WO2005032240A1

WO2005032240A1 - Provision of evapotranspiration drive technique to certain types of irrigation scheduling system

Info

Publication number: WO2005032240A1
Application number: PCT/AU2004/001350
Authority: WO
Inventors: James Dunstone Townsend
Original assignee: James Dunstone Townsend
Priority date: 2003-10-03
Filing date: 2004-10-04
Publication date: 2005-04-14

Abstract

A method for remote management of at least one irrigation site, the site having a receiving device (4) in communication with one or more switching devices for controlling irrigation of the site, the method comprising the steps of (a) making progressive estimates of soil moisture content of the site; (b) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (c) monitoring at least one evapotranspiration loss (ET) factor for the site over a period of time; (d) calculating an ET value for the site for that period of time; (e) comparing the ET value to the optimum irrigation event to establishing an ET drive factor; and (f) in response to the receiving device (4) receiving an irrigation signal to carry out an irrigation program, causing the switching devices to operate dependently upon the ET drive factor.

Description

Provision of evapotranspiration drive technique to certain types of irrigation scheduling system

Field of the Invention

This invention relates to a method of enhancing otherwise sophisticated switching control, such as for, but not limited to, the provision of irrigation control and/or scheduling services and other related purposes, to a site or to a series of sites.

Background to the Invention

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.

In respect of irrigation control technology, devices to start and stop irrigation cycles without human intervention (generally known, and referred to hereinafter as "controllers") are well known and are the subject of numerous patents and patent applications. These devices send an electric current (usually 24vAC in horticultural, agricultural or domestic use) to a remote solenoid valve causing that valve to open. Closure of the valve is usually effected by discontinuing the supply of electric current to the solenoid of the valve whereupon the valve is closed by a variety of means not material to this invention. Most controllers are able to accommodate a number of such valves; opening and closing them in a programmed succession for programmed times on programmed days of the week. This series of sequential valve opening and closing for specified times on specified days is generally referred to as a "program" or an "irrigation program". Many of the known devices are capable of storing and executing more than one such "program", thus adding a degree of flexibility into what may be accomplished. It is well known that a major problem and failing of all such devices is that they are capable only of repeating the "program" or "programs" without any ability to respond to weather conditions, rain etc. As a result of this problem and failing, the operation of the type of irrigation control device (controller) described above almost inevitably results in considerable wastage of water. This wastage of water by controllers that can only repeat a predetermined schedule is well known to the industry and a variety of methods and inventions have been used in an attempt to prevent such wastage. These include:

• rain switches which prevent controller operation during and shortly after rain

• soil moisture measuring devices which prevent controller operation when soil is judged to be sufficiently moist

• controllers which are able to schedule their operations using meteorological data

The inventor has previously taught methods intended as a considerable advance on the above- mentioned techniques, summarised in USP 6,076,740.

In these above-mentioned methods an appropriate program is entered into the "controller" which is told to apply this program to the site every night. Irrigation is then prevented or permitted by a device usually (but not always) including a relay which enables/disables controller activity and/or output of the controller in response to, in one form of the invention, radio signals. This device is hereinafter referred to as the "on-site electronic device".

The controller and the on-site electronic device accordingly work together to ensure that, under ideal conditions:

• only a calculated amount of irrigation water is applied to an irrigation site under such control

• such application occurs only when required; and

• should rain fall at the site while irrigation is occurring, the irrigation activity will be stopped.

Detailed descriptions concerning the manner in which this is accomplished can be found in other teachings of the inventor, particularly USP 6,076,740 and AU 772133. The contents of those prior patents are incorporated by reference in the current specification.

In contrast to these systems, in the USA, many use an "ET drive" system. For the purposes of this specification the "ET drive" means the ability to replace on a frequent, usually but not invariably daily, basis the water lost from the soil by evapo-transpiration (ET) on that, or the previous, day.

ET is usefully explained in USP 5,870,302 (Oliver). An ET value is calculated using a combination of meteorological data. These factors include temperature, solar radiation, wind speed, vapour pressure or humidity, and barometric pressure. A change in one or more of these parameters can have a direct effect on the ET value used to determine when and how much to water is to be applied to a site. Specific types of vegetation at the site are also weighted to allow further refinement to the calculation of the ET. Those weightings are well known and are used in conjunction with other coefficients to determine how much water to apply to replenish the water lost from the soil. Those other coefficients include: (i) type of vegetation; (ii) soil type; (iii) root depth; (iv) topography; (v) micro-climate; and (vi) density of vegetation. The following is an explanation given in Oliver.

