IRRIGATION CONTROL SYSTEM
RELATED APPLICATIONS
The present application claims the benefit under 35 U.S.C. 119(e) of US Provisional
Application 60/935,571 filed August 20, 2007, the disclosure of which is incorporated herein
by reference.
FIELD
The invention relates to systems and apparatus for controlling irrigation systems.
BACKGROUND OF THE INVENTION
Irrigation systems that deliver water, often containing plant nutrients, pesticides and/or
medications, to plants via networks of irrigation pipes are very well known. In some irrigation
systems, external sprinklers, emitters or drippers, are connected to the irrigation pipes to divert
water from the pipes and deliver the water to plants. In many such irrigation networks, water
from the pipes is delivered to the plants by emitters or drippers that are installed on or
"integrated" inside the irrigation pipes. For convenience, any of the various types of devices
used in an irrigation system to divert water from an irrigation pipe in the system and deliver
the diverted water to the plants is generically referred to as an emitter. Spacing between
emitters, and emitter characteristics are often configured to respond to different irrigation
needs of plants that the irrigation system is used to irrigate.
For a given configuration of irrigation pipes and emitters, quantities of water delivered
by the irrigation system may be controlled by controlling any of various water flow control
devices, such as water pumps, flow valves and check valves, and/or combinations of flow
control devices known in the art. Flow control devices may operate to control water from a
source that provides water to all of, or a portion of, irrigation pipes in an irrigation system or
to control water from individual emitters in the irrigation system.
Israel Patent Application 177552 entitled "Irrigation Pipe" filed August 17, 2006, the
disclosure of which is incorporated herein by reference, describes an irrigation system having
irrigation pipes comprising integrated emitters having different pressure thresholds at which
they open to deliver water from the pipes. Which emitters open to deliver water, is controlled
by changing pressure in the irrigation pipes. US Patent 5,113,888, "Pneumatic Moisture
Sensitive Valve", the disclosure of which is incorporated herein by reference, describes a
spray device having its own valve that is opened and closed to control amounts of water that
the device sprays on plants.
Various automatic and/or manual methods and systems are used to determine when
and how much water to supply to plants irrigated by an irrigation system and to control water
flow devices in the system accordingly. US Patent 5,113,888 noted above, controls the water
flow valve in the spray device described in the patent responsive to soil moisture. The spray
device comprises an element located in the soil that has pores, which are blocked when soil
water moisture is above a predetermined amount and that are open when soil moisture is
below a predetermined amount. When the pores are open, air is released from a chamber in the
valve relieving pressure that keeps the flow valve closed to allow the valve to open and water
to flow to and be sprayed from the spray device. US Patent 6,978,794, the disclosure of which
is incorporated herein by reference, describes controlling an irrigation system responsive to
soil moisture determined by at least one time domain reflectometry sensor ("TDRS") located
in the soil. The patent describes using multiple TDRS's at a different soil depth to provide
measurements of soil moisture content. US 6,314,340, the disclosure of which is incorporated
herein by reference, describes controlling water responsive to diurnal high and low
temperatures.
For many agricultural and scientific applications, soil water matric potential is used as
a measure of soil moisture content and suitability of soil conditions for plant growth and
irrigation systems are often controlled responsive to measurements of soil matric potential.
Water matric potential, conventionally represented by "?", is a measure of how strongly
particulate soil matter attracts water to adhere to the particulate surfaces. The drier a soil, the
stronger are the forces with which soil particles attract and hold water to their surfaces and the
greater is the water matric potential. As matric potential of a soil increases, the more difficult
it is for plants to extract water from the soil. When soil gets so dry that plants cannot extract
water from the soil, plant transpiration stops and plants wilt.
Matric potential has units of pressure, is typically negative, and is conventionally
measured using a tensiometer. A tensiometer usually comprises a porous material that is
connected by an airtight seal to a sealed reservoir filled with water. The porous material is
placed in contact with soil whose matric potential, and thereby moisture content, is to be
determined and functions to couple the reservoir to the soil to allow water but not air to pass
between the reservoir and soil. The forces that attract water to soil particles draw water
through the porous material from the reservoir and generate a vacuum in the reservoir. The
drier the soil, the greater are the forces that draw water from the reservoir through the porous
material and the greater is the vacuum, i.e. the pressure of the vacuum decreases. As soil
moisture increases, the forces that attract water to the soil particles decrease and water is
drawn from the soil through the porous material into the reservoir and pressure of the vacuum
increases. The vacuum increases (pressure decreases) or decreases (pressure increases) as
water content of the soil respectively decreases or increases. A suitable pressure monitor is
used to determine pressure of the vacuum and thereby provide a measure of the soil matric
potential.
The porous material in a tensiometer is usually a ceramic and is often formed having a
cuplike or test tube-like shape. However, US Patent 4,068,525, the disclosure of which is
incorporated herein by reference, notes that the porous material "may be formed from any of a
wide variety of materials, including ceramics, the only requirement being that the 'bubbling
pressure', the pressure below which air will not pass through the wettened pores of the
material, must be greater than normal atmospheric pressure, to prevent bubbles of air from
entering the instrument". It is noted that bubbling pressure is generally maintained only when
the porous material is saturated with water.
Additionally, the porous material should provide good hydraulic contact between the
soils and the water reservoir. The latter constraint with respect to soil contact generally
requires that the porous material be in relatively intimate mechanical contact with soil
particles. Whereas such contact can usually be provided by a surface of a ceramic, for coarse
soils or gravels, such mechanical and resulting hydraulic contact can be difficult to obtain
using a ceramic material. Gee et al, in an article entitled "A Wick Tensiometer to Measure
Low Tensions in Coarse Soils"; Soil Sci Soc. Am. J. 54:1498-1500 (1990) describes a
tensiometer for use in coarse soils in which the porous material "is constructed from paper
toweling or other comparable wicking material rolled tightly into a cylinder (-0.7 cm in
diameter and ~ 7 cm long)." The authors note that the tightly rolled wicking material when
wetted was pressure tested for suitable bubbling pressure.
