Measuring salinity of irrigation water

One common measure of water salinity is electrical conductivity (EC). Basically, EC measures the capacity of a solution to transfer an electrical current between two electrodes.

Water with a high concentration of salts is a good medium for electricity movement and will have a higher EC than water with a low salt concentration. Laboratory water analyses will measure EC and provide other information to help assess the quality of a water source.

However, it is not feasible to conduct these analyses every year. EC can also be monitored in the field using a handheld EC meter. The most common portable EC meters are the stick type, which consist of a small metal probe attached to a digital display. These EC meters are very useful in that they are easy to use, small, relatively inexpensive, and provide measurements with a reasonable level of accuracy.

To ensure correct readings, verify the units and calibrate the device before use.

• Units

There are several units used to measure EC. The most common is deciSiemens per meter (dS/m). When using your EC meter, make sure you know what units are being displayed. This information can be found in the manual or can be inferred during the calibration process (by using a solution of known conductivity).

• Calibration

Handheld EC meters require calibration prior to use. This is usually done with solutions of known conductivity provided by the manufacturer. The device manual should contain instructions for correct calibration. Calibrate your EC meter as often as possible, at least once before the season starts.

Calibration should be made considering that, for agricultural purposes, EC readings will range from 0.5 to 4 dS/ m. Very low or very high values are not common in agriculture (for example, the EC of distilled water, water without any salts, is approximately 0.002 dS/m, and the EC of sea water is 58 dS/m).

Rice is most susceptible to salt damage during the seedling and pollination stages. Sacramento river water is low in salts, with EC ranging from 0.13 to 0.37 dS/m. However, drain and well water can have much higher salinity content. Also, water management can affect salinity of water in rice fields.

Yield reductions have been observed when the EC of irrigation water was higher than 1.9 dS/m. Typically, water in lower basins has higher salinity, and EC values tend to peak during holding periods. Reducing salinity levels can be achieved by adding fresh water (or water with a lower EC value) to fields with salty water.

Continued water flow will not let salts accumulate in the field. Water holding requirements may render continued flow difficult. If using saline water, avoid using herbicides that require long term water holding periods.

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