Constanze Mueller, Cornelius Jantschke, Wolfram Spreer, Karlheinz Köller:
Correction Factors for TDR-soil Moisture Measurements Within the Tree's Root Zones


University of Hohenheim, Agricultural Engineering in the Tropics and Sub-tropics, Germany

Irrigation systems has been established all over the world to secure crop production and therefore cash income during dry seasons. But water scarcity prompts farmers to budget water in order to avoid a total crop failure and loss of their major source of income. The less water, the higher the potential for conflicts about water.

In order to investigate the efficiency of irrigation systems, the root zone of mango trees has been equipped with TDR (Time Domain Reflectometry) soil moisture detection devices in an orchard, close to Chiang Mai, Northern Thailand. The TDR-technique has been adapted to detect the velocity, an electromagnetic wave is spreading within a medium. Waves' velocity is only dependent on relative permittivity er besides magnetic permeability mr of soil. Magnetic permeability of most soils can be set to 1, therefore the wave-velocity is only dependent on the relative permittivity. Permittivity of dry, porous material (er < 5) is significantly lower than the one of water (er = 81). So transit time and soil moisture are directly related, which results in soil moisture content.

The trial is meant to control the soil moisture within the relevant root-zone in order to optimise irrigation efficiency with different irrigation-designs and -scenarios. Simultaneously the TDR-technique is combined with a high resolution soil-temperature recording of the portrayed root"=zone. The combination of the data"=sets of irrigation amount, weather data, soil moisture and soil temperature draws a clear picture of ideal irrigation conditions. The TDR-signal is effected by soil temperature, therefore it is additionally verified by laboratory drying of soil samples. The combined information of soil samples, soil"=temperature, soil moisture and soil density creates a temperature correction factor for TDR-measurements, which enables to improve TDR-measurement's accuracy in field trials with alternating soil temperatures.

Keywords: Correction factors, irrigation, Mango, soil temperature, Thailand, Time Domain Reflectometry (TDR)


Contact Address: Cornelius Jantschke, University of Hohenheim, Department of Agricultural Engineering in the Tropics and Sub-TropicsGarbenstraße 9, 70593 Stuttgart, Germany, e-mail:
Andreas Deininger, September 2004