US Patent 5,156,179, the disclosure of which is incorporated herein by reference,
describes an irrigation system that is controlled using a tensiometer responsive to water matric
potential. The system comprises a "flow controller device" that includes a valve assembly
connected with the tensiometer to "provide automatic control of flow of water for irrigation".
Changes in pressure in the tensiometer move a piston in the valve to provide "variable control
of the rate of flow" through the valve assembly "according to the matric tension of the soil for
water".
SUMMARY OF THE INVENTION
An aspect of some embodiments of the invention relates to providing a tensiometer for
measuring matric potential of a soil, for which functions of providing hydraulic contact with
the soil and sealing a water reservoir used with or comprised in the tensiometer against ingress
of air through the hydraulic contact are provided by different components of the tensiometer.
According to an aspect of some embodiments of the invention, a septum, hereinafter a
"sealing septum" interfaces the tensiometer water reservoir with a component of the
tensiometer formed from a porous material that provides hydraulic contact between the
tensiometer reservoir and the soil and when wet substantially seals the reservoir against
ingress of air through the porous material. For convenience of presentation, the component
formed from the porous material is referred to as a "hydraulic coupler".
In an embodiment of the invention, because the sealing septum substantially provides
appropriate sealing of the water reservoir, the porous material of the hydraulic coupler is
generally not required when wet to have a bubbling pressure greater than an absolute value of
a minimum matric potential of the soil in which the tensiometer is to be used. (As noted above,
matric potential is usually a negative pressure, and a minimum matric potential is negative
pressure having a greatest absolute value. Bubbling pressure of a material is the negative of a
minimum matric potential at which air will not pass through the material, generally when the
material is properly wetted.) By substantially separating the function of providing hydraulic
contact with a soil and the function of sealing against passage of air, a relatively broad
spectrum of materials can be used for the hydraulic coupler and a tensiometer can
advantageously be configured for specific agricultural applications while also providing
relatively improved hydraulic contact with the soil.
For example, according to an aspect of some embodiments of the invention, the
hydraulic coupler comprises a porous material in which plant roots are able relatively easily to
grow. Optionally, the porous material comprises a woven and/or non-woven geotextile and/or
fiberglass. Practice of the invention is not however, limited to such materials and a tensiometer
in accordance with an embodiment of the invention may, for example, comprise any
hydrophilic material characterized by suitable porosity and may of course comprise relatively
rigid materials such as ceramics.
It is noted that the roots of many plants are able to generate hydraulic pressure
equivalent to about 15 atmospheres in order to extract water from soil. Such pressure can
cause relatively steep gradients in soil moisture for which soil in a near neighborhood of a
plant's roots is substantially drier than soil outside of the near neighborhood. Since plant
growth and health are generally relatively sensitive to the soil environment near to their roots,
a tensiometer for which plant roots are able to grow inside the tensiometer's hydraulic coupler
can provide water matric potential measurements advantageously sensitive to soil conditions
in the near neighborhoods of plant roots. Such measurements can be particularly advantageous
for use in controlling an irrigation system that provides water to the plants.
An aspect of some embodiments of the invention relates to providing a tensiometer that
is relatively inexpensive and simple to make and use.
In an embodiment of the invention, a tensiometer comprises a housing having a first
housing part formed having an inlet orifice for communication with a sealed water reservoir
and a second housing part formed to mate with the first part. The mated parts are assembled
sandwiching a sealing septum between the orifice and a first region of a porous hydraulic
coupling material that is located in the tensiometer housing when the tensiometer is
assembled. A second region of the hydraulic coupling material is located outside of the
assembled housing and provides hydraulic coupling of the tensiometer to soil for which the
tensiometer provides matric potential measurements. Optionally the first and second housing
parts are formed by injection molding plastic. Optionally, the sealing septum is formed from
materials readily available in the market such as a plastic, ceramic, or sintered metal
characterized by a porosity having suitable uniformity and pore size. Optionally, the pore size
has a characteristic dimension having an average between about 0.5 micron and about 1
micron. Optionally, the hydraulic coupling material comprises a geotextile. The tensiometer
may rapidly be assembled by any of various methods known in the art, such as by ultrasonic
welding, gluing, or snap locking the first and second housing parts together.
An aspect of some embodiments of the invention, relates to providing a configuration
of tensiometers that provides a measurement of water matric potential responsive to water
matric potential conditions over a relatively large area.
According to an aspect of some embodiments of the invention, a plurality of
tensiometers is distributed over the area and the tensiometers in the plurality are coupled to a
same common water reservoir. Pressure of a partial vacuum in the common reservoir is
responsive to the water matric potential at each of the locations at which a tensiometer of the
plurality of tensiometers is located. At equilibrium, pressure of a partial vacuum in the
common water reservoir provides a measure, hereinafter a "representative matric potential", of
water matric potential in the area that is intermediate a highest and lowest value for water
matric potential provided by the tensiometers. A suitable pressure or vacuum gauge is used to
provide a measurement of pressure in the reservoir and thereby a measure of the representative
matric potential.
An aspect of some embodiments of the invention relates to providing an improved
water management algorithm for controlling irrigation of a field responsive to water matric
potential.