Vegetation Type.

For a particular type of vegetation, such as cool season grass, the ET value represents the amount of water that has to be spread over the vegetation to replace the moisture lost by the natural and ongoing process of evaporation and transpiration. Accordingly, ET values are usually normalized to a specific type of plant or crop. For example, various plants require different amounts of moisture in the soil to sustain an optimal appearance and healthy growth environment. Plants which are drought-tolerant require less water than a baseline crop, such as grass, while lush plant types require more water. A crop coefficient (Kc) value is used to adjust the baseline ET value for a particular plant type. For example, the crop coefficient Kc for shrub-type plants might be 0.5, while the Kc for cool-season grass might be 0.8. In addition, the Kc is also dependent on the time of year. That is, the Kc function is cyclic in nature, with the maximum generally occurring during the spring and the minimum during the winter.

Soil Type. The ability of soil to absorb and retain applied water is an important consideration in determining how much and how often to water. Sandy soils do not retain water well, so less water with more frequency is needed, or water will percolate beyond the root zone and be wasted. On the other hand, clay soils retain water well, meaning more water with less frequency can be applied. In applying water, the absorption rate also needs to be taken into account to avoid water run off. Sandy soils have a high absorption rate as compared to clay soils. In the latter case, the total amount of water to apply needs to be divided into multiple watering cycles with each cycle having a relatively short watering time with a waiting time between cycles. Root Depth. The root zone depth of plants to be watered must also be taken into account. If too much water is applied, the water will percolate beyond the root zone and be wasted. Root zone depth also affects the frequency of watering. A plant with a deep root zone needs less frequent but longer watering times. A plant with a shallow root zone needs more frequent but shorter watering times. Topography.

Topography is an important consideration in watering, since a steep slope will have a higher amount of run off than a shallow slope. Steeper slopes require multiple cycles with short watering times and wait times between cycles to allow penetration of the applied water into the soil.

Micro-climate.

Micro-climate takes into account existing conditions immediately surrounding the area which is to be watered. These conditions can include fully or partial shaded areas, parking lot areas, park areas with trees, etc. Since shaded areas do not require as much water as sunlit areas, less water is needed. A micro-climate coefficient (Kmc) value is used to adjust the baseline ET value for a particular site. Vegetation Density.

Density of the vegetation which is to be watered is also used in determining the amount of water to be applied. As density of vegetation increases, more water will transpire from the leaf area, requiring an increase in the amount of water needed. A vegetation density coefficient (Kd) value is used to adjust the baseline ET value for a particular plant density.

In particular, our system disclosed in 6,076,740 whilst manage irrigation very efficiently, does not emulate the "ET drive" functionality. Our prior system, determined the frequency and amount of irrigation as an Optimum Irrigation Event dependent upon whether the soil moisture content has fallen below a defined Refill Point and not having regard to short term fluctuations in ET.

Accordingly, the present invention seeks to use ET data in combination with optimum irrigation event data, to control irrigation of a site.

Summary of the invention In one form of the invention, a method is provided for remote management of at least one irrigation site, the site having a receiving device in communication with one or more switching devices for controlling irrigation of the site, the method comprising the steps of:

(a) making progressive estimates of soil moisture content of the site;

(b) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (c) monitoring at least one evapo-transpiration loss (ET) factor for the site over a period of time;

(d) calculating an ET value for the site for that period of time;

(e) comparing the ET value to the optimum irrigation event to establish an ET drive factor; and

(f) in response to the receiving device receiving an irrigation signal to carry out an irrigation program, causing the switching devices to operate dependently upon the ET drive factor.

