基于热脉冲方法的南方红壤蒸发原位监测

基于热脉冲技术的能量平衡方法是一种原位实时监测土壤蒸发速率技术。热脉冲方法的准确性受地表气象条件、土壤类型等因素影响，目前该技术多应用于质地较砂北方旱地土壤。为进一步验证热脉冲方法在质地较黏的南方红壤上监测蒸发速率的适用性，该研究利用热脉冲传感器与微型蒸渗仪开展了为期30 d的监测试验，对比了热脉冲能量平衡方法与微型蒸渗仪测得日蒸发速率以及累积蒸发量，并通过监测土壤含水率变化和气象因子，评价了红壤区土壤蒸发的主要影响因素。结果表明，热脉冲方法测得的日蒸发速率与微型蒸渗仪监测值变化趋势相同，试验期间两种方法测得日蒸发速率最大相差0.80 mm/d，最小相差0.02 mm/d，决定系数R2=0.52。两种方法测得的累积蒸发量随时间变化值之间具有更强的相关性（R2=0.99），30 d内微型蒸渗仪监测的总累积蒸发量为19.3 mm，热脉冲方法监测的累积蒸发量为24.4 mm。由于降水影响，微型蒸渗仪漏测了7 d的蒸发速率，剔除该7 d数据后，二者仅相差4.6%。相比于传统的微型蒸渗仪法，热脉冲方法具有自动化的优点，且能够实时监测剖面微尺度（mm尺度）上蒸发动态。研究区红壤的日蒸发速率与各因子的相关程度由大小为土壤含水率、净辐射、风速、气温。研究结果表明热脉冲能量平衡方法能够准确地监测南方红壤区土壤蒸发动态，研究可为红壤区水热循环研究提供技术和理论支撑。

Evaluation of the heat pulse method for determining evaporation of a red soil in southern China

A heat-pulse and sensible heat balance approach has been used for in-situ monitoring soil evaporation at a fine depth scale. However, the accuracy of the heat pulse sensor depends mainly on the meteorological conditions and soil properties. This technique has been tested for the dryland sandy soils in northern China. This study aims to further evaluate the accuracy of the heat pulse method on the red soils (Ultisols) in southern China. A 30-day field experiment was conducted from October 27thto November 25th, 2020, in order to compare the performance of the heat pulse method and commonly-used micro-lysimeter. A three-needle heat pulse sensor was installed in a bare soil profile located at the Huazhong Agricultural University, Wuhan, China. The soil temperature and thermal properties were then determined by the heat pulse sensors, and then the soil evaporation rate was calculated using the sensible heat balance. A net radiometer was installed to monitor the net radiation dynamics for the correction of the surface (Stage I) evaporation errors. The independent daily evaporation rates were measured by micro-lysimeters, which were made of PVC pipes with 10 cm in diameter and 9 cm in depth. The soil water content and meteorological data were collected to evaluate the red soil evaporation from a local weather station. The results showed that the daily evaporation rate by the heat pulse method was agreed better with the measurement of micro-lysimeter, indicating the maximum deviation of 0.80 mm/d and the minimum deviation of 0.02 mm/d during the study period. The coefficient of determination was found to be 0.52 for the daily evaporation rate between the two approaches. Overall, the daily evaporation rate presented an average bias of 0.04 mm/d and the root mean square error of 0.40 mm/d. The reliable cumulative evaporation was obtained with a coefficient of determination of 0.99, compared with the micro-lysimeter. The 30-day cumulative evaporation values were 24.4 and 19.3 mm on average (7-day measurements were not available due to rainfall) measured by the heat pulse method and micro-lysimeters, respectively. The heat pulse method can be expected to automatically capture the fine-scale evaporation dynamics. The heat pulse method can also perform the time-intensive (Sub-hourly) evaporation measurements, rather than the daily measurement in the labor-intensive micro-lysimeter. There was a downward migration of the dry surface layer in the heat pulse method, which was undetectable for the commonly-used lysimeter. Consequently, the daily evaporation rate was the most affected by the soil water content, followed by the net radiation, the wind speed, and the air temperature, according to the soil water content and meteorological records. Anyway, the heat pulse method can reliably determine the evaporation rate of red soil in southern China.