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United States Department of Agriculture

Agricultural Research Service

Assessment of Salinity and Irrigation/Drainage Practices
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1 - Abstract
2 - Introduction
3 - Mobile Four-Electrode Sensing System
4 - Mobile Electromagnetic Sensing System
5 - Mapping Theory and Software
6 - Conclusions
7 - References
Mobile Electromagnetic Sensing System
Figure 5: mobile electromagnetic sensing system
Figure 5. mobile salinity assessment vehicle with combined electromagnetic induction and four-electrode soil conductivity sensing systems
This system involves a Geonics EM-38 instrument mounted in front of the transport vehicle within a vinyl ester pipe, as well as two sets of four-electrode arrays mounted underneath the vehicle.


The "EM-pipe" can be rotated, to enable the EM-38 readings to be made in both horizontal (EMh) or vertical (EMv) configurations. The tube and "rotator" are mounted on a hydraulic apparatus which elevates the EM-38 sensor sequentially to various heights above ground and translates it sequentially in the horizontal direction, so as to allow both EMh and EMv measurements to be made sequentially at various heights above both the furrow and seedbed. These changes in the height and orientation of the EM sensor are undertaken in order to alter the depth and distribution of the EM signal in the soil and, thus, to permit the determination of the salinity-distribution in the rootzone in two dimensions. The four-electrode arrays are mounted on a hydraulically operated scissor-action mechanism which includes a sensor and control mechanism to insert the probes into the soil and to measure ECa at both 1-m and 2-m array spacings in both the furrow and seed bed. In the picture the EM-sensor and four-electrode arrays are in the "up", or "travel", position.
An automated control system was developed to carry out the sequence of 52 operations involved in the full range of possible sequential "EM-38 and four-electrode" measurements. The engineering design of this system is described elsewhere (Carter, et. al., 1993). The control system is operated via an interface control panel with enable-buttons for activating EM and four-electrode sensor measurements and a 6-position selection switch for positioning the sensors over (and at various heights above for the EM sensor) the furrow and seed-bed. When the EM button is enabled, the EM sensor is rotated to the vertical (EMv) configuration and the carriage moves both the EM and four-electrode sensors to the selected position. The EM "start" button then initiates the following automated sequence:
  1. the EMv reading is made and logged
  2. the EM-38 sensor is rotated to the horizontal position
  3. the EMh reading is made and logged
  4. the EM-38 sensor is rotated back to the vertical position
This sequence is repeated for each Y-Z position selected. Depressing the four-electrode "start" button initiates the following automated sequence:
  1. the scissors apparatus inserts the electrodes into the soil
  2. ECa is measured at the 1-m array spacing
  3. a delay is provided for data logging at the 1-m spacing
  4. the meter/logger is switched to the 2-m array
  5. the ECa is read after a delay at the 2-m array spacing
After completion of the last logging, the scissors apparatus lifts the electrodes from the soil and stores them in the travel position. A small printed circuit board provides the necessary time delay functions. The mobile unit then moves to the next measurement site. All measurements at each site can be made in about 30-45 seconds. A Cooperative Research and Development Act contract has been developed by USDA/ARS with AG Industrial Manufacturing Inc. of Lodi, California to commercialize this system.

Example 1

With the EM equipment, salinity distributions within the rootzone can be inferred.
Figure 6: two-dimensional pattern of salinity
Figure 6. The average two-dimensional pattern of salinity in the soil profiles of the transect of figure 3
Figure 6 shows the average two-dimensional distribution of salinity in the soil (Imperial clay) profiles along a transect across a furrow-irrigated, tile-drained alfalfa field located in the Imperial Valley of California.
Salinity in the center of the seed-bed of the fine-textured soil is not as high as might be expected. A likely reason for this is the presence of an extensive network of cracks within the bed which allowed water movement through it, especially in the later stages of the irrigation season. This "inter-flow" likely leached out salts which otherwise would have accumulated by capillarity and upward flow in the bed, if it was completely isolated from the furrows. The patterns of salinity within the soil profiles were very similar at various points along the transect; however, in relation to the average profile shape, salinity increased in the upper part of the profile and decreased in the lower part of the profile with distance towards the down gradient end of the furrow-irrigated field, as shown in the next figure.

Example 2

Figure 7: changes in salinity distribution down a furrow
Figure 7. Changes (percentage basis) in salinity distribution, with reference to the mean profile, within soil profiles along a transect across a furrow-irrigated, tile-drained alfalfa field (Imperial clay soil) located in the Imperial Valley of California.
  • The data in figures 6 and 7 show that the pattern of salinity within the bed and throughout the soil profile varied systematically in response to the imposed irrigation system.
  • Salinity distribution in the rootzone can also be affected by the drainage system; this was observed in the Coachella Valley field previously discussed. Lower salinities occurred in this field in the soil overlying the tile-lines and higher salinities occurred in the soil located in between the tile lines.

Example 3

Additionally in this field, as shown below, the distribution of salinity in the soil profile varied with the mean level of salinity.
Figure 8: relation between salinity distribution and mean level of salinity
Figure 8. Relation between salinity distribution and mean level of salinity in a tile drained field (silty loam soil) located in the Coachella Valley of California
These distributions imply that salinity is high in areas where the net flux of water has been upward in the region of the field located in between the drain lines and is low in the areas where the flux has been downward, that is where leaching has occurred in the soil overlying the tile lines.

Example 4

The salinity distribution in the upper part of the rootzone (0-0.5 m) of the Coachella Valley field is shown below:
Figure 9: two-dimensional distributions of salinity
Figure 9. Two-dimensional distributions of salinity in the upper half-meter of the soil profiles of a field located in the Coachella Valley of California, as influenced by mean (0-0.5 m) salinity level
  • These data indicate that the salinity levels and patterns within the seed bed of this field are also related to the mean profile salinity levels, which in turn are related to the drainage pattern.
  • As shown in Figure 9, the salinity distributions in this silty-loam soil are clearly two-dimensional in contrast to the one-dimensional profiles observed for the clay textured Imperial Valley soil (Figure 6).
  • This difference in salinity distribution is thought to be due to differences in the cracking properties of the two soils.
  • Taken together, all these data Figures 4,8 & 9 indicate that the drainage system in this field is inadequate given the manner of irrigation, or geohydrologic situation, or both, existing there.
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Last Modified: 4/7/2006
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