Measuring Evapotranspiration Of Wood Stand With Heat-pulse Sensors

Quantifying evapotranspiration(ET) and partitioning it into its individual components, evaporation(E) and transpiration(T), is essential to the understanding of eco-hydrological systems and to guide agricultural productive practices. However, E and T are rarely measured separately. Our objective was to determine the ET of a small tree stand by measuring E and T with heat-pulse-probes(HPP) and applying the sensible heat balance(SHB) and heat ratio based sap flow methods(HRM), respectively, and to investigate sap flow discrepancy mechanism of Chinese willow(Salix matsudana) under two different soil conditions, in Loess Plateau, China.Experiments were conducted on two different sites, a sandy soil field and a loess soil stand, in Loess Plateau, during July to October in 2014. Three three-needle heatpulse probes were installed in three sample Chinese willow trees in sandy soil stand, and a combination of heat-ratio method(HRM), T-max method(T-max) and single heat-pulse probe method(SHPP) was used to calculate sap flux density(Vs), and Vs comparable measurement was conducted with thermal diffusion probes(TDP), afterwards different heat-pulse based sap flow methods evaluation was carried out. Simultaneously, two HPP-arrangements, using three and five HPP-sets, were used to measure E from subsurface soil layers with sensible heat balance method(SHB), and E comparable measurement was done with micro-lysimeters(MLS). Transpiration was determined by upscaling HPP-Vs to stand level, and ET was then determined by summing its two individual components, E and T, and ET comparable measurements were done using water budget method. Experiments were also performed in sandy soil stand and loess soil stand to obtain soil profile(depth 0-6 m) volumetric water content(VWC) with neutron probes, VWC dynamic with EC-5 sensors(depth 0-2 m), E with MLSs and meteorological data, in addition with leaf water potential, root embolism degree, and TDP-Vs measurements, to investigate Chinese willow growth and sap flow discrepancy of two different soil conditions. Results were as follows:It took five week to reach a stable measurement statue after HPP installation, Vs measured with three methods in early stage after installation is 135%~220% higher than Vs in stable measurement stage. HRM-Vs and SHPP-Vs were high significant linearly correlation(P<0.001) with TDP-Vs, with R2 of 0.93 and 0.91, respectively. T-max-Vs was significant linearly correlation with TDP-Vs, with R2 of 0.73, lower than other two methods. Characteristics and applicability were distinctly unlike for different methodologies, and proper methods and a combination of them should be selected to sap flow measurements based on different study purposes and characteristics.Daily evaporation determined by HPP-set with five HPPs were significant linearly correlation(P<0.001) with MLS-E, with R2 of 0.53. During Aug. 18 to Oct. 21 in 2014, Chinese willow sandy soil stand total HPP-ET was 157 mm, within which HPP-E was 157.4 mm, and HPP-T was 75.1 mm determined by upscaling from HPP-Vs, and HPPET was slightly higher than SHB-ET(135±23.5 mm).In the study sits, Chinese willow sap flow in loess soil stand was 2.6 times the amount of that in sandy soil stand, and daily sap flow amount in two sits were significant linear correlation(P<0.01) with reference crop ET0, with R2 of 0.58 and 0.52, respectively. Loess soil had a high field moisture capacity and poor infiltration capability, while sandy soil had a low field moisture capacity and good infiltration capability. The sandy soil stand moisture capacity was only half of loess soil stand, and sandy soil stand MLS-E was 1.5 times the amount of loess soil stand, and in most cases loess soil infiltration depth wouldn’t exceed 20 cm, while sandy soil could exceed more than 1 m. Sandy soil evaporation amount was 1.5 times as that of loess soil stand. Sandy soil stand Chinese willow leaf water potential was remarkably lower than loess soil stand, and root embolism degree was about twice as large of loess soil stand, indicating sandy soil stand Chinese willow suffered from heavy soil water stress. To sum up, soil water condition differences caused by soil texture, in addition to willow physiological morphology differences adapted for local soil water conditions, was the key to Chinese willow growth status and sap flow discrepancy under two different soil conditions in Loess Plateau, China.