Comparison of calibration techniques of an Impedance Probe for near surface soil moisture measurement

摘要

An Impedance Probe (IP) was used during the Southern Great Plain 1997 field experiment to support the remote sensing soil moisture mapping effort in Oklahoma, USA. Soil moisture sampling techniques used as ground-truth for remote sensing are most often not adequate due to the time and cost associated with them. The need for fast, accurate, less expensive, and less destructive measurement techniques within the remote sensing discipline is increasingly growing. The IP offers an alternative for measuring soil moisture in remotely-sensed regions where the logistics of gravimetric sampling can restrict the number of samples obtained. The gravimetric technique has been the traditional and most accurate technique used to validate remotely measured soil moisture. This technique is very destructive and time consuming, thereby limiting our ability to apply it over large areas expected by coarse resolution microwave remote sensing products. Recent advances in the Time Domain Reflectometery (TDR) techniqueshave resulted in sensors with less noise and require minimum human intervention. Field sampling and laboratory experiments were conducted to evaluate and calibrate soil moisture data measured using an IP. The objectives were: to examine the best calibration technique that can be used to estimate the spatial distribution of soil moisture and/or a representative mean moisture level for a given location, and to evaluate the correlation of soil moisture values using the probe and gravimetric measurement techniques. Results indicated that a site specific calibration curve developed by using a pressure plate apparatus provided a representative mean value was equivalent to the gravimetric technique. Both, the simulated and manufacturer calibration techniquesprovide soil moisture values that are spatially correlated with the gravimetric measurement technique. Our results indicated that, the instrument can provide reproducible results comparable to the gravimetric technique. This instrument can be utilized tomeet the requirement of measuring the soil moisture content in the upper 6-cm of the soil depth.