Remote soil water measurement

This new approach to monitoring soil water movement can lead to the more efficient use of fertilizer, a reduction in nutrient waste and waterway contamination and indicate the presence of coliform bacteria.

When rain reaches the ground it mixes with, and dissolves many of the physical and chemical substances it comes into contact with as it begins its downward journey. Rain and irrigation water can transport fertilizer, farm chemicals, animal waste and pathogens to unknown destinations underground, in various proportions and concentrations.

Where did that last application of fertilizer go?

Chances are that the only way you know how effective your last fertilizer application was is from indirect observation – is the pasture or crop noticeably thriving since its application?

As water-soluble nutrients are absorbed into the ground they effectively become ‘invisible’. This leaves many unanswered questions. How fast are nutrients transported through the soil? What proportion is taken up by plants? How much carries on down below the root zone and is wasted, or worse, finds its way to ground and surface waterways? Does sufficient nutrient even get to the roots, or was the application too light and likely to limit growth?

Soil water sampling can help answer these questions, but…

Conventional soil water sampling can’t answer all the questions

A tool that’s often used for sampling soil water is the Suction Lysimeter. It generally has a long tube with a porous cup attached at the bottom and is inserted into the ground. An operator uses a hand pump to create a vacuum inside the tube to suck soil water through the porous cup. The operator returns regularly to withdraw the sample and restore the vacuum.

But these conventional Lysimeters have significant limitations; they:

  • start sampling immediately – a sample can’t be initiated by, say, a rainfall, irrigation, or drainage event
  • don’t account for soil moisture status
  • can’t differentiate between mobile (freely draining) and immobile (held between soil particles) water
  • don’t maintain constant suction – the vacuum decays slowly and so the sample volume collected diminishes
  • can only take single samples
  • can’t be controlled or monitored remotely
  • are therefore labour intensive and data-deficient.

These limitations are significant, but are being overcome with the help of some new technology.

New technology provides many of the answers

NIWA’s ‘new generation’ suction lysimeter, the Lysimate, helps overcome these limitations and helps answer these questions. Lysimate was originally prototyped by Landcare Research, developed into a product by NIWA and has evolved from a single-sample tool to a multi-sampler.

You can think of Lysimate as having two broad functions; one performed above ground, the other below.

Above ground, the Lysimate enclosure contains:

  • a regulated reversible pump
  • measurement and control circuitry
  • wireless communications
  • sample storage bottles.

Below ground, sampling tubes, 'tipped’ with a porous cup to admit the water sample, are inserted in the ground at representative sampling locations.

To overcome the limitations of conventional Lysimeters, Lysimate:

  • starts the sample automatically, when you want it to start - you choose and programme: a future time, a rainfall event (with or without a following delay), a soil moisture threshold or a percentage change in either of these variables
  • continuously measures and records the soil moisture profile using collocated sensors buried at different depths
  • can detect a ‘wetting front’, following a rainfall event or from irrigation, enabling it to differentiate between mobile and immobile water
  • maintains a constant rate of sample collection, by regulating the suction
  • version 3 (the multi-sampler) can take up to six individual sequential samples, and purge sample lines between samples
  • time-stamps the start and end of each sample collected
  • stores the sample in an easy-to-retrieve container
  • enables you to view all recorded data, status, and remotely control the operation of individual Lysimates using any Internet connected device such as a Smartphone, providing a satisfying ‘soil-water-to-web’ experience - the results of any lab analysis come later of course, so this can’t be displayed
  • is easy to install, and after installation you don’t have to be there… that is until Lysimate sends a ‘sample ready’ message to your Smartphone - when you do go to collect samples the ‘quick swap’ sample bottles make it easy and it won’t interfere with day-to-day paddock management... just leave the sampling tubes in the ground between sampling campaigns, while cattle continue grazing, and retain the integrity of the sampling environment.

Extracting Lysimate’s visible benefits

Lysimate is a useful tool that is ideal for a number of scientific, commercial and compliance purposes.

It can be deployed as a large wirelessly-networked array to improve the spatial resolution when sampling large areas such as paddocks or around sewage ponds. An array like this could operate unattended for weeks.

Here are two examples of how Lysimate may be used:

Assess the effectiveness of fertilizer application

Lysimate sampling and subsequent analysis can indicate if you’re applying too much fertilizer, applying it during unsuitable weather conditions or maybe not applying enough. By adjusting the amount, or frequency of application, you could improve your returns on pasture or crop production. You can use Lysimate in conjunction with NIWA’s farm weather forecasting service to help you make better weather-based decisions.

NIWA Farm Weather Forecasting Service 

Obtain warning of potential water contamination

Lysimate remote sampling and subsequent analysis can warn of potential water quality contamination, for example, where nutrient loads and bacteria, such as E.coli from farm runoff, may breach ‘water usage values’ set by Regional Councils. Lysimate sampling can become a ‘sentinel’ that enables detection of sewage, farm runoff water or leachate from a landfill that may seep into and degrade the quality of a waterway.

Lysimate’s recorded soil moisture, rainfall, vacuum data and status can be viewed remotely, at any time. Lysimate will tell you when to come and retrieve your soil water samples. Then ‘visibility’ of suspended solids and dissolved chemicals is only ‘a lab analysis away’.

Next steps for Lysimate

Lysimate has the potential to help support both government initiatives to categorise and monitor the quality of New Zealand's water resources, and also optimize compliance, productivity and efficiency for landowners.

This year we will carry out trials with the new multi-sampler Lysimate 3 while developing a variant that will enable the use of up to three underground sampling tubes, arrayed horizontally or vertically.

If you have a soil water question that you think might be ‘answered’ by Lysimate, we’d be pleased to help.

Contacts

For more information please contact the following: 

NIWA Instrument Systems

Jeremy Bulleid
Jeremy.Bulleid@niwa.co.nz 

NIWA Science 

MS Srinivasan
MS.Srinivasan@niwa.co.nz

NIWA’s first ‘single-sample’ Lysimeter, Lysimate 2. The sample is collected in the tube on the left, the evacuated sampling tube is in the middle. Two collocated, buried soil moisture sensors are connected. Lysimate 2 is powered by a solar-charged Lithium battery. [NIWA]
The initial computer-aided design of Lysimate 3, to date the most recent multi-sampling version, supports up to six independent flow-rated samples from a single underground sampling tube. [NIWA]
A simulation of a typical underground view showing the arrangement of sensors and sampling tube. The most advanced Lysimate will support a vertical array of three sampling tubes. A vertical array of soil moisture sensors enables detection of the ‘wetting front’ and determination of how ‘mobile’ the water is. [NIWA]
A group of NIWA’s second generation ‘single-sample’ Lysimeters, Lysimate 2A, on test. The buried vacuum sampling tube is now separated from the controller. The sample is collected in the sample storage bottle attached to the left side of the controller. The raingauge (not shown) is being used to trigger sampling, at, or sometime after the start, of a rainfall event. [NIWA]
In this rainfall event the ‘wetting front’ arrives at the upper soil moisture sensor first. Approximately two hours later it arrives at the lower sensor. [NIWA]
Research subject: Instrumentation