Improvement Of Sensible Heat Balance Theory For Determining Soil Water Evaporation And Partitioning Field Evapotranspiration

The individual components of evapotranspiration include soil water evaporation and plant transpiration. Research on the proportion of evaporation and transpiration in evapotranspiration is important in order to increase water use efficiency. However, in the present study of evapotranspiration partitioning, few methods can monitor soil water evaporation in situ and continuously. Based on the sensible heat balance theory, three-needle heat-pulse sensors could be used to measure soil water evaporation, while, the positional deviation of three-needle heat-pulse sensors would cause great error and the evaporation above4-mm depth could not be determined. In view of the above problems, the main objective of this study was:improving the design of three-needle heat-pulse sensor and reducing the error of field soil thermal property measured by heat-pulse technique in order to increase the measurement accuracy of soil water evaporation by sensible heat balace theory and further applied to the evapotranspiration partitioning research.The first part described design of the improved multi-needle heat-pulse sensor and showed its field performance. The multi-needle heat-pulse sensor was constructed by seven temperature needles and four heater needles, equivalent of four three-needle heat-pulse sensors and with two more temperature needles at1and2mm depths. The multi-needle heat-pulse sensor can capture soil temperature dynamic, even at the top1and2mm depths. It can measure the soil thermal property and further determine evaporation rate based on the sensible heat balance theory. Compared with the three-needle heat-pulse sensor, the multi-needle heat-pulse sensor could decrease the errors caused by sensor installation and the undetectable zone was reduced from0-4mm layer to0-1.5mm layer.The second part improved the surface soil thermal property calculations during the heat-pulse measurements. When the heat-pulse technique was used to measure surface soil thermal property in the field, the results would be influenced by the diurnal changes of soil ambient temperature. The heat-pulse thermal properties would be underestimated (overestimated) when the ambient temperature increased (decreased). At11:00when the ambient temperature was increasing on Day258in the year of2012, the volumetric heat capacity (C) and thermal conductivity (λ) in the6-12mm soil layer were underestimatied by8%and13%, respectively. Following Jury and Bellantuoni (1976), the change of ambient temperature during heat-pulse measurements could be extrapolated linearly from the ambient temperature dynamic befor heat-pulse measurement, and used to calibrate the influence of ambient temperature changes. But thermal property couldn’t be calibrated at some daytime (e.g.11:00to15:00on Day258). Compared to thermal properties estimated by the de Vries model, the average deviation of heat-pulse thermal properties below6mm depth were small after calibrating the ambient temperature effect. The average deviation were within0.30MJ m-3℃-1for C and within0.22W m-1℃-1for λ, While, the thermal property errors were larger above6mm depth, because it was also influenced by the soil-air interface. The soil-air interface effect could be calibrated by using the Pulse Infinite Line Source-Adiabatic Boundry Condition (PILS-ABC) modle to calculate thermal properties instead of the Pulse Infinite Line Source (PILS) model. After calibrating by the PILS-ABC model, the average deviation was reduced from0.46to0.32MJ m-3℃-1for C and from0.54to0.12W m-1℃-1for μ in the0-6mm soil layer.The third part examined the field evapotranspiration partitioning using the sensible heat balance theory for soil water evaporation. From Aug.1th to Sep.25th in the year of2012, the cumulative evaporation at interrow location was10mm larger than the cumulative evaporation at within row location. Therfore, the spatial variability of evaporation in the corn field could be considered. Compared with the daily evapotranspiration by lysimeter, the daily evapotranspiration determined by heat-pulse technique and sap flow stem heat balance was obvious larger and most values were between1to1.5times of the lysimeter evapotranspiration.The sensible heat balace theory can determine soil water evaporation rate and location without known the soil water and heat transport parameters and hydraulic properties. It has potential to improve the understanding of evaporation and vapor movement in the shallow subsurface soil and increase the field water use efficiency.