In an embodiment of the invention, an irrigation cycle defined by the algorithm
comprises a period of active irrigation during which the algorithm controls an irrigation
system to provide pulses of water to a field responsive to measurements of water matric
potential in the field. Optionally, the cycle is a diurnal cycle. Optionally pulses of water are
provided responsive to comparing measurements of water matric potential to a calibration
water matric potential measurement. In an embodiment of the invention, the calibration water
potential measurement is acquired prior to the active irrigation period at a time for which
plants in the field have a relatively small demand for water. Generally, plants exhibit a
minimum in water demand at night, often in the early dawn hours and it is at such hours that
calibration matric potential measurements are, optionally, acquired. Optionally, the water
matric potential measurements are acquired using a tensiometer.
In an embodiment of the invention, an algorithm controls an irrigation system to
provide water to a field continuously during an active irrigation period. The duration of the
active irrigation period is determined by the algorithm responsive to a comparison of a
measurement of water matric potential for the field with a calibration water matric potential.
There is therefore provided in accordance with an embodiment of the invention, a
tensiometer for use in determining matric potential of a soil comprising: a water inlet; a
hydraulic coupler comprising a porous material for providing hydraulic coupling between
water that enters the inlet and the soil; and a septum that seals water that enters the inlet
against ingress of air via the porous material. Optionally, the porous material comprises a
geotextile. Additionally or alternatively, the porous material is adapted to enable growth of
plant roots therein.
In some embodiments of the invention, the septum comprises a septum surface, at least
a part of which is contiguous with water that enters the inlet. Optionally, the tensiometer
comprises a water labyrinth having baffles. Optionally, a portion of the septum surface
contacts the baffles.
In some embodiments of the invention, the septum comprises a membrane and the
septum surface is a surface of the membrane. Optionally, the membrane comprises a plurality
of layers. Optionally, the layers comprise a first layer having a bubbling pressure greater than
about a maximum absolute value of the matric potential of the soil in which the tensiometer is
used. Optionally, the first layer is supported by at least one support layer. Optionally, the first
layer is sandwiched between two support layers.
In some embodiments of the invention, the septum has a bubbling pressure greater than
about a maximum absolute value of the matric potential of the soil in which the tensiometer is
used.
In some embodiments of the invention, the bubbling pressure is about equal to one
atmosphere.
In some embodiments of the invention, a tensiometer comprises an elastic member that
resiliency presses the porous material to the septum.
In some embodiments of the invention, a tensiometer comprises a water reservoir
coupled to the water inlet.
In some embodiments of the invention, a tensiometer comprises a device for providing
a measure of pressure in the water reservoir.
There is further provided in accordance with an embodiment of the invention, an
irrigation system comprising: an irrigation pipe having at least one output orifice for
outputting water from the pipe; at least one tensiometer according to an embodiment of the
invention coupled to the irrigation pipe so that water output from an orifice of the at least one
orifice is constrained to pass substantially directly from the orifice through the hydraulic
coupler. Optionally, the irrigation pipe comprises at least one emitter and an output orifice is
an orifice of the at least one emitter. Additionally or alternatively, the at least one emitter is an
integrated emitter. Additionally or alternatively, the at least one emitter comprises a plurality
of emitters.
In some embodiments of the invention, each of the at least one tensiometer is coupled
to a same water reservoir.
There is further provided in accordance with an embodiment of the invention,
apparatus for use in determining matric potential of a soil comprising: a plurality of
tensiometers; and a same water reservoir to which all the tensiometers are hydraulically
coupled. Optionally, the plurality of tensiometers comprises a tensiometer in accordance with
an embodiment of the invention. Additionally or alternatively, the apparatus comprises a valve
adapted to connect the irrigation system to a water source and operable to enable water from
the water source to enter the reservoir and remove air therefrom.
There is further provided in accordance with an embodiment of the invention, an
irrigation system comprising: an irrigation pipe having at least one output orifice for
outputting water from the pipe; at least one tensiometer comprising a hydraulic coupler for
coupling the tensiometer to soil irrigated by the irrigation system; and a valve adapted to
connect the irrigation system to a water source and operable to enable water from the water
source to enter the at least one tensiometer and flush air from the tensiometer and coupler.
There is further provided in accordance with an embodiment of the invention, a
tensiometer for use in determining matric potential of a soil comprising: a water inlet; a
hydraulic coupler comprising a porous material for providing hydraulic coupling between
water that enters the inlet and the soil; a valve adapted to connect the tensiometer to a water
source and operable to enable water from the water source to enter the tensiometer and flush
the hydraulic coupler.
There is further provided in accordance with an embodiment of the invention, a
method of irrigating a field, the method comprising: acquiring a calibration water matric
potential for the field; and irrigating the field with an amount of water responsive to the value
of the calibration matric potential. Optionally, irrigating a field comprises performing an
irrigation cyclically. Optionally, irrigating the field cyclically comprises irrigating the field in
diurnal cycles. Optionally, acquiring a calibration water matric potential comprises acquiring a
calibration water matric potential at least once a day.
In some embodiments of the invention, the field comprises plants and acquiring the
calibration water matric potential comprises acquiring the matric potential when the plants
exhibit relatively small water demand.
In some embodiments of the invention, providing an amount of water comprises
providing a pulse of water. Optionally, providing an amount of water comprises acquiring a
water matric potential measurement for the field in addition to the calibration water matric
potential, comparing the additional water matric potential measurement to the calibration
water matric potential, and providing an amount of water responsive to the comparison.
Optionally, comparing the additional water matric to the calibration matric potential comprises
determining their difference. Optionally, providing a pulse of water comprises providing the
pulse responsive to the difference.
In some embodiments of the invention, providing water comprises providing water
continuously. Optionally, providing water continuously, comprises determining an irrigation
period responsive to the calibration water matric potential and providing water continuously
for the determined irrigation period. Optionally, determining the irrigation period comprises
determining the irrigation period responsive to a difference between the calibration water
matric and a previously determined calibration water matric.