In another form of the invention, a distributed system is provided for remote management of at least one irrigation site, the system comprising:

(a) a receiving device in communication with one or more switching devices for controlling irrigation of the site;

(b) a host computer system having memory containing a computer program and being adapted to execute the computer program to: (i) make progressive estimates of soil moisture content of the site; (ii) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (iii) calculate an ET value for the site for a period of time; and (iv) compare the ET value to the optimum irrigation event to establish an ET drive factor; the host computer system being adapted to communicate an irrigation signal to the receiving device of the site to cause the switching devices to operate dependently upon the ET drive factor. In another form of the invention, a host computer system is provided for remote management of at least one irrigation site, the system having memory containing a computer program and being adapted to execute the computer program to:

(a) make progressive estimates of soil moisture content of the site; (b) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site;

(c) calculate an ET value for the site for a period of time; and (d) compare the ET value to the optimum irrigation event to establishing an ET drive factor; the host computer system being adapted to communicate an irrigation signal to a receiving device of the site to cause the switching devices to operate dependently upon the ET drive factor. In another form of the invention, a receiving device is provided for communicating with one or more switching devices and being adapted to control irrigation of an irrigation site, the receiving device being adapted to receive an irrigation signal from a host computer to cause the switching devices to operate dependently upon a ET drive factor, wherein the ET drive factor has been derived by that host computer: (a) making progressive estimates of soil moisture content of the site;

(b) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site;

(c) calculating an ET value for the site for a period of time; and (d) comparing the ET value to the optimum irrigation event to establish an ET drive factor.

Preferably, the ET drive factor defines a ratio of the relative times the switching devices are open and closed. Typically the calculation of the ET value over a period of time is a 24 hour period.

The estimated soil moisture content is typically determined by a calculation that produces substantially the same result as estimating a maximum soil moisture content and making adjustments for moisture loss from, and for moisture addition to, the site. In one form, the soil moisture content estimates are dependent on an estimated root zone depth and the soil texture identifier for the site. The estimated root zone depth may also be dependent on the vegetation type identifier. In another variation, the maximum soil moisture content is dependent on the product of the estimated root zone depth and an estimated soil moisture holding capacity for the site. Dependence may also be had on meteorological data measured for the site (eg solar radiation data). As such, this invention allows certain types of irrigation management and irrigation scheduling systems the ability to cause an on-site irrigation controller to operate its outputs for varying, calculated run times in order to match application to a loss incurred as a result of EN during a previous designated period, usually, but not limited to, the current or previous day. More particularly, this approach is different to that disclosed in the inventor's prior patent USP 6,076,740. That patent previously taught a method in which a system controls the operation of an on-site controller (usually an irrigation controller) by allowing or disallowing its operation on certain nights. In one embodiment of that system, operation of the on-site controller would be delayed until the moisture content of the root zone is reduced to a pre-determined Refill Point at which point irrigation is allowed and the root zone is refilled. The size of this optimum irrigation event is considerably greater than would be needed if the evaporative losses were to be replaced on a daily (or similar) basis.

The definitions of Refill Point and Optimum Irrigation Event used in this specification are the same as in USP 6,076,740 but for the sake of clarity those definitions and their bases appear below:

Each site is surveyed with a view to accurately establishing the following:

• Area (sq.m) to be irrigated.

• Root Zone Depth (RZD). This is a sensible site range.

• Precipitation Rate of the irrigation system. • Soil texture within the root zone.

With the data from the above, calculations can now be done to establish the following:

1 Total Available Water (TAW (mm) = RZD(cm).times.SMHC) where SMHC (Soil moisture holding capacity is typically 0.75 mm/cm for sand; 1.00 mm/cm for sandy loam; 1.40 mm/cm for loam; 1.60 mm/cm for clay loam and 1.80 mm/cm for clay). 2 Refill Point (RFP(mm)=TAW(mm).times.f) where f is a factor, 0.4 has been found satisfactory for most soils

3 Optimum Irrigation Event (OIE(mins)=((TAW(mm)-RFP(mm))/PR(mm/hr).times.60) Where PR is Precipitation rate (mm/hr).

With the present invention, the calculated run time requirement to replace the amount of water lost to the root zone by evapo-transpiration in the (usually daily) period (this amount of water being of fundamental importance in ET driven management systems) can be expressed as decimal fraction of the Optimum Irrigation Event. This decimal fraction can be referred to as the ETD (evapo-transpiration drive) factor and is calculated:

ETDf = (calculated ET/OIE) Where:

• ETDf = ET drive factor

• OIE = Optimum irrigation event, expressed in the same units as the ET for the period in question, for the site in question (OIE = the amount of water needed to refill the root zone at the site to field capacity from the designated Refill Point) The ETDf can be calculated for the site or for a more general area which contains the site in question as well as possibly other sites under similar management. Calculating the ETDf for a more general area may be particularly useful in some embodiments of this invention.

