Evaluation and monitoring methods for seeps management in Northern Victoria
03 Evaluation and monitoring for seeps management in Northern Victoria 03 Piezometers / Water Ec (Electrical Conductivity) 04 GIS mapping 05 Elevation 06 EM38 What is EM38? What does EM38 measure? 07 NDVI What is NDVI? How do you calculate NDVI?
Evaluation and monitoring methods for seeps management in Northern Victoria
This guide provides information on five diagnostic tools for characterising seeps. After gaining an understanding of seep processes at each site management practices can be chosen that will have the greatest chance of success.
Piezometers / Water EC (Electrical Conductivity)
Installing piezometers for each site is essential to the learning's required for effective decision making. Surveying the site and graduating the installation of the bores gives the most accurate account of the site you are dealing with. A hand auger is the easiest and cheapest approach with PVC pipe inserted to maintain the integrity of the bore. From there we can establish watertable depth and also the pH and EC level of the water at the appropriated elevation.
Ryan Fox and Ash Wright using the hand auger to install a piezometer and the installation of the PVC
A video of how to install a piezometer is included on the Mallee Sustainable Farming Website https://www.youtube.com/watch?v=Tjg0VgBhmgk
Geographic Information Systems (GIS) is a digitized system framework for the capture, storage, checking and display of data related to positions on the earth's surface. This information falls squarely in the seep management wheelhouse and helps greatly in a whole farm planning analysis of the situation for the grower. There is an abundance of freely available data and increasingly farmers own data that can assist decision making about managing seeps. Google Earth is a great starting point for this type of work along with other programs you may already be using as part of your farming operation. Links to access GIS mapping tools are: Google Earth. https://www.google.com/earth/versions/ Vic-map Elevation Digital Elevation Model (DEM) 10m. https://www.land.vic.gov.au/maps-andspatial/spatial-data/vicmap-catalogue/vicmap-elevation
Screen capture of programs readily accessible by farm managers
Elevation is a good indicator of the localised effects and cause of saline and scalded areas. Used in conjunction with piezometers knowledge of the sites elevation (nAHD) gives the grower the ability to identify the scale and the potential of the problem based on the total area that is visually affected, in coordination with other data the grower may have. A generalised groundwater contours map of the Victorian Mallee can help identify the locations of the associated highly saline regional groundwater saline outcrops. Elevations range from a high of 80m in the SE to a low of 20m in the NW of the Region. Extremely saline areas associated with this aquifer must be identified and treated differently to dune seeps. Sometimes dune seeps can be located contributing to edge expansion of regional salt-pans.
Figure 1. Hydrological processes modelled in salt loads study 1
The chosen Vic-map Elevation Digital Elevation Model (DEM) 10m is a product freely available. It is a raster representation of Victoria's elevation. DEM 10m has a spatial resolution of 10 metres with respective horizontal and vertical accuracy of 12.5m and 5m or better. The DEM is constructed from source data of various resolutions, accuracies and ages to produce an improved DEM containing increased detail in localised areas. The DEMs are hydrologically enforced and correctly defines the natural surface drainage and hydrological flow. This product has been quality assured by DELWP and 2 additional third-party consultants . 1
https://www.mdba.gov.au/sites/default/files/archived/mdbc-salinity-reports/439_Salinity_audit_of_MDB_100_year_perspective.pdf 2 https://www.land.vic.gov.au/maps-and-spatial/spatial-data/vicmap-catalogue/vicmap-elevation
EM38 What is EM38?
EM stands for Electro Magnetic induction, a technique used to measure soil conductivity. This is often measured with the Geonics EM38 (see diagram from Lesch et al. 2005 below). The EM38 reader has two coils spaced 1m apart. One coil generates a primary magnetic field in the soil. The second coil measures both the primary field, and the secondary magnetic fields that are induced in the soil and relate to its electrical conductivity. In an EM survey the EM38 equipment is dragged over a paddock and regular measurements recorded together with GPS positions.
What does the EM38 measure?
The EM38 measures electrical conductivity of soil, known as ‘apparent EC’ or ECa. The electrical conductivity that the EM38 ‘sees’ is a complex combination of conductivity at different soil depths, depending on the orientation of the EM38 (‘horizontal’ or ‘vertical’). Soil electrical conductivity is influenced by salt, water, temperature, and the composition and arrangement of soil particles. Soils that are warmer, have more salt, water, clay or organic matter content will conduct more electricity and have a higher ECa (EM reading). Salinity is the other major influence on the EM signal and is also very important for crop growth. A level of 0.6 dS/m (60 ms/m) is generally regarded as having an influence on plant growth; the effect is greater with higher EC1:5 and when shallower in the soil profile. High EC1:5 can indicate a discharge salinity area; very low EC1:5 can indicate an area prone to recharge. Measuring both the major influences on the EM signal allow us to separate out whether Mallee EM maps are of salt (generally the case), water (less often), or salt and water together (quite often). In Mallee soils salt and water tend to be related because of soil texture – sands drain well, leach salt and have low water content, whereas clays drain poorly, accumulate salt and have high water content. Where the relationship is poor this can indicate other soil factors (e.g. different soil origins) that may also be influencing the EM signal. Measuring salt (EC1:5) and water together also allow us to ‘sample’ for unusual combinations e.g. abnormally high salinity and high water together, indicating an effect of salinity on crop water use or the presence of a water table. The information gathered from the EM38 data set will hopefully give some indication of the scalded site area and the extent of the problem with higher conductivity associated with the presence of surface salt and its relationship to the watertable EC.
Figure 2) Principle of EM38 operation 20053 3
Picture 3) EM38 at Geoff Vines property east of Ouyen
NDVI What is NDVI?
NDVI stands for Normalised Difference Vegetation Index. The NDVI is used in agriculture extensively for the assessment of crop biomass. NDVI for seeps assessment is used mostly over summer to identify areas that are becoming a seep or even those that are intermittent and the area or extent of influence of the seep.
How do you calculate NDVI?
Healthy vegetation (chlorophyll) reflects more near-infrared (NIR) and green light compared to other wavelengths. But it absorbs more red and blue light. This is why our eyes see vegetation as the colour green. If you could see near-infrared, then it would be strong for vegetation too. Satellite sensors like Landsat and Sentinel-2 both have the necessary bands with NIR and red. The result of this formula generates a value between -1 and +1. If you have low reflectance (or low values) in the red channel and high reflectance in the NIR channel, this will yield a high NDVI value. And vice versa. Overall, NDVI is a standardized way to measure healthy vegetation. When you have high NDVI values, you have healthier vegetation. When you have low NDVI, you have less or no vegetation. Generally, if you want 4 to see vegetation change over time, then you will have to perform atmospheric correction.
Figure 3) Illustration of how vegetation reflects light Picture courtesy of NASA
Figure 4) Example NDVI image of two known seep areas
Seep sites can be mapped using NDVI data5. This data can assist with basic whole paddock assessments that can be follow up with grower ground truthing. NDVI data is freely available from the ESA portal and 6 produced in readily useable formats . Chris McDonough’s (Insight Extension for Agriculture, SA) work on NDVI as a diagnostic tool for seep areas can be found at: https://msfp.org.au/use-drone-satellite-ndvi-imaging-early-detection-seepsmallee. 4 https://gisgeography.com/ndvi-normalized-difference-vegetation-index/
5 https://msfp.org.au/use-drone-satellite-ndvi-imaging-early-detection-seeps-mallee/ 6 https://sentinel.esa.int/web/sentinel/missions/sentinel-2