AU2016201306A1 - A method of irrigation - Google Patents

Abstract

A method of irrigating a field comprising acquiring a calibration water matric potential for the field, and irrigating the field during an active irrigation period with an amount 5 of water responsive to the value of the calibration matric potential. Acquiring a calibration water matric potential comprises acquiring a calibration water matric potential at least once a day when plants in the field exhibit relatively small water demand. -20 7455970_1 (GHMatters) P83166.AU.3 AJM .- 300 SET TB,TE,TalTdiff 30 i ... 302 ACQUIRE Tclock clock =-, Tcal N CALIBRATE Mo 3 -04 ..ACQUIRE Tcloc~k 305 ? <*YES SET Tp= Tclock 3j-07 SET AT= Tclock -Tp J08 NO T df RRIGAT) ON PULSE 1,9 ACQUIRE TclockJ1 FSET Tp= TcIockJ J12 MEASURE MI31 A " :: B C

Description

A METHOD OF IRRIGATION RELATED APPLICATIONS The present application is divided from application no. 2014202521 filed 9 May 5 2014 divided from application no. 2012238263 filed 9 October 2012, which is divided from application no. 2008290202 filed 5 August 2008, which is based on and claims the benefit of the filing and priority dates of US Provisional Application No. 60/935,571 filed 20 August 2007, the disclosures of all of which are incorporated herein by reference. 10 FIELD The invention relates to a method of irrigation. BACKGROUND OF THE INVENTION 15 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 20 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 25 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 30 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. 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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 15 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 20 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. 25 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 "Y", is a measure of how strongly particulate soil matter attracts water to adhere to the 30 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 -2 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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. 15 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 20 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 25 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 30 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 -3 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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". 15 SUMMARY OF THE INVENTION According to a broad aspect of the invention, there is provided a method of irrigating a field, the method comprising: acquiring a calibration water matric potential for the field; and 20 irrigating the field during an active irrigation period with an amount of water responsive to the value of the calibration matric potential, wherein acquiring a calibration water matric potential comprises acquiring a calibration water matric potential at least once a day when plants in the field exhibit relatively small water demand. 25 In an embodiment, irrigating a field comprises performing an irrigation cyclically. In an example, irrigating the field cyclically comprises irrigating the field in diurnal cycles. In another embodiment, prior to the active irrigation period is when the acquiring of the calibration water matric potential is performed. 30 In one embodiment, acquiring the calibration water matric potential comprises acquiring the matric potential at night or in the early dawn hours. In an embodiment, providing an amount of water comprises acquiring a water -4 7455970_1 (GHMatters) P83166.AU.3 AJM 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. In an example, comparing the additional water matric to the calibration 5 matric potential comprises determining their difference. For example, the difference may be determined between absolute values of the additional water matric potential measurement and the calibration water matric potential. In an example, the method includes providing the amount of water responsive to the difference only if the absolute value of the additional water matric potential measurement is greater than the absolute 10 value of the calibration water matric potential. In another embodiment, the method comprises determining a maximum time lapse parameter being a maximum elapsed time between provision of amounts of water to the field; and, upon the time that has passed since provision of an amount of water reaching the maximum time lapse parameter, providing an additional amount of water 15 to the field. In an example, if immediately after providing an additional amount of water due to the maximum time lapse parameter, also responsive to the value of the calibration matric potential an additional subsequent amount of water is required, then the additional subsequent amount of water is also provided. In an embodiment, the method includes acquiring the additional water matric 20 potential measurement and comparing it to the calibration water matric potential continuously during the active irrigation period. In another embodiment, providing an amount of water comprises providing a pulse of water. 25 BRIEF DESCRIPTION OF FIGURES In order that the invention may be more clearly ascertained, 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 30 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. -5 7455970_1 (GHMatters) P83166.AU.3 AJM 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; 5 Fig. IC 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 10 Figs. 1A and iB, 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 15 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 20 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 25 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. 1B to 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 30 tensiometer, even though, as shown in Figs. 1A to ID, 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 -6 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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. 15 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. iB, 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 20 30 in Fig. IC, 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. 25 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 30 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 -7 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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 15 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. 20 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 25 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. 30 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 -8 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 10 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 15 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 Y7 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 20 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 25 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 30 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 -9 7455970_1 (GHMatters) P83166.AU.3 AJM (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 5 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 10 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 x. In order to operate reliably, advantageously, septum membrane is maintained properly wetted and does not have air trapped in its pores. However, during operation, 15 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 20 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 25 in a region of a field, a plurality of tensiometers, optionally of a type shown in Figs. 1A to 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 30 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. - 10 7455970_1 (GHMatters) P83166.AU.3 AJM 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 5 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 to 3 having hydraulic couplers 70 formed from a geotextile in which roots 244 10 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. 