These calculations could in one embodiment of the invention be performed in a central server, a computer managing a number of sites but located remotely from them. Such a system has, as part of the system, a series of on-site electronic devices that typically receive wireless messages typically containing instructions as to which sites are to be enabled to irrigate.

To the software code of these on-site electronic devices can be given additional capability, allowing the device to operate in "ET Mode". These additions may include but not be limited to: • Ability to receive instruction to operate in ET or other mode Ability to receive and store site ETDf Ability to receive and store ETDf for a larger area Ability to receive and store site "group of seconds" size Ability to increment and or decrement a group of counters to divide operational time into "ON" and "OFF" seconds

The device can, in one embodiment of the invention, be wirelessly instructed to operate in "ET Mode" or in such other mode or modes as it may be capable of doing for such periods as may be required from time to time.

When in "ET Mode" the device will, in one embodiment of the invention, when instructed to enable an irrigation, also receive the ETDf. Upon receiving the irrigation enabling signal and the particular ETDf it contains in this embodiment, or having received such a signal, at the start of an appropriate, designated irrigation operational window, the on-site electronic device will in at least one embodiment, divide a group of seconds into "ON" seconds and "OFF" seconds. This operation is shown in flow diagram 1.

For example, if the ETDf happened to be 0.4 (i.e. the day's ET loss represented 40% of the site's OIE) then the on-site electronic device would calculate that, if there were to be 60 seconds in the group then (60 x 0.4 = 24) 24 seconds would be "ON" seconds and (60 -24 = 36) 36 seconds would be "OFF" seconds. If the group was 100 seconds then the "ON" seconds would be 40 and the "OFF" seconds would be 60 and so on.

Operationally, the preferred embodiment and in respect of the example above, the on-site electronic device would enable irrigation for 24 seconds and disable irrigation for 36 seconds before enabling for a further 24 seconds and so on through to the end of the operational irrigation window.

The size of the groups of seconds could be fixed or varied on any appropriate basis. For example the size of the group of seconds could be varied from site to site such as to strike a balance between effective division of the potential operational period and minimizing the number of disablements. Regardless of the operational detail, the overall effect of the addition of such capability to such a system is the new ability to match daily station runtimes to the water lost to evapo- transpiration during the previous appropriate period, so that the lost water would be replaced.

Description of the drawings

The invention will now be further explained and illustrated by reference to the accompanying drawing in which:

FIG. 1 is a flow diagram of one form of the invention.

FIG. 2 is a schematic diagram of one practical embodiment of the invention.

In overview, the system depicted includes a main computer 1 which is connected to fifteen weather stations 2. Each sub-area 3 has at least one weather station 2. Each weather station 2 carries out a dual purpose in this particular illustration of the system of the invention. Firstly, it can measure weather conditions such as solar rays and temperature. Secondly it can measure and record rainfall in the sub-area.

Main computer 1 monitors the weather stations 2 and can calculate the moisture content value for a particular sub-area 3. Main computer 1 also communicates to one or more switches 4 typically by a paging network. Each of these switches 4 is associated with a pre-existing irrigation controller 5 for a particular tract of land. Each switch 4 is controlled by the main computer 1 and by closing or opening switch 4 may permit or prevent the controller 5 from irrigating the tract of land according to its own programmed cycle. Where users require external access to the system, to review and/or alter the irrigation control system settings for a particular site, a dial up facility is provided. External users 10 will access the main computer 1. The user 10 enters by security identification and identification of the particular site. The user 10 is then given access to appropriate information including but not limited to the weather information and the settings for that particular site. The user 10 may change the settings and this information is then transmitted to the main computer.

A typical implementation procedure for the irrigation control system as depicted is as follows.

The area in which the irrigation is to be controlled is defined. In most cases this will be the greater metropolitan area and environs of a large city, or the general area and environs of a provincial city, or the area covered by a town. Once the overall area is defined, it is further divided into sub-areas 3. A sub-area 3 may be defined by a common or similar microclimate. This division is necessarily subjective and will usually contain inaccuracies, however, this does not markedly affect the operation of the system and does not interfere with the system achieving efficient irrigation management outcomes. Typically, a large metropolitan area and environs of a city, eg one million people, may contain 10-15 sub-areas 3. These sub-areas 3 will be identified by a number.

