TDR测定喀斯特地区石灰土含水量的标定研究

摘要

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.%土壤含水量是气候、水文、土壤侵蚀等研究中的一个重要参数.TDR法因具有方便、快速、精确且不扰动土壤等优点而得到广泛应用,但因受土壤质地、容重、温度等物理因素的影响而必须对其进行标定.本研究采用室内标定(壤土)和田间标定(粗、中、细质土以及三者的组合)相结合的方式,对用TDR法测定的喀斯特地区不同质地石灰土含水量进行了标定.结果表明:(1)TDR法测定的土壤含水量比烘干法测定值要小,两者最大绝对偏差在室内标定条件下为10.6％,田间标定为12.2％;相应的相对偏差最高分别可达60.3％和32.8％.因此,必须在使用前对TDR进行标定.(2)标定后TDR法测定的土壤含水量精度明显提高,平均绝对偏差低至1.4％～3.1％;室内标定曲线精度略低于田间标定曲线.(3)不同质地组合的TDR法土壤含水量标定曲线的标定精度较高(平均绝对偏差为1.5％～2.6％),可用于不同质地土壤含水量标定.本研究结果可用于修订校正研究区及类似区域TDR法测定土壤含水量的结果.