There is further provided in accordance with an embodiment of the invention, an
irrigation system comprising: an irrigation pipe having at least one output orifice for
outputting liquid from the pipe; at least one hydraulic coupler coupled to the irrigation pipe so
that liquid output from an orifice of the at least one orifice passes through the hydraulic
coupler; and at least one sensing means coupled to the hydraulic coupler to sense a property
associated with liquid in the hydraulic coupler, responsive to which property output of water
via the at least one orifice is controlled. Optionally, the sensed property comprises matric
potential. Additionally or alternatively, the sensed property comprises moisture content of the
hydraulic coupler.
Optionally, the irrigation system comprises a controller that controls output of water
via the at least one orifice responsive to the sensed property.
BRIEF DESCRIPTION OF FIGURES
Non-limiting examples of embodiments of the invention are described below with
reference to figures attached hereto and listed below. Identical structures, elements or parts
that appear in more than one figure are generally labeled with a same numeral in all the figures
in which they appear. Dimensions of components and features shown in the figures are chosen
for convenience and clarity of presentation and are not necessarily shown to scale.
Fig. 1A schematically shows an exploded view of a tensiometer, in accordance with an
embodiment of the invention;
Fig. IB schematically shows details of a top housing part of the tensiometer shown in
Fig. 1A, in accordance with an embodiment of the invention;
Fig. 1C schematically shows a plan view of the top housing part shown in Fig. IB, in
accordance with an embodiment of the invention; *
Fig. ID schematically shows a perspective view of a bottom housing part of the
tensiometer shown in Fig. 1A, in accordance with an embodiment of the invention;
Fig. 2 schematically shows an assembled view of the tensiometer shown in Figs. 1A-
1B, in accordance with an embodiment of the invention;
Fig. 3 schematically shows a side cross-sectional view of the tensiometer shown in Fig.
1A and Fig. 2 connected to a sealed water reservoir, in accordance with an embodiment of the
invention;
Fig. 4 schematically shows a configuration of tensiometers distributed in the soil of an
agricultural field in which plants are grown, in accordance with an embodiment of the
invention;
Figs. 5A and 5B show a flow diagram of an algorithm for controlling irrigation of a
field responsive to water matric potential in accordance with an embodiment of the invention;
and
Fig. 6 shows a flow diagram of another algorithm for controlling irrigation of a field
responsive to water matric potential in accordance with an embodiment of the invention.
DETAILED DESCRIPTION
Figs. 1A schematically shows an exploded view of a tensiometer 20 for measuring
water matric potential in a soil, in accordance with an embodiment of the invention. Figs. IB-
ID schematically show enlarged views of components of tensiometer 20 shown in Fig. 1A.
Fig. 2 schematically shows an assembled view of tensiometer 20. For convenience of
presentation, apparatus 20 is referred to as a tensiometer, even though, as shown in Figs. 1A-
1D, it optionally, does not comprise a water reservoir and apparatus for providing a measure of
pressure in the reservoir.
Tensiometer 20 optionally comprises a housing 22 having first and second housing
parts 30 and 50, hereinafter referred to for convenience as housing top 30 and housing bottom
50, a sealing septum 60, a hydraulic soil coupler 70 formed from a porous material and a
resilient element 80.
Hydraulic coupler 70 is formed having a soil-coupling region 72 that extends outside
of housing 22 when tensiometer 20 is assembled (Fig. 2) and is a part of tensiometer 20 that
contacts soil for which the tensiometer provides water matric potential measurements and
hydraulically couples the tensiometer to the soil. Optionally soil-coupling region 72 enlarges
with distance from tensiometer housing 22. Hydraulic coupler 70 optionally comprises a neck
region 74 and an optionally circular reservoir-coupling region 76 that are discussed below and
are located inside housing 22. Hydraulic coupler 70 is optionally formed from a flexible
porous material and is optionally such that plants that are to be grown in a soil for which
tensiometer 20 is to be used to monitor water matric potential can intrude their roots.
Optionally, hydraulic coupler 70 is formed from a material comprising a geotextile.
Housing top 30 comprises a tubular stem 31 having a lumen for connecting tensiometer
20 to a sealed tensiometer water reservoir and is formed having a septum recess 33, shown in a
perspective view of first housing part 30 from a side opposite that of stem 31 in Fig. 1B, that
seats sealing septum 60. A bottom surface 34 of septum recess 33 is formed having an inlet
hole 35, clearly shown in a plan view of housing top 30 in Fig. 1C, through which water from
a reservoir connected to stem 31 enters tensiometer 20. Bottom surface 34 of septum recess 33
is optionally formed having a water flow labyrinth 36 comprising an entrance, "detour" baffle
37 that covers portions of inlet hole 35 and a plurality of raised cylindrical baffles 38. Detour
baffle 37 is optionally "starfish shaped" comprising five angularly, equally spaced arms 39.
Labyrinth 36 is surrounded by an annular, optionally planar surface 40 devoid of labyrinth
components. Housing top 30 optionally comprises a neck 41 formed having a channel 42 for
receiving neck region 74 of hydraulic coupler 70 and optionally comprises an assembly ridge
44 for mounting housing top 30 to housing bottom 50.
Sealing septum 60 optionally comprises a porous septum membrane 61 supported by
an annular septum frame 62, which optionally protrudes on either side of the plane of the
septum membrane. When tensiometer 20 is assembled, the annular septum frame seats on
annular region 40 of bottom surface 34 and septum membrane 61 optionally rests on and is
supported by detour and cylindrical baffles 37 and 38.
