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.