15 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 20 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 25 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 30 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 - 11 7455970_1 (GHMatters) P83166.AU.3 AJM "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 5 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 10 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 15 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 "Ti", in soil region 240, the water matric potential is Vi. At equilibrium, a partial vacuum in water reservoir 202 settles down to a pressure equal to that of a representative matric potential "yo". At the representative matric potential, as much 20 water enters water reservoir 202 from tensiometers Ti at locations for which matric potentials Vi > yo as exits the water reservoir from tensiometers Ti at locations for which Vi < yo. Assume that water flow into or out of a tensiometer Ti is proportional to (Vi - yo)/R where R is a resistance to water transport of soil in soil region 240, which is the same for all locations of tensiometers Ti, and is independent of (Yi - YO). N N 25 Then at equilibrium, Z(Wi-Wo)/R = 0 and yo = (1/N)Z i, so that yo is an / / average of all the Vi. 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 (Yi - YO). 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 - 12 7455970_1 (GHMatters) P83166.AU.3 AJM 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 Y is 5 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 10 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 MO. Mo is optionally acquired at night after a period of time during which irrigation was not provided and water demand by plants in the field is minimal. Optionally, Tcal is about 0500. Tdiff is 15 an optionally fixed, maximum time lapse allowed by algorithm 300 between provision of pulses of water to field 240. Optionally, Tdiff is equal to about 5 hours. TB is a time following time Tcal 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. TE is a time at which the active irrigation period ends. Optionally, TB is about an hour later 20 than Tcal and TE 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, "Tclock". In a decision block 303 the time Tclock is checked to see if it is about equal to Tcal. If it is not, then the algorithm returns to block 302 to acquire a new reading for Tclock. If on the other hand Tclock is about equal to Tcal, algorithm 25 300 advances to a block 304 and acquires a calibration reading, M 0 , of the soil matric potential x. The algorithm then proceeds to acquire another reading, Tclock, of the system clock in a block 305 and then proceeds to a decision block 306. In decision block 306 algorithm 300 determines if Tclock is greater than or equal to time TB at which active irrigation of field 240 is to commence. If Tclock is less than TB, the 30 algorithm returns to block 305 to acquire another reading for Tclock. If on the other - 13 7455970_1 (GHMatters) P83166.AU.3 AJM hand Tclock is greater than or about equal to TB, algorithm 300 advances to a block 307 and sets a variable time parameter Tp equal to Tclock, and in a block 308 optionally sets AT equal to (Tclock - Tp), which initializes AT to zero. Optionally, in a decision block 309, algorithm 300 determines if AT is greater 5 than Tdiff 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 |MIl is greater than the absolute value Ml acquired in 10 block 304. If |MIl is greater than M 0 l, 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 litres of water per m 2 of field 240. The inventors have determined that aforementioned amount of water per pulse is 15 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 |MIl increases relatively rapidly, indicating a requirement for 20 irrigation pulses every 0.25 hours, optionally the algorithm increases the amount of water provided by an irrigation pulse. Optionally, the algorithm increases water provided by a pulse to about 0.9 litres/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 25 316 and acquires a new reading for Tclock and resets Tp to Tclock in a block 317. It is noted that in decision block 314, if |MIl is less than Ml, 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 Tdiff, algorithm 300 does not skip 30 to block 314 where it measures MI, but rather, optionally, proceeds to a block 310 and - 14 7455970_1 (GHMatters) P83166.AU.3 AJM provides a pulse of irrigating water to field 240. Thereafter the algorithm proceeds to a block 311, acquires a new reading for Tclock, and in a block 312 resets Tp to Tclock. It then proceeds to block 314 to measure MI and via blocks 315-317 eventually to decision block 318. 5 In decision block 318 algorithm 300 determines if Tclock is greater than or equal to TE, the time set in block 301 at which the active irrigation period ends and a new irrigation cycle begins. If Tclock is less than TE, algorithm 300 returns to block 308 and resets AT, otherwise, the algorithm returns to block 302 to begin the cycle again. 10 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. 15 In a block 401 of algorithm 400, optionally parameters TB, TE, Tdiff , Tirr, Tcal, and Mdiff are set. As in algorithm 300, TB and TE are begin and end times of active irrigation and Tcal is a calibration time. Tirr is an initial value for duration of the active irrigation period, and Tdiff is an adjustment to Tirr, which algorithm 400 makes subject to certain water matric potential conditions of field 240. Mdiff is an optionally 20 fixed, maximum change in water matric potential for which algorithm 400 does not adjust Tirr. Effects 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. Mdiff is optionally a positive number having value equal to a fraction less than one of a typical matric potential for 25 the field being irrigated with the irrigation system. Optionally, Mdiff is equal to about 5% of a calibration matric potential acquired for the field. Optionally, for a given day, Mdiff is equal to 5% of a calibration matric potential for a previous day. In a block 402, algorithm 400 acquires a value for Tclock, and optionally in a decision block 403 determines if Tclock is equal to Tcal. If it is not it returns to block - 15 7455970_1 (GHMatters) P83166.AU.3 AJM 402 to acquire a new value for Tclock. On the other hand, if Tclock is equal to Teal the algorithm proceeds to a block 404 and acquires a reading "Mn" for the water matric potential 17 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 5 404, 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-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 AM is 10 greater than or equal to Mdiff 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 Tirr by an amount Tdiff and then proceeds to a block 410 to acquire time Tclock. If AM is less than zero, the algorithm proceeds from block 407 to a block 409 where it increases Tirr by an 15 amount Tdiff and then proceeds to a block 410 to acquire time Tclock. If in decision block 406 the absolute value of AM is less than Mdiff, then algorithm 400 skips directly from block 406 to block 410 to acquire Tclock, skipping blocks 407, 408 and 409. From block 410, the algorithm proceeds to decision block 411. In decision 20 block 411, algorithm 400 determines if Tclock acquired in block 410 is greater than or equal to the active irrigation begin time TB. If it is not, it returns to block 410 to acquire a new value for Tclock and then to block 411 to test the new Tclock. If in block 411 the algorithm determines that Tclock is greater than or equal to TB, the algorithm proceeds to a block 412 and begins continuous irrigation of field 240. 25 From block 412 the algorithm continues to a block 413 to acquire a new value for Tclock and in a decision block 414 determines if (Tclock - TB) is greater than or equal to Tirr. If it is not, the algorithm returns to block 412 to continue continuous irrigation of field 240. If on the other hand, (Tclock - TB) > Tirr then the algorithm ends continuous irrigation and returns to block 403. - 16 7455970_1 (GHMatters) P83166.AU.3 AJM 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; rather, the verb is 5 used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 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 10 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 15 occur to persons of the art. Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge in any country. - 17 7455970_1 (GHMatters) P83166.AU.3 AJM