Septum membrane 61 transmits water but is characterized by a bubbling pressure,
hereinafter referred to as an "operating bubbling pressure", when wet that is equal to a
maximum water matric potential, typically between about -0.2 bar to about -0.7 bar, expected
to be encountered in a soil in which tensiometer 20 is to be used. Optionally, the operating
bubbling pressure of porous membrane 61 is equal to about 1 atmosphere. As a result, water
can pass through membrane 61 relatively easily, but for a pressure differential across the
membrane less than or equal to about a maximum water matric potential of soil in which
tensiometer 20 is used, membrane 61 is substantially impervious to air. Optionally, membrane
61 is a layered structure, schematically shown in an inset 66 in Fig. 1A, and optionally
comprises a porous layer 63, which transmits water but when wet is impervious to air for
pressures less than an appropriate operating bubbling pressure, sandwiched between two
support layers 64. Optionally, porous layer 63 is formed by way of example from a ceramic,
and/or a sintered metal and/or a suitable woven or non-woven fabric having suitable porosity.
Support layers 64 are optionally meshed, or screen-like layers formed from any suitably rigid
and strong material. Optionally, porous layer 63 is characterized by an average pore size from
about 0.5 to about 1 micron. Optionally, support layers are formed from a metal and/or plastic.
Housing bottom 50 is formed to mate with housing top 30 and is optionally formed
having a mating ridge 51 that is matched to fit inside recess 33 (Fig. IB) formed in housing
top 30 so that it aligns the housing top and bottom. Mating ridge 51 defines a portion of a
boundary of a recess 52 that seats reservoir-coupling region 76 (Fig. 1A) of hydraulic coupler
70. The housing bottom also comprises a neck 54 formed having a channel 55 that matches
neck 41 and channel 42 respectively of housing top 30. A bottom surface 56 of recess 52 is
optionally formed having a cavity 57 for receiving resilient element 80, optionally in a shape
of a sphere, formed from an elastic material. An outer, optionally planar peripheral border 58
surrounds mating ridge 51 and channel 55,
When tensiometer 20 is assembled, assembly ridge 44 of housing top 30 contacts and
is bonded to peripheral border 58 of housing bottom 50 and mating ridge 51 presses annular
septum frame 62 to annular surface 40 of housing top 30 to secure septum 50 in septum recess
33 of the housing top. Resilient sphere 80 is slightly compressed and urges reservoir-coupling
region of hydraulic coupler 70 to resiliency press on septum membrane 61 and the septum
membrane to rest securely on water labyrinth baffles 37 and 38. Because of the secure contact
between septum membrane 61 and labyrinth baffles 37 and 38, water that enters tensiometer
20 is distributed substantially equally over the surface of septum membrane 61 that contacts
the labyrinth baffles. Starfish detour baffle 37 operates to direct substantially equal portions of
water that enters inlet hole 35 to flow radially in each of five different sectors defined by the
starfish baffle arms 39. Cylindrical baffles 38 disperse radially flowing water azimuthally. As
a result, water that enters tensiometer 20 through inlet hole 35 wets substantially equally all
regions of septum membrane 61 and the membrane becomes substantially impervious to
passage of air for the bubbling pressure for which it is intended.
Fig. 3 schematically shows a side cross-sectional view of tensiometer 20 shown in Fig,
1A and Fig. 2 connected to a sealed water reservoir 100 partially filled with water 120 and
being used to determine a value for the water matric potential y of a soil region 130, in
accordance with an embodiment of the invention. It is noted that whereas water reservoir 100
is shown above the surface of soil region 130, in practice, the water reservoir is generally
located below the surface of soil for which the tensiometer is used to measure water matric
potential.
Tensiometer 20 is positioned in soil region 130 so that soil-coupling region 72 of
hydraulic coupler 70 is in contact with soil in the soil region. A pressure gauge 102 is coupled
to water reservoir 100 to measure pressure in the reservoir. In Fig. 3, by way of example, the
pressure gauge is shown as a manometer having a left hand branch 103 coupled to water
reservoir 100 and a right hand branch 104 exposed to atmospheric pressure. The manometer is
assumed to comprise mercury 125 as a manometer fluid, and left hand branch 103 between the
mercury and water 120 in reservoir 100 is filled with water. Whereas in Fig. 3 pressure gauge
102 is shown as a manometer, in practice any suitable pressure gauge or sensor known in the
art may be used to provide a measure of pressure in reservoir 100.
Hydraulic coupler 70 provides a hydraulic coupling between soil in soil region 130 and
water in water reservoir 100 via contact between reservoir-coupling region 76 (Fig. 1 A) of the
hydraulic coupler and sealing septum 60. The soil draws water from or introduces water into
water reservoir 100 via the hydraulic coupler depending on whether the water matric potential
of soil region 130 is greater than or less than the pressure in water reservoir 100. Equilibrium
is established for which there is substantially no water flow from or into the reservoir when
pressure in the reservoir is equal to the soil water matric potential. Since the matric potential is
almost always negative, there is a vacuum in reservoir 100 above a waterline 121 of water 120
in the reservoir. In Fig. 3 mercury 125, is higher in left hand branch 103 of the manometer
connected to water reservoir 100 than in right hand branch 104 of the manometer exposed to
atmospheric pressure. A difference between the height of mercury in the left and right hand
branches provides a measure of the partial vacuum in water reservoir 100 and thereby of the
matric potential vj/.
In order to operate reliably, advantageously, septum membrane is maintained properly
wetted and does not have air trapped in its pores. However, during operation, air might leak
through hydraulic coupler 70 or seep through water 120 and be trapped by the membrane or in
spaces between baffles 37 and 38 of labyrinth 39. In order to purge septum 61 and/or labyrinth
36 of air that they may trap, a purge valve 105 is optionally connected to reservoir 100. Purge
valve 105 is connected to a suitable source of water (not shown) and in accordance with an
embodiment of the invention is periodically opened to flush water from the water source
through the reservoir, septum membrane 61, and labyrinth 36 to purge the septum and
labyrinth of air they may have trapped. Advantageously, the space above waterline 121 is
substantially a vacuum and water provided via purge valve 105 is used to remove air from
reservoir 100.
