A close look at a few millimeters of soil: Determining evaporation from soil temperature and thermal property measurements

Soil water evaporation intimately couples the hydrological cycle with the land-surface energy balance, making it an important driver for a wide range of terrestrial processes. To date, few methods are available to characterize soil water evaporation in detail, which has, in turn, limited capability to predict soil water evaporation processes in local and large scale models and contributes to uncertainty in surface energy balance closure. Recent work has developed a measurement-based soil sensible heat balance (SHB) approach, capable of quantifying evaporation time and depth dynamics at a vertical scale of millimeters, near the soil surface. The SHB approach does not require detailed characterization or estimation of soil hydraulic properties, which has been a limitation in previous work. Rather it utilizes detailed measurements of the soil temperature profile and soil thermal properties from heat-pulse sensors in order to derive evaporation front dynamics. Laboratory experiments indicate that the SHB approach is precise to within <10% of independent mass balance measurements. Field tests indicate that the SHB is well correlated to lysimeter and Bowen ratio measurement approaches for bare surface conditions with a near 1:1 relationship. Numerical modeling and laboratory experiments have also shown that the SHB approach can provide in situ sub-surface evaporation estimates at relatively short time steps (e.g., sub-daily). Most recent advances in heat-pulse sensor design and variations on the simple soil SHB calculation have extended the measurement zone to capture sub-surface soil water evaporation from its inception and extended measurements for application in evapotranspiration partitioning. In this presentation, we describe the principle of measurement, discuss accuracy and limitations for implementation, and provide example applications.