Figure 1. Mobile "fixed-array" four-electrode system
with GPS antenna mounted on top of the mast

• The electrodes are combined into the "heels" of tillage shanks and mounted on a hydraulically controlled tool-bar attached to a tractor via a conventional three-point hitch
• The electrodes run at a depth of about 10 cm in the soil as the tractor moves across the field
• A Global Positioning System (GPS) antenna is positioned above the tractor cab and used to determine the spatial position of each sensor reading
• The ECa and the GPS signals are sensed at adjustable frequencies (as often as every second) and logged into memory for later analysis of salinity condition and spatial relations
• The four-electrode conductivity meter and the GPS receiver, their respective power supplies and their data loggers are contained in the water-tight, stainless steel box mounted behind the tool-bar shown in Figure 1.
• The tractor operator is provided with a remote monitor displaying time, ECa reading and logging status
• The analysis of the spatial data is carried out at the side of the field in a mobile office equipped with a computer work station and soil-salinity testing facilities

Examples of output data obtained with the mobile four-electrode sensing system

Figure 2. Relation between bulk soil electrical conductivity and distance along a transect
across a furrow-irrigated, tile-drained alfalfa field (Imperial clay soil)
located in the Imperial Valley of California

• Figure 2 shows ECa readings collected every 1 m apart as the tractor moved across a furrow irrigated, tile-drained alfalfa field in the Imperial Valley of California
• The "minimum" in the ECa readings occurring at about 380 meters from the irrigation-intake end of the field corresponds to the position of a suite of subsurface drains
• Otherwise, the ECa values increased toward the "tail end" of the field, presumably due to reduced application and infiltration of irrigation water with distance "down" the furrows

Figure 3. Average rootzone soil salinities on a laboratory, soil sample-extraction basis (ECe), as predicted from the measured ECa data along the transect

• Figure 3 show correspondence between soil salinity predictions (ECe basis) based on soil electrical conductivity measurements obtained by mobile, electromagnetic induction (EMh) and four-electrode systcms along a transect across a furrow-irrigated, tile drained alfalfa field (Imperial clay soil) located in the Imperial Valley of California
• Also shown are salinities predicted from the EM-sensor system discussed later; the accuracy of these predictions is generally excellent
• These data suggest that much of the variability in average rootzone salinity across the field is caused by the interactive, effects of the drainage and irrigation systems

Example of the marked effect that a subsurface drainage system can have on average rootzone salinity

Figure 4. Relation between bulk soil electrical conductivity (ECa) and distance along a transect crossing two sets of tile-drains in a field (silty loam soil) located in the Coachella Valley of California

• Figure 4 shows an example of a field of silty loam soil in the Coachella Valley which has two sets of buried "tile-lines"; one set being about 2.7 m deep and spaced about 90 m apart and another set being about 1.7 m deep and located at one-third and two-third distances between the deeper lines
• Soil salinity levels "mimicked" the drainage system, with high values of ECa measured in the soil located between tile-spacings and low values in the soil overlying them
• Concurrently, salinity tended to increase in the direction of irrigation (to the left in the figure), although the trend is "tempered" somewhat by the effect of the drainage system