To define sub-areas 3, a number of empirical factors are used including:

General orientation (North, South etc.) Landform (plain, valley area, slope) Overall land use

Density of buildings, etc. Once the overall area has been defined and then further sub-divided into (10-15) sub-areas 3, the following external support network is put in place. At least one rain-gauge and its supporting data accumulating logger is positioned in a convenient location in each sub-section and connected by communication means 8 (usually telephone or radio or a combination of both) to the main computer 1. As indicated above, at least one weather station 2 may be installed within the sub-area 3. In one alternative (not shown) at least one weather station 2 may be installed in the overall area and not specifically in the sub-area 3. In another alternative (not shown) there may be a combination of those alternatives.

Once the external support network is in place and connected to the main computer 1 the surveying of the individual irrigation sites within each sub-area can begin. Each individual site is now registered on the main computer 1 with its basic factors indicated above (TAW, RFP, OIE, PR) and its identification number which tells the system within which sub-area 3 it lies. The switch 4 is now connected to the controller 5 at each site. Switch 4 is connected across the common wire (or wires) 9 of controller 5. Typically the switch 4 will take power (24vAC) from the controller 5.

Now a program is entered into the controller 5 which calls for the calculated Optimum Irrigation Event to be calculated for each site.

Similarly, the evapo-transpiration loss (ET) factor for the site is monitored over a period of time and an ET value calculated for the site for that period of time Thereafter, the system compares the ET value to the Optimum Irrigation Event to establish an ET drive factor and in response to the controller 5 receiving an irrigation signal to carry out an irrigation program, causing the switching devices 4 to operate dependently upon the ET drive factor.

From then on the programming in the main computer 1 maintains the soil moisture budget for each of the sites registered onto it. It does this by communicating with the weather station 2 and the network of rainfall sensors and establishes how much water will have been transpired by plants in each sub-area 3. This amount is deducted from the soil moisture budget of each site, with additions to the soil moisture budget for each site being made where rain falls in a particular sub-area 3. Should rain fall it will be detected by the rainfall sensors positioned in the sub-area 3. If it is outside the programmed operating window of controller 5, it will be passively recorded by the rainfall sensors and up loaded as data each day by the main computer 1. It will then be added to the soil moisture budget of sites in the main computer 1 which recorded the rainfall. Rainfall will therefore delay irrigation until it has been transpired or harvested by the plants or evaporated.

If the rainfall is within the operating window of the controller 5 (that is when irrigation is likely to be occurring) the data accumulating logger connected to the rainfall sensor detecting rainfall will contact the main computer 1 and advise that rain is falling in that sub-area 3. In this case all sites from that sub-area 3 on that evening's action list will be sent a data string causing the switch 4 to open, thus bringing irrigation within that sub-area 3 to an immediate halt. This may in some circumstances be done by a broadcast call which will be acted upon by all switches 4 in the designated sub-area 3. The main computer 1 will then track the rainfall event and add it to the soil moisture budget of the relevant sites. If the total rainfall event is significant, irrigation will not resume that evening. If the system deems that the rainfall event is not significant, then irrigation may resume that evening.

The switch 4 operates in the common wire 9 of controller 5. This associated controller can be an inexpensive controller which may have been installed on the irrigation system to be managed prior to adopting the irrigation control system of the invention.

The switch 4 typically consists of, but is not limited to, the following means: paging system receiver means microprocessor means memory area means clock means one or more switching relay means.

Switch 4 is capable of receiving a detailed program containing switching instructions for the operation of one or more relay means. It is capable of receiving a particular string which is intended for it alone, or depending on the structure or content of the transmitted data stream, it can also respond to a broadcast type call intended to simultaneously give rise to a specific action or group of actions within an entire group of switches 4.

The relay means of the switch 4 may be either of the normally open or normally closed type depending upon the circumstances.

Further switch 4 is capable of receiving, processing and storing data strings including (but not limited to) the following types of information, which would normally be transmitted (but not necessarily) in the following order:

1. General call or broadcast recognition characters 2. Sub-area identification number.

3. Specific unit recognition or capture code (characters).

4. Specified task designation characters (normally used to designate tasks the subject of a broadcast call). 5. Program definition characters of the general type (but not limited to)~relay one close/open at (time) for duration (minutes); on (date);~relay n, close/open at (time); for duration (minutes); on (date).

