Abstract:
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.

Description:
RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application 60/935,571 filed Aug. 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 Aug. 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. U.S. Pat. No. 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. U.S. Pat. No. 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. U.S. Pat. No. 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&#39;s at a different soil depth to provide measurements of soil moisture content. U.S. Pat. No. 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 “ψ”, 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, U.S. Pat. No. 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. 
     U.S. Pat. No. 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&#39;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&#39;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 resiliently 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. 1B  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. 1B , in accordance with an embodiment of the invention; 
         FIG. 1D  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 
       FIG. 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. 1B-1D  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. 1B ) 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 resiliently 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 ψ 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. 1A ) 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 ψ. 
     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 ψ 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 “T i ”, in soil region  240 , the water matric potential is ψ i . At equilibrium, a partial vacuum in water reservoir  202  settles down to a pressure equal to that of a representative matric potential “ψ o ”. At the representative matric potential, as much water enters water reservoir  202  from tensiometers T i  at locations for which matric potentials ψ i &gt;ψ o  as exits the water reservoir from tensiometers T i  at locations for which ψ i &lt;ψ o . Assume that water flow into or out of a tensiometer T i  is proportional to (ψ i −ψ 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 T i , and is independent of (ψ i −ψ o ). Then at equilibrium, 
                   ∑   i   N     ⁢       (       Ψ   ⁢           ⁢   i     -     Ψ   ⁢           ⁢   o       )     ⁢     /     ⁢   R       =       0   ⁢           ⁢   and   ⁢           ⁢     ψ   o       =       (     1   ⁢     /     ⁢   N     )     ⁢       ∑   i   N     ⁢   Ψi           ,         
so that ψ o  is an average of all the ψ 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 (ψ i −ψ 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 ψ 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 T cal , T diff , T B  and T E  are determined. T cal  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 M o . M o  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, T cal  is about 0500. T diff  is an optionally fixed, maximum time lapse allowed by algorithm  300  between provision of pulses of water to field  240 . Optionally, T diff  is equal to about 5 hours. T B  is a time following time T cal  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. T E  is a time at which the active irrigation period ends. Optionally, T B  is about an hour later than T cal  and T E  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 clock ”. In a decision block  303  the time T clock  is checked to see if it is about equal to T cal . If it is not, then the algorithm returns to block  302  to acquire a new reading for T clock . If on the other hand T clock  is about equal to T cal , algorithm  300  advances to a block  304  and acquires a calibration reading, M o , of the soil matric potential ψ. The algorithm then proceeds to acquire another reading, T clock , of the system clock in a block  305  and then proceeds to a decision block  306 . In decision block  306  algorithm  300  determines if T clock  is greater than or equal to time T B  at which active irrigation of field  240  is to commence. If T clock  is less than T B , the algorithm returns to block  305  to acquire another reading for T clock . If on the other hand T clock  is greater than or about equal to T B , algorithm  300  advances to a block  307  and sets a variable time parameter T P  equal to T clock , and in a block  308  optionally sets ΔT equal to (T clock −T P ), which initializes ΔT to zero. 
     Optionally, in a decision block  309 , algorithm  300  determines if ΔT is greater than T diff . 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 M I  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 |M I | is greater than the absolute value |M o | acquired in block  304 . If |M I | is greater than |M o |, 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 2  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 |M I | 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/m 2  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 clock  and resets T P  to T clock  in a block  317 . It is noted that in decision block  314 , if |M I | is less than |M o |, 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 ΔT is greater than T diff , algorithm  300  does not skip to block  314  where it measures M I , 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 clock , and in a block  312  resets T P  to T clock . It then proceeds to block  314  to measure M I  and via blocks  315 - 317  eventually to decision block  318 . 
     In decision block  318  algorithm  300  determines if T clock  is greater than or equal to T E , the time set in block  301  at which the active irrigation period ends and a new irrigation cycle begins. If T clock  is less than T E , algorithm  300  returns to block  308  and resets ΔT, 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 T B , T E , T diff , T irr , T cal , and M diff  are set. As in algorithm  300 , T B  and T E  are begin and end times of active irrigation and T cal  is a calibration time. T irr  is an initial value for duration of the active irrigation period, and T diff  is an adjustment to T irr , which algorithm  400  makes subject to certain water matric potential conditions of field  240 . M diff  is an optionally fixed, maximum change in water matric potential for which algorithm  400  does not adjust T irr . Affects of the parameters set in block  401  on decisions of algorithm  400  are clarified below. In some embodiments of the invention, T irr  and T diff  have values equal to about 3 hours and 0.2 hours, respectively. M diff  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 diff  is equal to about 5% of a calibration matric potential acquired for the field. Optionally, for a given day, M diff  is equal to 5% of a calibration matric potential for a previous day. 
     In a block  402 , algorithm  400  acquires a value for T clock , and optionally in a decision block  403  determines if T clock  is equal to T cal . If it is not it returns to block  402  to acquire a new value for T clock . On the other hand, if T clock  is equal to T cal  the algorithm proceeds to a block  404  and acquires a reading “M n ” for the water matric potential ψ 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 M n  in a suitable memory. In a block  405  the algorithm optionally assigns a value to ΔM equal to a difference between of the current reading M n  of the water matric potential and a value of a reading, M n-1 , 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 ΔM is greater than or equal to M diff . If it is, the algorithm proceeds to a decision block  407  to determine if ΔM is greater than or equal to zero. If ΔM is greater than zero, the algorithm proceeds from block  407  to a block  408  where it decreases T irr  by an amount T diff  and then proceeds to a block  410  to acquire time T clock . If ΔM is less than zero, the algorithm proceeds from block  407  to a block  409  where it increases T irr  by an amount T diff  and then proceeds to a block  410  to acquire time T clock . 
     If in decision block  406  the absolute value of ΔM is less than M diff , then algorithm  400  skips directly from block  406  to block  410  to acquire T clock , skipping blocks  407 ,  408  and  409 . 
     From block  410 , the algorithm proceeds to decision block  411 . In decision block  411 , algorithm  400  determines if T clock  acquired in block  410  is greater than or equal to the active irrigation begin time T B . If it is not, it returns to block  410  to acquire a new value for T clock  and then to block  411  to test the new T clock . If in block  411  the algorithm determines that T clock  is greater than or equal to T B , 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 T clock  and in a decision block  414  determines if (T clock −T B ) is greater than or equal to T irr . If it is not, the algorithm returns to block  412  to continue continuous irrigation of field  240 . If on the other hand, (T clock −T B )&gt;T irr  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.