Study on the Different Methods of Calibrating Shallow Soil Moisture Content Measured with Neutron Instrument

Neutron instrument is a high-tech,intelligent and advanced instrument,it is extensively used in measuring soil moisture content without disturbing soil in fieldwork,can be easily operated to successively monitor soil moisture content at any depths at the same measuring site,the measurement does not be limited by seasons,change of physical and chemical properties of soils,and can be connected with automatically recording system and computer.Neutron instrument is extensively used in many domains,such as pedology,geography,agriculture,forestry,meteorology,hydrogeology,and water conservancy and building projects.In this paper,the calibrating equation of soil moisture content measured with neutron instrument is developed,and the methods for increasing the measuring precision of shallow soil moisture content are researched,compared and verified. The traditional dying method is regarded as the criterion in measuring soil moisture content.The test results are calibrated and verified in field.The result shows that the hierarchical-section method is optimal in calibrating shallow soil moisture,and the relative errors are lower than 10%,which can satisfy the requirements of measuring shallow soil moisture content;the measuring precision of shallow soil moisture content can not be increased by the indirect derivation using deep soil moisture content;the results of shallow soil moisture content calibrated with the regressed equation of nonlinear least square method are not stable,and the test errors are high.Generally,it is considered that the detecting radius of neutron instrument is related to soil properties and moisture content.