6. ET drive factor

7. Test time. 8. Lock/unlock code (prevent all irrigation operations until receipt of particular unlock code).

Also included in switch 4 may be an accessible momentary switch means which, if pressed or otherwise operated, will allow irrigation operations in the absence of system authorisation to do so for a programmable "Test" time. In other words the switch 4 will restore the integrity of the common wire 10 of the controller 5 with which it is associated. This is to allow the associated controller 5 and its in-field irrigation system to be tested.

The word 'comprising' and forms of the word 'comprising' as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 A method for remote management of at least one irrigation site, the site having a receiving device in communication with one or more switching devices for controlling irrigation of the site, the method comprising the steps of: (a) making progressive estimates of soil moisture content of the site; (b) determining an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (c) monitoring at least one evapo-transpiration loss (ET) factor for the site over a period of time; (d) calculating an ET value for the site for that period of time; (e) comparing the ET value to the optimum irrigation event to establish an ET drive factor; and (f) in response to the receiving device receiving an irrigation signal to carry out an irrigation program, causing the switching devices to operate dependently upon the ET drive factor.

2 A method according to claim 1 wherein the ET drive factor defines a ratio of the relative times the switching devices are open and closed.

3 A method according to claim 1 wherein the calculation of the ET value over a period of time is a 24 hour period.

4 A method according to claim 1 wherein the estimated soil moisture content is determined by a calculation that produces substantially the same result as estimating a maximum soil moisture content and malting adjustments for moisture loss from, and for moisture addition to, the site. 5 A method according to claim 2 wherein the soil moisture content estimates are dependent on an estimated root zone depth and the soil texture identifier for the site.

6 A method according to claim 3 wherein the estimated root zone depth is also dependent on the vegetation type identifier. A method according to any one of claims 4 wherein the maximum soil moisture content is dependent on the product of the estimated root zone depth and an estimated soil moisture holding capacity for the site. A method according to claim 2 wherein the soil moisture loss is estimated in dependence on meteorological data measured for the site. A method according to claim 6 wherein the meteorological data includes solar radiation data. A distributed system for remote management of at least one irrigation site, the system comprising: (a) a receiving device in communication with one or more switching devices for controlling irrigation of the site;

(b) a host computer system having memory containing a computer program and being adapted to execute the computer program to: (i) make progressive estimates of soil moisture content of the site; (ii) determine an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (iii) calculate an ET value for the site for a period of time; and (iv) compare the ET value to the optimum irrigation event to establish an ET drive factor; the host computer system being adapted to communicate an irrigation signal to the receiving device of the site to cause the switching devices to operate dependently upon the ET drive factor. A distributed system according to claim 10 wherein the ET drive factor defines a ratio of the relative times the switching devices are open and closed. A distributed system according to claim 10 wherein the calculation of the ET value over a period of time is a 24 hour period. A host computer system for remote management of at least one irrigation site, the system having memory containing a computer program and being adapted to execute the computer program to: (a) make progressive estimates of soil moisture content of the site;

(b) determine an optimum irrigation event comprising a measure of irrigation necessary to increase the estimated soil moisture content from a Refill Point for the site to a maximum soil moisture content for the site; (c) calculate an ET value for the site for a period of time; and

(d) compare the ET value to the optimum irrigation event to establish an ET drive factor; the host computer system being adapted to communicate an irrigation signal to a receiving device of the site to cause the switching devices to operate dependently upon the ET drive factor. A host computer according to claim 13 wherein the ET drive factor defines a ratio of the relative times the switching devices are open and closed. A host computer according to claim 1 wherein the calculation of the ET value over a period of time is a 24 hour period. A receiving device for communicating with one or more switching devices and being adapted to control irrigation of an irrigation site, the receiving device being adapted to receive an irrigation signal from a host computer to cause the switching devices to operate dependently upon a ET drive factor, wherein the ET drive factor has been derived by that host computer; (a) making progressive estimates of soil moisture content of the site;

(c) calculating an ET value for the site for a period of time; and (d) comparing the ET value to the optimum irrigation event to establish an ET drive factor. A receiving device according to claim 16 wherein the ET drive factor defines a ratio of the relative times the switching devices are open and closed. A receiving device according to claim 16 wherein the calculation of the ET value over a period of time is a 24 hour period. A receiving device according to claim 16 adapted to be retrofitted to an existing irrigation controller.