Soil water monitoring choosing the right device

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AGFACTS                                                      Soil water
AGFACTS                                                      choosing the right
                                                             Agfact AC.27, July 2002
                                                             David Williams
                                                             Irrigation Officer

This Agfact is for irrigators who need to choose             • extent of any watertables within or just below a
a soil water monitoring device.                                crop’s rootzone
Best management practice in irrigation includes              • irrigation timing and forecasting based on water use
regularly monitoring soil water levels throughout the          (known as irrigation scheduling)
irrigation season. NSW Agriculture encourages farmers        What is the key information that a device must
to adopt and use long-term soil water monitoring to          provide?
help achieve efficient water use, protect the                As a minimum, you need the device to provide soil
environment, and maximise crop yields and quality.           water readings for the plant rootzone of a block before
We also strongly recommend that, before you purchase         and after an irrigation event. A reading following a
any soil water monitoring device, you identify your          rainfall event is also of benefit.
individual requirements.
                                                                Don’t buy any device until you answer each of
Sadly, too many decisions to purchase soil water                these questions for your enterprise:
monitoring equipment are made on price alone. After a
                                                                 1   What information can I get from a soil water
short period of use, the equipment is abandoned, a
wasted investment and a poor advertisement for soil                  monitoring device?
water monitoring.                                               2    How labour-intensive is the device?
This Agfact can help you choose a device that matches           3    How usable is the information from the
your management style, crop and situation, the amount                device?
of information you need, and your budget.
                                                                4    What level of accuracy do I need?

                                                                5    Does soil type affect my choice?
    FROM A SOIL WATER                                           6    Does the irrigation system I use limit my
    MONITORING DEVICE?                                               choice?

Soil water monitoring devices can provide a range of            7    Does crop type limit my choice of device?
information. Some tools give simple 'wet/dry'
                                                                8    What other site factors affect my choice?
measurements, which gives a basic guide to reducing
plant stress and minimising irrigation water losses in          9    How durable is the product?
the field.                                                      10 How much maintenance will it need?
Other tools can gather more complex information:                11 Can I afford it?
• depth and amount of irrigation
                                                                12 What are the next steps to take?
• root activity and development

AGDEX 533/753
• The reading before an irrigation shows how dry             Some devices come with software to view and interpret
  the soil is.                                               data on date/time, soil water value and depth of
                                                             reading, or sometimes this software is available as an
• The reading after the irrigation or rainfall event
                                                             optional purchase. Most devices that are continuously
   shows how deep the water has gone, and can be
                                                             logged come with comprehensive software which
   used to indicate how much was applied.
                                                             converts the data and presents this information in a
• A reading taken below the active rootzone of a             convenient format. For other products you may need
  crop can indicate whetherover-irrigation has               to use spreadsheet software to work with the data.
                                                             You may need training in operating the device and
• If you take additional readings between irrigation         interpreting and analysing the data. Check with the
   events, you can determine a pattern of water use.         supplier whether training is provided, and to what
Many products (currently around forty) are on the            degree.
market which in some way determine the water                 Try and choose a device where you can get ongoing
content of soil. They range in price, complexity, use        support and software updates. Are there grower
and methods of data collection (although only a few          support groups where results and other issues can be
are in commercial use.)                                      discussed?
                                                             Is any equipment needed to access and interpret the
2   HOW LABOUR-INTENSIVE IS THE                              data (especially the data from electronic devices)? This
    DEVICE?                                                  equipment may be as simple as a pen and paper or as
                                                             complex as a mobile phone or radio telemetry and
Devices where the irrigator has to collect information
                                                             computers. Is the equipment included in the
manually are more labour-intensive than devices that
                                                             purchase? Can the data be printed or emailed?
collect or log data automatically.
Labour availability is usually the least considered
factor in the purchasing decision but lack of time or        4   WHAT LEVEL OF ACCURACY DO I
labour may be important reasons why irrigators                   NEED?
abandon manual monitoring.                                   You need the device you select to have a level of
Manually read devices need to be read regularly              accuracy that matches your irrigation system and the
throughout an irrigation season. Any missed readings         degree of control it allows.
result in incomplete data, which makes the data far          For example, if your system is capable of delivering
less useful. The irrigator may then assume there is no       varying amounts of water precisely, the device you
benefit in collecting data, and abandons the                 choose should be accurate enough to provide detailed
monitoring operations midseason. The purchase cost           and frequent soil water use information. You need less
is written off and the equipment, although sound, is         detail when system management is more basic or
left to gather dust. The soil water monitoring process       where water can only be supplied on roster.
has ceased because regular readings were not given
                                                             The level of accuracy of a device is not always related
                                                             to cost, and some low-cost devices are quite accurate.
If you choose to purchase a manually read device,            Note that hand-feel readings will not be as accurate as
you have to commit the labour required to                    a device which directly and repeatedly measures soil
undertake the readings. If you cannot guarantee this         water content.
labour commitment, then you should consider
other options such as automatic logging devices or a
                                                             The device's ability to obtain consistent readings every
contract service.
                                                             time (repeatability) can be improved by calibration.
                                                             In calibration, the readings from the device are
3   HOW USABLE IS THE                                        compared to independent measures of soil water
    INFORMATION FROM THE                                     content: the method of calibration depends on the
    DEVICE?                                                  type of device.
The information from soil water monitoring devices is        Calibration can also be used to determine a volumetric
often most useful when presented as graphs or charts:        reading for the soil water (often given in mm).
this makes it easier to compare and interpret data.

