Rapid soil water monitoring using EM38
Author: Neil Huth, CSIRO Agriculture Flagship, Toowoomba, Qld. | Date: 25 Jul 2014
Take home message
Total plant available soil water can be monitored quickly and easily in multiple paddocks using EM38 techniques. This allows farmers to make better decisions in situations where soil moisture is important.
Introduction
Effective monitoring of soil water is important for informing management decisions in cropping systems. Many different measurement approaches have been used depending on the type of soil and cropping system being employed. However, these can be labour intensive and so cost and logistical constraints on the number and placement of monitoring sites is always a concern for farm managers. Electromagnetic Induction (EMI) has potential advantages over other methods for soil water monitoring including speed and ease of use, no radioactive source, and its non-invasive nature. Use of the EM38 does not require wiring, electronic equipment or access tubes to be installed into the field. For these reasons, an EMI technique has been developed to enable managers to repeatedly monitor a large number of sites over an extended period in cropped fields. Our team in Toowoomba has been using EMI for soil water monitoring in scientific trials for over 10 years and some collaborating agronomists have been evaluating it for several seasons.
EMI provides a measure of the soil electrical conductivity of the soil profile, which is affected by variation in salt, clay content, organic matter, temperature, and soil moisture. Huth and Poulton (2007) showed that through careful selection of sites, and consistent reading of the same sites, the impacts of spatial variation of clay, salt and organic matter upon conductivity measures could be avoided. Furthermore, published tables help to account for any seasonal variation in soil temperatures. The remaining variation in EMI measurements correlates strongly with variation in soil moisture. This provides a rapid method for estimating soil moisture content at a large number of field locations. The following describes how such EMI techniques can be employed for soil water monitoring for informing crop management.
The EM methodology
The EM38 (Geonics Ltd, Canada, Figure 1) measures the apparent bulk soil electrical conductivity (ECa) by inducing a small current within the soil via a primary electromagnetic field from a transmitting coil and measuring the resultant secondary field back from the soil via a receiving coil. The EM38 allows two depth responses through simple changes in the orientation of the instrument (vertical, horizontal). The depth response for the vertical dipole is much deeper than for the horizontal dipole (Figure 2) which is heavily influenced by surface EC.
Figure 1. EM38-MkII in the field. The unit is sitting in the vertical dipole position. Horizontal dipole is obtained by placing the unit on its side. |
Figure 2. Depth response functions for EM38 used in either the vertical or horizontal dipole. |
The approach to applying the EM38 for soil water monitoring is simply this:
1) Paired EM38 readings in the vertical (ECv) and horizontal dipoles (ECh) are taken at chosen representative sites in a field. Consistent monitoring of the same sites minimises the impact of spatial variation in soil salts, clay and organic matter, which can affect ECa readings.
2) Each ECv and ECh reading is corrected for seasonal temperature variations using published tables of temperature correction factors from Huth and Poulton (2007) (Table 1).
3) A combined total EC (ECt) is calculated from the temperature corrected values using the equation ECt = 0.77 * ECv + 0.23 x ECh. This combines the two signals into a value that represents a measurement that is more evenly distributed with depth.
4) ECt can then be used as a measure of total soil water by comparing to wet and dry readings to provide a field-specific calibration for the EMI soil water monitoring technique.
Previous studies have shown that values of ECt obtained in this way correlate closely with soil water accumulated to depths of 90cm or deeper. Figure 3 shows data for a field near Warra, Qld where ECt was used to estimate soil water to a depth of 90cm. This data includes soil profiles that were close to the crop lower limit (i.e. 0 mm Plant available water (PAW)) or at drained upper limit (i.e. 153 mm PAW to 90cm in this soil). A linear fit to the data explains 93% of the variation (Figure 4).
Experience shows that good estimates of ECt for wet and dry soil profiles alone provide a good reference for ongoing monitoring of soil water using EM38. For example, if the dry and wet readings were known from previous monitoring on the site, the relative value of any EM38 reading between these two extremes can be used to estimate the fractional PAW for the field (e.g. 50% full). In Figures 3 and 4, a profile that is half full is shown in both cases.
Figure 3. Soil water profiles used in the calibration of the EM38. Dry readings correspond to crop lower limit. Wet readings correspond to drained upper limit. |
Figure 4. Plot of Total Soil Water to 90mm versus ECt. Note linear relationship between soil water and ECt. If wet and dry readings are known, fraction of plant available water can be calculated very easily |
Techniques such as these have been used by the CSIRO Toowoomba team for over a decade now for the rapid monitoring of soil water in trials where a large number of measurements are required. For example, Figure 5 shows estimates of total soil water to 90 cm for a deep black vertosol at Nangwee, Qld in 2013. This trial included three times of sowing. The first time of sowing had some restrictions on early growth and the second sowing ultimately yielded higher. Note that the EM38 readings were able to show that water use from the second sowing caught up to, and then exceeded, the first sowing.
