Calibration of soil water content of calcareous soil using Time Domain Reflectometry (TDR) method in karst regions

Soil water content is an important parameter in studying climate, hydrology and soil erosion. Among many soil moisture measurement methods, Time-Domain Reflectometry (TDR) technology has been widely used because of its unique advantages. For example, it is convenient, fast, accurate, and does not need to disturb the soil. Under low accuracy requirement, this method is not affected by soil physical factors, such as soil texture, bulk density and temperature. However, these soil physical factors affect the measured results under high requirement. Thus, calibration of TDR measurement is necessary. In this study, the soil water content of calcareous soil with different textures was measured by TDR in karst regions, combining laboratory and field calibrations. In laboratory, we used loam soil as the calibration medium. Soil pillars with volumetric water content from 6.3% (water content of air dried soil) to 60% with a gradient of 5%. Oven dry and TDR methods were used to measure the soil water content under each gradient. In the field, three types of soil, coarse-, medium- and fine-grained soils, were calibrated accordingly. Five calibration curves, including one obtained from laboratory, three from different types of soil and one from combination of the three types of soil in field calibration, were established. The accuracies of different calibration curves were evaluated by a model of effective coefficients. The results show that, (1) Soil water content measured with TDR is smaller than that measured with the oven dry method. The largest absolute errors between soil water content measured by the two methods are 10.6% in laboratory calibration and 12.2% in field calibration, respectively. The corresponding relative errors are 60.3% and 32.8%, respectively. Therefore, calibration is necessary before using TDR for soil water content measurement in the study area. (2) After calibration, the accuracy of TDR measurement is obviously improved with mean absolute errors reduced to 1.4%-3.1%. The accuracy of calibration curve obtained in laboratory is a little lower than that in the field. (3) The calibration curve obtained from the calibration of combination of different types of soil shows a high accuracy with mean absolute errors of 1.5% to 2.6%, which could be used to calibrate the TDR measurement of different types of soil. In sum, the most suitable calibration curve for the soil water content measured with TDR in the study area is that obtained from the calibration of the combination of three types of soil. The results in this study could be employed to calibrate the TDR measurement in other similar areas.