5    DOES SOIL TYPE AFFECT MY                                       How is the device powered? Is this power
     CHOICE?                                                        available, and can the power source be protected
                                                                    in the field or in transit?
Soil type can affect which device you can choose, as
some of the sensors used by soil water monitoring
devices may be inaccurate in some soil types. For               9    HOW DURABLE IS THE PRODUCT?
example, gypsum blocks in sand and capacitance
probes in cracking clays may give inaccurate readings.          Both portable and permanent products need to be
Salinity may also affect the accuracy of some sensors.          assessed for durability.

Variations in soil type may occur over your farm or                 Will the device stand up to damage from
even over a block. You need to check that you have                  ultraviolet rays, moisture and extreme temperature?
enough soil water monitoring sites to get                           Livestock, pests and machinery traffic can damage
representative data for the area being irrigated at that            fixed devices. Will water get into electronic parts,
time.                                                               for example, or will the seals weather? What about
                                                                    lightning strikes?
                                                                    Portable products need to withstand possible
6    DOES THE IRRIGATION SYSTEM I                                   damage in transport.
The characteristics of the irrigation system can affect         10 HOW MUCH MAINTENANCE WILL
what device is chosen or how it is installed.                      IT NEED?
Surface irrigation may impede access for manual                 Some devices may have particular maintenance needs
readings, and may also cause problems by inundating             or particular difficulties in servicing, and these have to
sensors or access tubes, so waterproofing is important.         be considered during your selection process.
The distribution uniformity of a system affects how                 Look at mid-season and end of season
sensors may be placed. Drip and micro irrigation in                 maintenance requirements. Can you maintain it,
particular require correct selection of representative              or is dealer servicing required? Does it need to be
monitoring sites.                                                   sent away, and if so, how long for?
                                                                    Does the product come with adequate dealer
                                                                    support? Back-up service is crucial. If the product
                                                                    needs to be sent away, is a replacement product
                                                                    available? What is the likely turnaround for
The profile and placement of the device must match                  product repairs?
the requirements of the crop at the monitored site.
    Deep-rooted plants may need more sensors, or
                                                                11 CAN I AFFORD IT?
    single sensors giving readings at multiple depths.
    For annual crops, sensors have to be installed              In answering this question, assess both the initial and
    after emergence and removed at the end of the               the annual costs of a product.
    season before harvest.                                      Initial cost is usually the most important factor
    The machinery and human traffic in the crop                 considered by irrigators when a product is purchased.
    affect how sensors can be placed. Lucerne is a              Cheaper products tend to be manually read and so can
    particularly difficult crop to monitor because of the       be more labour-intensive. If labour can be provided
    traffic during haymaking.                                   easily and cost-effectively, then this will not be an
8    WHAT OTHER SITE FACTORS                                    Labour can become an issue when its true cost over
     AFFECT MY CHOICE?                                          many full seasons is considered. The cost of labour,
                                                                mainly in data collection, can be quite high, as
    Livestock grazing on crops also affects how data
                                                                readings may need to be taken every two to three
    can be collected.
                                                                days at each site throughout the season.
    Reading sensors manually could damage some
    crops and compact the soil around the sensor                It is important to look at methods such as the
    site: automatically logged devices may be a better          automatic collection of data as a means of reducing
    choice for these situations.                                the labour cost of data collection. The trade-off is

usually the increased initial purchase cost of the            References
product, but in some cases, and on some crops,      
savings can be achieved between 5 and 10 years after
                                                              Charlesworth, P 2000, Information Package on
                                                              Soil Water Monitoring, CSIRO Land and Water
The issue of annual costs relates to maintenance              Consultancy Report 03/00, prepared for
both during and after the season, and re-installation         LWRRDC’s National Program for Irrigation
costs in annual crops. Here the variation in cost             Research and Development, February.
between products lies with differing labour
requirements and the need for dealer or outside
support.                                                      Acknowledgments
                                                              The author wishes to acknowledge the
                                                              assistance of these colleagues in compiling this
12 WHAT ARE MY NEXT STEPS?                                    publication:
   Talk to an Irrigation Officer from NSW                     Dr N Austin, Mr J Giddings, Ms R
   Agriculture before any purchase.                           Greenslade, Mr R Hoogers and Ms I Quarisa.

   Consider doing an irrigation management course,
   such as the free WaterWise on the Farm
   Introduction to Irrigation Management course
   offered by NSW Agriculture. This details how to
   determine soil variability, soil water
   characteristics and crop water use, evaluate
   irrigation system performance, and use soil- or
   climate-based methods for scheduling irrigation.
   Consider what level of scheduling you want to
   Match the device to your enterprise and your
   management style.
   Make sure adequate labour is available to read
   manually-read devices regularly: if labour isn't
   available, consider automatically read devices, or
   Talk to another irrigator who has successfully
   used the product or products that you are
   considering, and discuss the questions we have
   listed in this Agfact. (The supplier may have
   other purchasers who are happy to be contacted.)
A list of soil water monitoring devices (categorised
by method) and contact details for manufacturers
and distributors are available from NSW Agriculture
(List of devices and distributors).
                                                            The information contained in this document is
                                                            based on knowledge and understanding at the time
                                                            of writing (July 2002, revisions October 2003).
                                                            However, because of advances in knowledge, users
                                                            are reminded of the need to ensure that information
                                                            upon which they rely is up to date and to check
                                                            currency of the information with the appropriate
                                                            officer of New South Wales Department of
                                                            Agriculture or the user’s independent adviser.


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