Determining Evapotranspiration of a Chinese Willow Stand with Three-Needle Heat-Pulse Probes

Abstract

Quantifying evapotranspiration (ET) and partitioning it into its individual components, evaporation (E) and transpiration (T), is essential to understanding eco-hydrological systems and guiding agricultural production practices. Our study tested the possibility of using heat-pulse probes (HPP) to determine E, T, and ET. Values of E and T were determined using the sensible heat balance theory and heat ratio method (HRM), respectively. Field experiments were conducted in a small stand of 10 Salix matsudana trees growing in a sandy soil on the Loess Plateau, China. An arrangement of five three-needle HPP sets determined E throughout the 3- to 33-mm soil layer. Sap flux density (vₛ) was determined by three HPP-sets inserted in the sapwood of three sample trees at breast height; up-scaling was used to determine T at the stand level. Summing E and T gave the stand ET. The HPP E, vₛ, and ET values were compared with those obtained from microlysimeters (MLS), thermal diffusion probes (TDP), and the water budget method, respectively. Daily HPP E had a moderate, but significant (R² = 0.47; P < 0.001) linear relationship with MLS E. Total HPP E was 13.4% lower than MLS E. Hourly HPP vₛ had a highly significant linear relationship (R² = 0.87; P < 0.001) to TDP vₛ but was consistently underestimated by 49%. Adjusting the original wound-size correction factor removed this bias. Summing the HPP E (67.3 mm) and T (89.0 mm) gave the ET (156.3 mm), which was 21.0 mm higher than the water budget ET estimate. We concluded that using HPP to estimate ET, E, and T was a promising approach, suitable for providing long-term, continuous, in situ, and high time-resolution estimations, even though certain issues still need to be resolved.