In an embodiment of the invention, to provide a measure of matric potential y in a
region of a field, a plurality of tensiometers, optionally of a type shown in Figs. 1A-3, is
positioned in soil at different locations in the field and coupled to a common sealed water
reservoir. Pressure in the common water reservoir provides a measure, i.e. "representative
matric potential", of water matric potential in the field that is intermediate a highest and lowest
value for water matric potential provided by the tensiometers. Optionally, the field is an
agricultural field for growing plants and the plurality of tensiometers and representative matric
potential is used to control irrigation of the plants in the field.
Fig. 4 schematically shows a configuration of tensiometers 200 distributed in the soil
of an agricultural field 240 in which plants 242 are grown, in accordance with an embodiment
of the invention. The tensiometers are connected to a same water reservoir 202 connected to a
pressure gauge 204 used to provide a measure of a partial vacuum in the reservoir and thereby
of a representative matric potential of the region of agricultural field 240 in which the
tensiometers are located.
By way of example, in Fig. 4 plants 242 are irrigated using an irrigation pipe 210,
comprising integrated emitters 212 and tensiometers 200 are of a type shown in Figs. 1A-3
having hydraulic couplers 70 formed from a geotextile in which roots 244 of plants 242 are
able to grow. In accordance with an embodiment of the invention, each tensiometer 200
coupled to water reservoir 202 is located in a neighborhood of a plant 242 and has its
hydraulic coupler 70 wrapped around a region of irrigation pipe 210 in which an emitter 212 is
located. Some roots 244 of plants 242 are shown growing into the geotextile fabric of
hydraulic couplers 70 of tensiometers 200. Because of the close proximity of emitters 212 and
plant roots 244 to hydraulic couplers 70, each tensiometer 200 is responsive to soil water
matric potential to which plants 242 are relatively sensitive and to changes in the matric
potential produced by water emitted by emitters 212.
In an embodiment of the invention, measurements of changes in pressure in reservoir
202, and thereby of changes in representative water matric potential of field 240, provided by
pressure gauge 204 are used to control water emitted by emitters 212. When the representative
water matric potential provided by pressure gauge 204 falls below a desired lower threshold
for water matric potential, emitters 212 are controlled to release water to the soil. When the
representative water matric potential rises above a desired upper threshold, the emitters are
prevented from delivering water to the soil.
Optionally, emitters 212 release water to soil region 240 only after pressure in
irrigation pipe 210 rises above a release water threshold pressure and water released by
emitters 212 is controlled by controlling pressure in the irrigation pipe. In some embodiments
of the invention, water release is controlled by pulsing pressure in irrigation pipe 210 above
the emitter threshold pressure. In some embodiments of the invention, pressure pulses are
periodic and are characterized by a pulse length. The period and pulse length of the pressure
pulse are optionally determined responsive to a "hydration" relaxation time of soil in soil
region 240 characteristic of a time it takes the soil to reach a limiting water matric potential
following release of a quantity of water to the soil by an emitter 212 during a pressure pulse.
Controlling release of water in accordance with an embodiment of the invention by pulsing
water pressure responsive to a soil hydration relaxation time can be advantageous in providing
relatively accurate control of irrigation. For example, it can be advantageous in preventing
over irrigation of plants 242.
The inventors of embodiments of the invention have carried out irrigation experiments
in which plants were irrigated responsive to a representative matric potential in accordance
with an embodiment of the invention. The inventors found that they were able to achieve
relatively improved crop yields with relatively smaller quantities of water than would
normally be provided to the plants.
Under some conditions, a representative water matric potential provided by a plurality
of tensiometers in accordance with an embodiment of the invention is substantially equal to an
average of the measurements provided by the tensiometers. For example, assume that at a
location of an "i-th" tensiometer 200, for convenience represented by "Tj", in soil region 240,
the water matric potential is ?i. At equilibrium, a partial vacuum in water reservoir 202 settles
down to a pressure equal to that of a representative matric potential "?o". At the
representative matric potential, as much water enters water reservoir 202 from tensiometers Tj
at locations for which matric potentials ?i > ?o as exits the water reservoir from tensiometers
Tj at locations for which ?i < ?o. Assume that water flow into or out of a tensiometer Tj is
proportional to (?i - ?o)/R where R is a resistance to water transport of soil in soil region 240,
which is the same for all locations of tensiometers Tj, and is independent of (?i - ?o). Then at
equilibrium, , so that ?o is an average of all the ?i.
However, it is expected that, in general, R will not only not be the same for all locations of soil
region 130 but will be dependent on (?i - ?o). As a result, it is expected that a given
representative water matric potential will in general be some sort of weighted average of the
matric potential at the locations of each of tensiometers 200.
In some embodiments of the invention, provision of water to an agricultural field by an
irrigation system, such as agricultural field 240 and the irrigation system shown in Fig. 4,
which provides measurements of soil water matric potential ? is controlled in accordance with
an algorithm 300 having a flow diagram similar to that shown in Figs. 5A and 5B. The flow
diagram delineates an optionally diurnal water provision cycle in which the irrigation system
provides pulses of water to the field subject to certain "trigger" conditions, described below,
prevailing.