Claims (13)

1. A method of irrigating a field, the method comprising: acquiring a calibration water matric potential for the field; and 5 irrigating the field during an active irrigation period with an amount of water responsive to the value of the calibration matric potential, wherein acquiring a calibration water matric potential comprises acquiring a calibration water matric potential at least once a day when plants in the field exhibit relatively small water demand. 10

2. A method according to claim 1 wherein irrigating a field comprises performing an irrigation cyclically.

3. A method according to claim 2 wherein irrigating the field cyclically comprises 15 irrigating the field in diurnal cycles.

4. A method according to any one of the preceding claims wherein prior to the active irrigation period is when the acquiring of the calibration water matric potential is performed. 20

5. A method according to any one of the preceding claims wherein acquiring the calibration water matric potential comprises acquiring the matric potential at night or in the early dawn hours. 25

6. A method according to any one of the preceding claims wherein 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. 30

7. A method according to claim 6 wherein comparing the additional water matric to the calibration matric potential comprises determining their difference. - 18 7455970_1 (GHMatters) P83166.AU.3 AJM

8. A method according to claim 7 wherein the difference is determined between absolute values of the additional water matric potential measurement and the calibration water matric potential. 5

9. A method according to claim 8 including providing the amount of water responsive to the difference only if the absolute value of the additional water matric potential measurement is greater than the absolute value of the calibration water matric potential 10

10. A method according to any one of the preceding claims comprising determining a maximum time lapse parameter being a maximum elapsed time between provision of amounts of water to the field; and, upon the time that has passed since provision of an amount of water reaching the maximum time lapse parameter, providing an additional 15 amount of water to the field.

11. A method according to claim 10 wherein if immediately after providing an additional amount of water due to the maximum time lapse parameter, also responsive to the value of the calibration matric potential an additional subsequent amount of 20 water is required, then the additional subsequent amount of water is also provided.

12. A method according to any one of claims 6 to 9 including acquiring the additional water matric potential measurement and comparing it to the calibration water matric potential continuously during the active irrigation period. 25

13. A method according to any one of the preceding claims wherein providing an amount of water comprises providing a pulse of water. 30 - 19 7455970_1 (GHMatters) P83166.AU.3 AJM