
0
2
time [h]
 flow
 [l×h-1]
12:30 20:00
0
2
20:00
 
 
Fig. 5. Drain flow response of the controlled system 
 
 37
 
 
 
 
 
 13:15 18:15  23:15  4:15  9:15  14:15
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Time [h]
Flow
[l×h
-1
]
night
day day
 
 
 
Fig. 6. Drain flow, where the return of each supply cycle is averaged. 
 
 
 0
500
 1000
 radiation
 1
 1.5
 2
 2.5
 time [h]
 flow[l×h-1]
 12:30  18:00  24:00  06:00  12:00  18:00  24:00  06:00  10:50
1
2
    a    b
 
Fig. 7. Two consecutive days with radiation and flow data 4-5 June 1997: 
1. Radiation as function of time of day 
      2. Water supplied (a) and a constant level of the drain flow (b). 
 
 38
 
 
 
18:00 0:00 6:00 12:00
1
1.5
2
2.5
time [h]
 flow
 [l ×h -1 ]
0
500
1000
radiation
13:15 15:00
day
night
day
drain
supply
 
 
 
 
Fig. 8. Radiation and flow data recorded on 20-21 May 1997. 
 
 
+15V +5V
Parallel
port
47k
Gnd
Mettler balance
25
13
Serial
port
Sync Puls
generator
 PC
Gnd
2
5
 13  12 (Data I/O port)
4
6
5
tipping
bucket
12
 
 
 
 
Fig. 9. Measurement set up for calibration of tipping bucket sensor. 
 
 
 


Feedback control of water supply in an NFT growing system

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