In a block 301, optionally values for parameters that control the water provision cycle
Tcal, Tdiff, TB and TE are determined. Tcal is a time during the diurnal cycle at which the
irrigation system calibrates water matric potential measurements and acquires a calibration
water matric potential measurement M0. M0 is optionally acquired during night after a period
of time during which irrigation was not provided and water demand by plants in the field is
minimal. Optionally, Tcai is about 0500. T^ff is an optionally fixed, maximum time lapse
allowed by algorithm 300 between provision of pulses of water to field 240. Optionally, T&ff
is equal to about 5 hours. Tjj is a time following time Tca] at which the irrigation system
begins a period of "active irrigation" in which it provides a pulse of water to field 240 when a
trigger condition occurs. Tg is a time at which the active irrigation period ends. Optionally,
Tg is about an hour later than Tcai and Tg is a time at about dusk, for example about 1700.
In a step 302, algorithm 300 checks a system clock (not shown) to acquire a reading of
the time, "T^odc". In a decision block 303 the time T^^ is checked to see if it is about
equal to Tca]. If it is not, then the algorithm returns to block 302 to acquire a new reading for
Tclock- If on the other hand T^o^ is about equal to Tcai, algorithm 300 advances to a block
304 and acquires a calibration reading, M0, of the soil matric potential \\i. The algorithm then
proceeds to acquire another reading, TqIqq]^, of the system clock in a block 305 and then
proceeds to a decision block 306. In decision block 306 algorithm 300 determines if Tdock is
greater than or equal to time Tq at which active irrigation of field 240 is to commence. If
Tclock is less tnan Tjj' the algorithm returns to block 305 to acquire another reading for
Tclock- If on me other hand Tciock is greater than or about equal to Tg, algorithm 300
advances to a block 307 and sets a variable time parameter Tp equal to T^o^, and in a block
308 optionally sets AT equal to (T^o^ - Tp), which initializes AT to zero.
Optionally, in a decision block 309, algorithm 300 determines if AT is greater than
Tjiff. If it is not, (which at this stage, immediately after initialization, is the case) algorithm
300 optionally skips to a block 313. In block 313 algorithm 300 acquires a measurement Mi of
the water matric potential of field 240, optionally responsive to readings from tensiometers
200 (Fig. 4), and proceeds to determine in a decision block 314 if the absolute value of |Mj| is
greater than the absolute value |M0| acquired in block 304. If |MjJ is greater than |M0|,
algorithm 300 optionally proceeds to a block 315 and controls the irrigation system to provide
a pulse of water to field 240.
In some embodiments of the invention, a pulse of water provided by the irrigation
system is determined to provide about 0.6 liters of water per m^ of field 240. The inventors
have determined that aforementioned amount of water per pulse is convenient to maintain
appropriate irrigation, generally, if a time between pulses is greater than or about equal to 0.5
hours. In some embodiments of the invention, algorithm 300 increases an amount of water
provided by an irrigation pulse if time between pulses decreases to less than about 0.5 hours.
For example, if irrigation algorithm 300 "finds" that |Mt| increases relatively rapidly,
indicating a requirement for irrigation pulses every 0.25 hours, optionally the algorithm
increases the mount of water provided by an irrigation pulse. Optionally, the algorithm
increases water provided by a pulse to about 0.9 liters/m2 if it finds that demand for irrigation
pulses reaches a rate of about 4 pulses per hour.
Following provision of the pulse of water, algorithm 300 proceeds to a block 316 and
acquires a new reading for T^^ and resets Tp to Tciock in a block 317. It is noted that in
decision block 314, if |MtJ is less than |M0|, algorithm 300 skips blocks 315 to 317, does not
provide a pulse of water, and goes directly to a decision block 318 shown in Fig. 5B.
Returning to block 309 if AT is greater than T^ff, algorithm 300 does not skip to block
314 where it measures Mj, but rather, optionally, proceeds to a block 310 and provides a pulse
of irrigating water to field 240. Thereafter the algorithm proceeds to a block 311, acquires a
new reading for T^o^, and in a block 312 resets Tp to T^o^. It then proceeds to block 314
to measure M\ and via blocks 315-317 eventually to decision block 318.
In decision block 318 algorithm 300 determines if T^^ is greater than or equal to
Tg, the time set in block 301 at which the active irrigation period ends and a new irrigation
cycle begins. If Tcj0ck *s ^ess man Te» algorithm 300 returns to block 308 and resets AT,
otherwise, the algorithm returns to block 302 to begin the cycle again.
In some embodiments of the invention, an agricultural field, such as field 240 (Fig. 4)
is irrigated in accordance with an algorithm 400 having a flow diagram shown in Fig. 6.
Algorithm 400 controls an irrigation system to continuously provide water to agricultural field
240 during an active irrigation period instead of by pulsing water provision.
In a block 401 of algorithm 400, optionally parameters Tg, Tg, Tjjff, Tj^ Tcai, and
Mdiff are set. As in algorithm 300, Tg and Tg are begin and end times of active irrigation and
Tcai is a calibration time. Tjn- is an initial value for duration of the active irrigation period, and
Tdjff is an adjustment to T[n, which algorithm 400 makes subject to certain water matric
potential conditions of field 240. M^iff is an optionally fixed, maximum change in water
matric potential for which algorithm 400 does not adjust T[n. Affects of the parameters set in
block 401 on decisions of algorithm 400 are clarified below. In some embodiments of the
invention, Tirr and Tdiff have values equal to about 3 hours and 0.2 hours, respectively. M^iff
is optionally a positive number having value equal to a fraction less than one of a typical
matric potential for the field being irrigated with the irrigation system. Optionally, M^iff is
equal to about 5% of a calibration matric potential acquired for the field. Optionally, for a
given day, M^iff is equal to 5% of a calibration matric potential for a previous day.
