Submitted to: Irrigation and Drainage International Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 16, 2000
Publication Date: N/A
Interpretive Summary: The ability to determine accurately determine the amount of water in soil is necessary to improve irrigation management. Gravimetric methods, that is taking soil samples and drying them in an oven to determine the water content, are slow and labor intensive. Soil moisture also can be measured with nuclear gauges or with modified cable testing equipment known as time domain reflectance (TDR). Both of these methods require expensive equipmen and are difficult to automate so that soil moisture readings can be made continuously. Traditional methods for soil moisture determination also require frequent trips into the field causing damage to plants and soil compaction, which can affect the measurements. Recently, small packaged TDR sensors, which are relatively inexpensive and easily automated, have been introduced. Work conducted at Phoenix, Arizona, has shown that moisture readings from all of the packaged sensors tested were affected by salinity level, that is, how much salt the soil contains. Therefore, if these sensors are used where soil salinity is high, they will not give accurate measures of soil moisture. Models were developed that can be used to correct the soil moisture reading for the effects of salinity on the reading obtained from the sensors. This work should be of value to instrument designers, other researchers, and ultimately producers.
Encapsulated time domain reflectometry (TDR) and electrical capacitance soil moisture measurement systems can be used in a variety of irrigation management applications. These self-contained systems are relatively inexpensive, easily automated, and measure over a wide range of volumetric soil water contents. However, soil moisture readings are often unreliable in highly saline soils, and information on the effects of salinity on the sensor output is limited. Soil moisture measurements of four commercial devices, including a TDR cable tester (Trase), two encapsulated TDRs (Aqua- Tel and Trime), and one encapsulated capacitance sensor (Theta), were obtained over a wide range of water contents and salinity levels during laboratory studies with a sand medium. All four systems were affected by salinity and performed poorly at or near saturated conditions. The Trase produced a sufficient amount of reliable soil moisture data, which was used das a standard to evaluate the other sensors. The Aqua-Tel and Theta units did not lose sensitivity to soil moisture as the salinity increased, but the output was shifted upwards significantly. The Trime became less sensitive to salinity as it increased, while the output also shifted upward. To produce soil moisture values that are independent of soil conditions, all of the encapsulated systems need a method to correct the reading for soil salinity.