热脉冲技术监测近地表热特性和蒸发：土壤异质性的影响及矫正

The heat-pulse technique can be used to measure soil thermal property and monitor dynamic of soil evaporation based on the sensible heat balance theory. Near-surface soil shows great heterogeneity due to the effects of soil ambient temperature and soil-air interface. This violates assumption of the heat-pulse technique about the homogeneous and isothermal medium and lead to great errors in measured thermal property and evaporation. Our previous work showed that wetting front would also affect near-surface thermal property measurements and these effects were coupled. In this project, the measurement errors under various soil ambient temperatures, soil-air interface and wetting front locations were analyzed to reveal the influence mechanism of soil heterogeneity to near-surface soil thermal property measurements using the heat-pulse technique. Combined with numerical analysis of the PILS—ABC and Philip and Kluitenberg (1999) models, wetting front influence was quantified. And then methods of correcting the coupled effects of soil heterogeneity were set up and theory of measuring near-surface soil thermal property using the heat-pulse technique were improved. Finally the sensible heat balance theory, which was originally used to measure evaporation in homogeneous soil, was improved to increase the measurement accuracy of soil evaporation. The results are useful in improving measurement technique and theory of near-surface soil thermal property and evaporation, and can also be applied to establish reasonable water management and improve water use efficiency.

热脉冲技术常用于测定土壤热特性，并基于感热平衡原理动态监测土壤蒸发。受土壤背景温度和土壤—大气界面影响，近地表土壤易表现出较大异质性，不能满足热脉冲技术关于均质和等温介质的假设，导致获得的近地表热特性和蒸发误差较大。我们前期研究表明湿润锋也会对近地表热特性测定造成影响，且与其它影响相互耦合，本项目通过动态土壤背景温度、土壤—大气界面和湿润锋位置对测定结果的误差分析，揭示近地表土壤异质性对热脉冲技术测定过程的影响机理，结合PILS-ABC模型和Philip and Kluitenberg（1999）模型对湿润锋的影响进行定量化，建立矫正土壤异质性耦合影响的方法，发展热脉冲技术测定近地表土壤热特性的理论。在此基础上，改进基于均质土壤测定蒸发的感热平衡原理，提高热脉冲技术测定蒸发的准确性。结果有助于完善监测近地表土壤热特性和蒸发的技术与理论，对制定合理水分管理措施、提高水分利用效率有重要意义。

热脉冲技术常用于测定土壤热特性，并基于感热平衡原理动态监测土壤水分蒸发。但热脉冲技术用于野外监测近地表土壤热特性时，常受到土壤背景温度和土壤-大气界面的影响；热脉冲技术用于监测土壤水分蒸发时，仍需要借助其它方法对蒸发阶段进行区分，限制了热脉冲技术在野外近地表土壤水热因子及蒸发监测方面的应用。本项目利用自制的多针热脉冲传感器，组装了一套连接4个多针热脉冲传感器的土壤温度及热特性测定系统，对沙地樟子松人工林林下4个位置（行内、行间1/4行距、行间1/2行距、行间3/4行距）的近地表5cm以上多深度或层次的土壤温度和热特性进行了监测，并基于感热平衡原理得到了土壤水分蒸发。在利用热脉冲监测数据计算土壤热特性时，对土壤背景温度和土壤-大气界面的影响进行了矫正，将多针热脉冲测定的土壤水热因子及蒸发数据与EC5水分传感器水分数据、称重法土壤水分数据、红外热像仪测定的地表温度与微型蒸渗仪蒸发数据比较，均具有较高的一致性。本项目建立了一种基于热脉冲监测的土壤温度数据区分土壤水分蒸发阶段的方法，基于该方法与土壤感热平衡原理，热脉冲技术能够得到全阶段的土壤水分蒸发，提升了该方法用于监测野外土壤水分蒸发过程的准确性及便利性。项目重点分析了玉米冠层下及沙地樟子松人工林林下土壤水热因子及土壤水分蒸发的时空异质性，结果表明：沙地樟子松人工林下不同位置处土壤地表温度存在明显差异，2019年第233日，行内、行间1/4行距、行间1/2行距和行间3/4行距处日最高地表温度分别为38.9、38.7、23.7和24.6℃；一次时长为28天的蒸发过程中，玉米冠层下行内、株下、行间1/4行距和行间1/2行距位置的累积土壤水分蒸发分别为24.8、19.6、30.6和27.3mm。多针热脉冲传感器能够在微尺度连续原位监测近地表土壤水热参数及蒸发，为干旱半干旱地区农田及林地土壤水分管理及提高水分利用效率提供了技术、理论及数据支撑。