In a block 402, algorithm 400 acquires a value for Tciock, and optionally in a decision
block 403 determines if T^ck is equal to Tcaj. If it is not it returns to block 402 to acquire a
new value for Td^. On the other hand, if Tci0ck is equal to Tcai the algorithm proceeds to a
block 404 and acquires a reading "Mn" for the water matric potential y of field 240. The
subscript "n" refers to an "n-th" day, assumed a current day, of operation of the irrigation
system in providing water to field 240. In a block 404, algorithm 400 stores the value for Mn
in a suitable memory. In a block 405 the algorithm optionally assigns a value to AM equal to a
difference between of the current reading Mn of the water matric potential and a value of a
reading, Mn_], of the water matric potential acquired for the day before the current day.
In a decision block 406, algorithm 400 determines if an absolute value of AM is greater
than or equal to Mjjff. If it is, the algorithm proceeds to a decision block 407 to determine if
AM is greater than or equal to zero. If AM is greater than zero, the algorithm proceeds from
block 407 to a block 408 where it decreases Tin- by ^ amount T^iff and then proceeds to a
block 410 to acquire time Tci0ck- If AM is less than zero, the algorithm proceeds from block
407 to a block 409 where it increases T^ by an amount T^fff and then proceeds to a block
410 to acquire time Tciock
If in decision block 406 the absolute value of AM is less than Mjiff, then algorithm
400 skips directly from block 406 to block 410 to acquire Tci0ck> skipping blocks 407, 408
and 409.
From block 410, the algorithm proceeds to decision block 411. In decision block 411,
algorithm 400 determines if TciQCk acquired in block 410 is greater than or equal to the active
irrigation begin time Tq. If it is not, it returns to block 410 to acquire a new value for Tciock
and then to block 411 to test the new T^o^. If in block 411 the algorithm determines that
Tciock 1S greater than or equal to Tg, the algorithm proceeds to a block 412 and begins
continuous irrigation of field 240.
From block 412 the algorithm continues to a block 413 to acquire a new value for
Tclock an<* in a decision block 414 determines if (Tciock - Tg) is greater than or equal to T[n.
If it is not, the algorithm returns to block 412 to continue continuous irrigation of field 240. If
on the other hand, (Tciock - Tg) > T\n then the algorithm ends continuous irrigation and
returns to block 403.
In the description and claims of the present application, each of the verbs, "comprise"
"include" and "have", and conjugates thereof, are used to indicate that the object or objects of
the verb are not necessarily a complete listing of members, components, elements or parts of
the subject or subjects of the verb.
The invention has been described with reference to embodiments thereof that are
provided by way of example and are not intended to limit the scope of the invention. The
described embodiments comprise different features, not all of which are required in all
embodiments of the invention. Some embodiments of the invention utilize only some of the
features or possible combinations of the features. Variations of embodiments of the described
invention and embodiments of the invention comprising different combinations of features
than those noted in the described embodiments will occur to persons of the art. The scope of
the invention is limited only by the following claims.
CLAIMS
1. A tensiometer for use in determining matric potential of a soil comprising:
a water inlet;
a hydraulic coupler comprising a porous material for providing hydraulic coupling
between water that enters the inlet and the soil; and
a septum that seals water that enters the inlet against ingress of air via the porous
material.
2. A tensiometer according to claim 1 wherein the porous material comprises a geotextile.
3. A tensiometer according to claim 1 or claim 2 wherein the porous material is adapted
to enable growth of plant roots therein.
4. A tensiometer according to any of the preceding claims wherein the septum comprises
a septum surface, at least a part of which is contiguous with water that enters the inlet.
5. A tensiometer according to claim 4 wherein the tensiometer comprises a water
labyrinth having baffles.
6. A tensiometer according to claim 5 wherein a portion of the septum surface contacts
the baffles.
7. A tensiometer according to any of claims 4-6 wherein the septum comprises a
membrane and the septum surface is a surface of the membrane.
8. A tensiometer according to claim 6 wherein the membrane comprises a plurality of
layers.
9. A tensiometer according to claim 8 wherein the layers comprise a first layer having a
bubbling pressure greater than about a maximum absolute value of the matric potential of the
soil in which the tensiometer is used.
10. A tensiometer according to claim 9 wherein the first layer is supported by at least one
support layer.
11. A tensiometer according to claim 10 wherein the first layer is sandwiched between two
support layers.
12. A tensiometer according to any of claims 1-8 wherein the septum has a bubbling
pressure greater than about a maximum absolute value of the matric potential of the soil in
which the tensiometer is used.
13. A tensiometer according to any of claims 9-12 wherein the bubbling pressure is about
equal to one atmosphere.
14. A tensiometer according to any of claims 1-13 and comprising an elastic member that
resiliently presses the porous material to the septum.
15. A tensiometer according to any of claims 1-14 and comprising a water reservoir
coupled to the water inlet.
16. A tensiometer according to any of claims 1-14 and comprising a device for providing a
measure of pressure in the water reservoir.
17. An irrigation system comprising:
an irrigation pipe having at least one output orifice for outputting water from the pipe;
at least one tensiometer according to any of claims 1-16 coupled to the irrigation pipe
so that water output from an orifice of the at least one orifice is constrained to pass
substantially directly from the orifice through the hydraulic coupler.
18. An irrigation system according to claim 17 wherein the irrigation pipe comprises at
least one emitter and an output orifice is an orifice of the at least one emitter.
19. An irrigation system according to claim 17 or claim 18 wherein the at least one emitter
is an integrated emitter.
20. An irrigation system according to claim 18 or claim 19 wherein the at least one emitter
comprises a plurality of emitters.

A tensiometer for use in determining matric potential of a soil comprising:
a water inlet; a hydraulic coupler comprising a porous material for
providing hydraulic coupling between water that enters the inlet and the
soil; and a septum that seals water that enters the inlet against ingress of
air via the porous material.