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Soil water measurements for Grain & Graze Murrumbidgee

How the measurements are made

To find out how the measurements are made, either click on a component in the following diagram, or select from the menu below it. For further information, click Submit a Question .

Overview

Watermark gypsum blocks were installed at 8 depths (from 0.2 to 1.6 m) at each measurement location.  These were connected to a buried CSIRO Wireless micrologger located 1.5 m downslope from the sensors, the connecting cables being buried in a trench 0.4 m deep.

At pre-programmed intervals (every 12 hours in this case), each micrologger reads each sensor and radios the results to a receiver located near the shearing shed a few hundred meters away. The receiver is connected to a small computer with a built-in cdma telephone, called a pc-ephone. The computer stores the sensor results. Once each day connects to the internet via the cdma telephone and sends the day's data file to a CSIRO computer.  At 7 am each day, the previous day's data is automatically added to an excel spreadsheet, the charts are updated, and copied to the computer hosting this website.

If all goes well, the previous day's data is available for viewing after 7 am each day.

Occasional glitches occur when the radio link fails because of excessive interference, or the cdma signal is not strong enough for the call to be made.  Built-in fail-safe procedures keep these to a minimum. and prevent data loss when it does happen.

Watermark gypsum blocks

The Watermark sensor was used here because it provides an inexpensive way to measure soil water status. 

It measures soil water potential rather than soil water content, which has advantages and disadvantages.
Advantages include: inexpensive; easy to install; provides an absolute measure of soil wetness or dryness;
Disadvantages include: measurements cannot be directly related to soil water content or storage in mm; the limit of accurate calibration is 200 kPa, whereas the theoretical lower limit for water extraction by crops is 1500 kPa (see further discussion under sensor limit below)

The Watermark sensor is a type of gypsum block.  These are porous blocks that wet and dry as the soil they are in contact with wets and dries.  The water content of the block is measured by measuring its resistance.  The gypsum in the block provides a buffer against background soil salinity so that it does not affect the resistance measurement.

The water content of the block unfortunately does not equal the water content of the soil, but is related to the 'soil water potential' of the soil with which the block is in contact. Soil water potential provides an absolute measure of how wet or dry the soil is; a value of zero indicates that the soil is 'saturated', a value of ~10 kPa indicates that the soil is at 'field capacity' or 'drained upper limit', while a value of 1500 kPa  indicates that the soil is at 'wilting point' - ie so dry that plants cannot extract more water from it. [In contrast the water content at these limits needs to be determined independently for a measured soil water content value to be interpreted in this way.]

Sensors such as the Watermark, which measure soil water potential, require less care to be taken during installation.  They do not need to be in intimate contact with the soil, as is required for most sensors that measure water content. Instead, the sensor can be bedded in a contact  material (usually diatomaceous earth, better known as swimming pool filter material) that ensures good contact between the sensor and the soil and allow it to equilibrate with the soil's water potential. [In contrast, if a soil water content sensor is bedded in a contact material instead of being perfectly in contact with the soil, it measures the water content of the contact material instead of the water content of the soil   This means that it is relatively straightforward to install several sensors in one narrow (~ 50 mm diameter) hole, bedding each in a layer of contact material and separating them with a layer of bentonite that ensures that water dos not move down the installation hole between the sensors.

Sensor Limit

A disadvantage of Watermark sensors is that the limit of their accurate calibration is 200 kPa, whereas crops have the ability to dry the soil to 1500 kPa. This is not a particularly serious disadvantage because in most of the soils on the southwest slopes of NSW where we are applying this technique, very little water is available to crops between soil water potentials of 200 kPa and 1500 kPa. Based on data for soils in the region, this is less than 5 mm in a 10 cm layer near the soil surface, decreasing to 2 mm in a 10 cm layer at 1 m depth. In periods of high water demand by crops sufficient to see a potential of 200 kPa being reached, crops would use this amount of water in a day or two. Therefore, once the crop has dried the soil to 200 kPa, it can be assumed that 1500 kPa (the theoretical lower limit for water extraction by crops) will be reached within a few days. Hence, the time at which 200 kPa is reached is still a good estimate of when the soil has been dried to its lower limit by the crop.

More information about interpreting soil water potential measurements can be found at  How to interpret the measurements. Further details about the Watermark and about gypsum blocks in general, can be found at the following websites:

Manufacturer	Irrometer [External Link]
One of the Australian distributors	Measurement Engineering Australia (mea) [External Link]
Miscellaneous	Ag WA Farmnote 3/98 [External Link] Soil Water Content Sensors and Measurement Discussion Group [External Link]

Wireless microloggers

The Wireless micrologger has been custom-designed by CSIRO Land and Water. Each logger can measure up to 8 sensors. At a pre-programmed time (or times) each day, the logger automatically turns on, measures the sensors and  stores the data. Every time measurements are made, the logger has the capability to radio the results to the receiver, as described below. Alternatively, to save power, results from several measurement times can be radioed together.

The micrologger has been designed to be buried and left unattended for long periods of time. It has a low power requirement, and its high capacity batteries will last several years at daily or sub-daily measurement intervals  The data are stored in memory (up to 5,000 individual measurements can be stored), so that if radio transmissions are interrupted, the data can be recovered by manually interrogating the logger with a PC.

Each logger is sealed in a watertight PVC housing and lowered down a PVC casing. While the casing extends to the soil surface, the top of the logger is at least 30 cm deep.  Thus, although the top of the casing may occasionally be damaged by, for example, tillage operations, the logger remains protected. However, to transmit the radio signal, an antenna needs to protrude above the soil surface. This is designed to be easily disconnected and re-connected at sowing and harvest, and easily laid down on the soil surface to permit spraying operations.

Radio communication

The radio signal from the logger is transmitted through the antenna protruding 2 to 3 m above the soil surface, to a similar antenna connected to a receiver located at some distance away. This distance can be up to 5 km under ideal conditions, but  the maximum practical range is more usually only 2 or 3 km.

Telephone link

The  radio receiver is connected to a pc-EPhone, a combined cdma telephone and personal digital assistance running the Microsoft Windows CE operating system. Custom software running on the pc-EPhone receives and stores the measurements sent by the microloggers. At a pre-programmed time each day, the pc-EPhone connects to the internet and uploads the data to a CSIRO ftp site. Should the cdma link fail, the data is kept in protected storage and can be downloaded  to a PC or to a compact flash card.

Loading the data to the web page

Each morning, software running on a CSIRO server automatically copies any incoming data files from the ftp site, processes them, and adds them to an excel spreadsheet. This spreadsheet updates charts of the data and saves them as "gif" images, which are uploaded to the web page and are immediately available for viewing. While the quality of the charts is not extremely high, they are small files (< 15 kB) so that  can be downloaded quickly even on slow dial-up lines.

Occasional glitches occur when the radio link fails because of excessive interference, or the cdma signal is not strong enough for the call to be made.  Built-in fail-safe procedures keep these to a minimum.  If the cdma link fails repeatedly, then  providing the radio communication has taken  place, the data is stored  on the pc-ephone computer and can be retrieved manually. Similarly, if the radio communication fails, the data is not lost as it is still stored in the data logger, from where it can also be retrieved manually if necessary.

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