含水量和容重对旱地耕层土壤热导率的影响及预测

土壤热导率是研究地表能量平衡和土壤水热运移过程中的一个基础参数。受土壤耕作、干湿交替和根系生长等过程的影响,耕层土壤的含水率和结构呈现较强的变异特征,而目前缺乏关于定量分析耕层土壤热导率变异特征的研究。该研究利用田间定位试验,采用热脉冲技术测定了含水率和容重变化条件下耕层土壤热导率的变异特征,并利用传递函数模型对耕层土壤热导率进行了预测。结果表明:含水率和容重是影响耕层土壤热导率变异的主要因子,而耕作强度和干湿交替是这种变异的关键驱动力;与翻耕和旋耕处理相比,免耕处理提高了土壤容重和含水率,从而增大了土壤热导率;在干湿交替作用下,翻耕后土壤容重逐步增加,耕层热导率也呈现上升趋势,波动幅度与含水率的变化相关。基于含水率、容重和质地信息,土壤热导率传递函数模型可以给出可靠的田间土壤热导率估计值,其均方根误差和平均偏差分别为0.09和-0.01 W/(m·K);考虑耕层土壤容重的动态信息,可以提高该模型预测土壤热导率的准确性。

Effects of soil water content and bulk density on thermal conductivity of plough layer soil in arid land and its prediction

Abstract: Soil thermal conductivity (?) is a key parameter for studying surface energy balance and coupled heat and water transfer in soil. ? can be obtained by heat pulse method or semi-empirical or empirical models, with both models based on the information of soil texture, water content (?) and bulk density (?b).The pedotransfer model has the advantages of simple form and having no requirement of soil minerology information. This pedotransfer ? model, however, has not been applied comprehensively under field conditions where ? displays strong spatial and temporal variability. The objectives of this study are to determine the spatial and temporal changes of ? as related to ? and ?b in tilled soil layers, and to test the feasibilities of the pedotransfer ? model for estimating field ? with the information of soil texture, ? and ?b. Two independent field experiments were conducted: one study of different tillage treatment''s effect on ? variations and another post-tillage soil structure dynamic study on ? at 2 soil depths due to alternate wetting and drying. For the tillage method study, ? measurements were carried out in the field, and soil cores were taken to determine ? and ?b gravimetrically. For the soil structure dynamic study, in situ ? changes were monitored with time domain reflectometry (TDR) technique, the dynamic ?b values were determined gravimetrically after each rainfall event, and the corresponding ??data were obtained from the collected intact soil cores by heat-pulse sensors. The results showed that ? and ?b were the key factors that affected ? in tilled soil layers. In 0-10 cm soil layer, the ?, ? and ?b values in no tillage treatment plot were significantly higher than those of the moldboard and rotary tillage plots. Soil ? values of the 10-20 cm soil layer were higher than that in the 0-10 cm layer, and the trends were consistent with that of ? and ?b regarding tillage treatment and soil depth. For the post-tillage soil structure dynamic study, ?b was increased gradually with time and soil depth and became relatively stable after 4 wetting/drying (W/D) cycles, i.e., from 0.98 to 1.16 g/cm3 for the 0-5 cm layer, and from 1.09 to 1.28 g/cm3 for the 5-10 cm layer. The magnitude of the change was relatively small among the first 3 W/D cycles when the degrees of saturation were relatively low, and ?b in the 5-10 cm layer reached the maximum after the fourth W/D cycle when the soil was nearly saturated, with the change became less significant thereafter. Comparison between measured and modeled values showed that the pedotransfer ? model provided reliable ??with RMSE of 0.09 W/(m·K) and mean bias of -0.01 W/(m·K). Our analysis also highlighted the fact that when ?b varied over time due to soil structure change, using a constant ?b (measured either right after tillage or at the end of the experiment) would introduce larger errors for ? estimations. The pedotransfer ? model for estimating soil ? could be useful for simulating heat transfer in tilled soil layers.