Soil water monitoring using EM38 is also being trialled by agronomists in SE Queensland. Here, irrigation of cotton is often managed using soil water information monitored using neutron moisture meters. However, logistical considerations heavily influence the extent of the use of neutron probes. Side-by-side comparison (Huth et al, 2012) of EM38 with neutron probes have shown that the EM38 is able to explain well over 90% of the variation in soil water measured with probes, but without the need for access tubes or radioactive sources, and with a much smaller time requirement.
Figure 5. Time course of estimated total soil water to 90cm depth using EM38 for three times of sowing for wheat at Nangwee, Qld in 2013. |
Figure 6. Time course of estimated total soil water to 90cm depth using NMM (lines) and EMI (symbols) for irrigated cotton fields near Brookstead, Qld. |
What can I do with this information?
Information regarding your current soil water status can be useful in a range of crop management decisions. These could include the following scenarios:
1) To grow 3t/ha of sorghum I need 300mm of PAW + rainfall. How much of this is already available in the soil? How much of this will I need from rainfall?
2) The Bureau of Meteorology is predicting a very dry summer. If I know my stored soil water I can estimate a minimum likely yield. Is this yield enough to be worth the risk?
3) The forecast is for a good season and I have good stored moisture. I can use these to predict potential yield. What other management do I need to consider for reaching these yield levels?
Pros versus Cons
If you choose to use EM38 you will need to weigh up the pros and cons for this instrument versus other approaches available in the market.
Pros |
Cons |
Very Portable - nothing remains in field |
High up-front capital expenditure (~20k) |
Rapid – so can take more measures from more locations |
Point in time measurement – you cannot have it logging continuously. This may not be an issue. |
Large zone of measurement – some sensors only sense a small volume of soil |
Requires calibration for each soil type – as is the case for all probes |
Non-destructive |
Affected by high salt and salt variability |
Relatively simple to calibrate |
No detailed layered soil information |
Low ongoing operating costs |
|
Similar accuracy to neutron moisture probes |
|
Table 1. Example temperature correction factors calculated for a range of locations
(from Huth and Poulton, 2007).
Dipole |
Day of Year |
Dalby |
Warwick |
Moree |
Gunnedah |
1-Jan |
0.99 |
0.98 |
0.93 |
0.98 |
|
1-Feb |
0.97 |
0.97 |
0.91 |
0.96 |
|
1-Mar |
0.99 |
0.99 |
0.93 |
0.98 |
|
1-Apr |
1.02 |
1.03 |
0.98 |
1.03 |
|
1-May |
1.08 |
1.10 |
1.06 |
1.10 |
|
1-Jun |
1.14 |
1.18 |
1.15 |
1.19 |
|
Vertical |
1-Jul |
1.19 |
1.24 |
1.23 |
1.26 |
1-Aug |
1.21 |
1.26 |
1.27 |
1.28 |
|
1-Sep |
1.19 |
1.24 |
1.24 |
1.26 |
|
1-Oct |
1.14 |
1.18 |
1.16 |
1.19 |
|
1-Nov |
1.08 |
1.10 |
1.07 |
1.11 |
|
1-Dec |
1.03 |
1.04 |
0.99 |
1.03 |
|
1-Jan |
0.97 |
0.96 |
0.90 |
0.96 |
|
1-Feb |
0.96 |
0.96 |
0.90 |
0.95 |
|
1-Mar |
0.98 |
0.98 |
0.92 |
0.98 |
|
1-Apr |
1.03 |
1.04 |
0.99 |
1.04 |
|
1-May |
1.09 |
1.12 |
1.09 |
1.13 |
|
Horizontal |
1-Jun |
1.16 |
1.21 |
1.20 |
1.22 |
1-Jul |
1.22 |
1.27 |
1.28 |
1.29 |
|
1-Aug |
1.23 |
1.29 |
1.31 |
1.31 |
|
1-Sep |
1.20 |
1.25 |
1.25 |
1.27 |
|
1-Oct |
1.14 |
1.17 |
1.15 |
1.18 |
|
1-Nov |
1.07 |
1.08 |
1.05 |
1.08 |
|
1-Dec |
1.01 |
1.01 |
0.96 |
1.01 |
|
References
Huth, N.I., Poulton, P.L., 2007. An electromagnetic induction method for monitoring variation in soil moisture in agroforestry systems. Australian Journal of Soil Research 45, 63-72.
N Huth, G Boulton, N Dalgliesh, B Cocks, P Poulton (2012) Electromagnetic Induction methods for monitoring soil water in irrigated cropping systems. 16th Australian Agronomy Conference. (http://www.regional.org.au/au/asa/2012/precision-agriculture/8133_huthni.htm)
Acknowledgements
This work was funded by CSIRO and GRDC. We thank Graham Boulton for assistance in field testing the EM38.
Contact details
Dr Neil Huth
CSIRO Agriculture Flagship
Ph: 07 46881421
Email: neil.huth@csiro.au
GRDC Project Code: CSA0003,
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