Tuesday 15 September 2015

EM Surveying and Water Holding Capacity

In recent weeks we’ve been through the calendar of EMSurveying, A day in the life of an EM Surveyor and The value in ground-truthing your EM Survey in amongst other informative posts from Lindsay NZ and HydroServices. This week we’re going to shed some light on how an EM Survey by Agri Optics can be used to evaluate water-holding capacity of the surveyed area.
Predominately our clients ask us to conduct an EM survey for them to determine the differences in their soils with a view to varying their irrigation depths on the different soil types; however we can adapt the EM Surveys for many different uses, one of these being to evaluate water-holding capacity variation.

The DualEM sensor works by emitting an electro-magnetic field into the soil and measuring the returning conductivity of the soil. The conductivity of the soil in New Zealand is affected mostly by soil texture and the amount of water the different soil textures can hold (the more water the soil can hold the more conductive it is). The readings can also be influenced by salinity, however other than a few isolated areas in NZ we don’t have an issue with salinity affecting the readings. 

Figure 1: DualEM sensor being trailed behind our light weight Polaris

To fully relate the EM readings to water holding capacity (WHC), ground-truthing is needed to quantify the actual WHC at different locations as the EM Survey only measures relative difference of one area compared to another. As we have mentioned in previous blogs we create a map of EM zones and within that locate sample sites for each zone to be ground-truthed. The ground-truthing is then carried out by typically HydroServices using their neutron probe and our agreed protocols. They then provide us with the water-holding capacities for each different depth. From this we then create a map of WHC variation across the surveyed and provide a report back to the client of these additional maps with description on correlations of WHC and EM and recommendations on how these maps would be used to implement more efficient irrigation.  

In addition to getting accurate WHC maps and the associated report back the client can also use the ground-truthing sites to help site soil moisture probes and if the probes are installed before the ground-truthing is completed, the ground-truthing reading can also be used as one of the field calibration readings for the soil moisture probes.  

Figure 2: Water-holding capacity map created from an EM map

All of this information helps the client get the most of their EM data and make efficient use of their water by matching the water to the different zones and then monitoring moisture levels with their moisture probes. If the 2015/16 season is going to be as dry as forecast then making every drop count will be crucial. We not only conduct the EM surveying we can now also provide our clients with AquaCheck soil moisture probes to help manage your irrigation as efficiently as possible. Visit our website or give Jemma or Chris a call to discuss any of the above.