Soil Moisture Measurement by TDR Coil Probe in the Surface Thin Soil Layer in the Cold Steppe of Mongolia

TDR coil probe (CP) developed for soil moisture measurement in the micro-scale can be considered to be concretely useful to the soil moisture measurement in the thin soil near the soil surface and/or in the soil moisture movements. It is indispensable for using CP to know actually the durability, the stability and applicability of CP in the field. The soil moisture measurement in the surface thin soil layer is also important to make a validation of soil moisture measurement algorithm of AMSR-E and AMSR2. A CP was made of brass and enamel wires and the sensor size was 45mm in diameter and 40mm in length. We made a severe test of CP in the cold steppe of Mongolia after laboratory tests of probe error and temperature effect. Six CPs, two wire type TDR probes and some meteorological instruments were installed in Sanzai site covered with grass and highly wet soils in 2002 and started field tests. We had much snowfall in winter and air temperature changed widely from + 38 to - 36 degrees in C there. There was a permafrost soil less than about 1 m depth. The soils after finishing the tests in Sanzai site, we set continually the same CPs with two wire type TDR probes and some meteorological sensors at the Mandalgobi site (MGS) in the MAVEX (Mongol AMSR/AMSR-E/AMSR2 Validation Experiment) study area of the Mongolian plateau in 2008. MGS was very flat and mostly covered with pasture and dry soils. There was a slight snowfall and soils were frozen in autumn and winter. Air temperature varied frequently between + 38 to - 35 degrees in C there. Each two CPs were horizontally installed at the 1 cm, 2 cm, and 3 cm depths, respectively. The CPs set at the 3 cm depth were in the effective measurement area of a two wire type TDR probe (TRIME-IT, IMKO) with two 11cm in length stainless rods of 3.5 mm in diameter at the 3 cm depth for the investigation of the accuracy. The five tested CPs were survived and measured successfully and precisely soil moisture in the surface thin soil layer above the 4 cm depth.