Evaporation from the soil largely determines both water availability in terrestrial ecosystems, and the partitioning of solar radiation between sensible and latent heat. It is key to both hydrology and climate. The evaporation process is complex, involving movement and phase change of water, varying with depth and time. Following water inputs, evaporation occurs at the soil surface, controlled by atmospheric demand. As surface soil water is depleted, evaporation becomes soil-limited and shifts below the surface; nonetheless it is generally viewed as a strictly surface process. As a result, measurement methods and understanding of these near-surface phenomena have lagged behind demand for accurate data. Much current research emphasizes large-scale areal estimates of soil moisture and temperature, but poor understanding of the soil water evaporation process causes low accuracy in water and energy balances. This poor understanding is largely due to our current inability to make the needed measurements. The purpose of the proposed research is to develop and test a new approach to measure evaporation within the soil. Recently developed sensors and concepts enable us to quantify sensible heat transferred into and out of mm-scale near-surface soil layers, as well as the change in sensible heat stored within each layer. Combined with conservation of energy, these measurements can locally quantify subsurface evaporation, showing the temporal patterns of in situ evaporation. Research will test four hypotheses: (1) that a sensible heat balance method can accurately estimate the mass of water evaporated from subsurface soil layers, (2) that the heat balance method can be extended to determine the latent heat flux from the soil surface layer (0-3 mm), (3) that through combined heat and mass balance, estimates of other hydrological components (transpiration and soil water flow) will be quantified or constrained, and (4) that the sensible heat balance method can quantitatively partition ET into evaporation and transpiration. Hypotheses 1-3 will be tested with both laboratory and field experiments, and Hypothesis 4 only by field experiments. Laboratory experiments will measure soil thermal properties, water content, and water flux under a combination of 2 energy regimes, 3 surface conditions, and 3 soils. Calculated evaporative loss via heat balance will be compared to evaporation measured by mass balance. In the field experiments, independent measurements of evaporation and transpiration will allow rigorous testing of heat balance estimates of transpiration and soil water evaporation. The intellectual merit of the proposed work is a new measurement-based methodology for quantifying soil water evaporation. The proposed research addresses current knowledge gaps by developing and testing in situ soil water evaporation measurement with novel sensors and analysis. Information obtained in the study will elucidate important evaporative processes. The research will quantify observation of soil water evaporation at and below the soil surface. This represents a notable advancement over descriptions of evaporation as a surface-only process. The proposed work carries broader impact by providing educational, scientific, and societal opportunities. Fundamental experience is provided for an early-career scientist, graduate students (including a minority student who is a NSF AGEP Fellow), and undergraduates. Results will be widely disseminated to the scientific community via website and published articles, and measurement techniques will have immediate repercussions for weather, climate, and environmental monitoring. Achieving the project goals will significantly improve our understanding of fundamental critical-zone properties and processes, enable better environmental monitoring and management, and enhance our predictions of large-scale hydrological and climate dynamics.

Project Report

This research project had the goals of developing and testing a method for measuring soil water evaporation. Soil water evaporation is dynamic in time and depth. The project included development of special technology to better measure soil water evaporation processes, which dictate terrestrial water availability and land-atmosphere energy exchange. We designed and constructed a novel soil sensor that allowed us to measure water and energy balances with unprecedented millimeter-scale detail in the surface soil layers. Sensor testing was performed in the laboratory and in the field. In both cases, the sensors agreed well with independent soil water evaporation estimates. Our new measurement method provided new details about evaporation processes that could not previously be measured. Laboratory measurements showed that the evaporation zone shifts downward from the soil surface, where it is maintained by liquid flow, to the soil subsurface, where it is maintained by diffusion, as the evaporation rate of drying soil begins to slow. Field experiments allowed examination of evaporation processes occurring in both bare soil surface and cropped field conditions. For bare surface conditions, we tracked the migration of the evaporation front from the surface to the subsurface. In cropped conditions, we were able to quantify and separate the portion of total evapotranspiration generated by direct soil evaporation from the portion of evapotranspiration coming from plant transpiration. Outcomes from this project are both a new technology for measurement and improved understanding of soil water evaporation processes. Results and measurement technologies will have application for improving agricultural water use efficiency, assessing water availability in arid and water-limited environments, and for developing improved drying technologies in manufacturing and engineering. They will also be utilized by the scientific community for development of enhanced hydrology and climate models. Research also offered important opportunities for training new scientists and for engaging with scientific communities. Five graduate students were trained through research performed as part of the project, including three female and/or minority Ph.D. students. Results were disseminated at national and international conferences in the U.S., as well as in China, Israel and Brazil. Several scientific journal papers, based on project findings, were published.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Thomas Torgersen
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Iowa State University
United States
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