This title appears in the Scientific Report : 2014 
Monitoring of Soil Evaporation and Drying at Different Scales
Vanderborght, Jan (Corresponding Author)
Shahraeeni, Ebrahim / Pohlmeier, Andreas / Merz, Steffen / Jonard, Francois / Graf, Alexander / Rothfuss, Youri / Brüggemann, Nicolas / Vereecken, Harry
Agrosphäre; IBG-3
2014
2014
6th International Conference on Porous Media & Annual Meeting, Milwaukee, Wisconsin (USA), 2014-05-27 - 2014-05-30
Abstract
Terrestrial Systems: From Observation to Prediction
Modelling and Monitoring Terrestrial Systems: Methods and Technologies


Evaporation from bare soil surfaces is an important component of the water cycle over land surfaces. 40% of the global land surface is used for agriculture [1], much of which is bare during part of the year due to tillage practices. An accurate description of liquid water, vapor, and heat fluxes in the soil and across the soil surface into the atmosphere is therefore critical for the management of water resources and for the prediction of coupled atmospheric processes determining weather and climate. In this contribution, we present different experimental methods that can be used to investigate soil evaporation and drying. At the smallest scale, we used magnetic resonance imaging (MRI) to visualize soil water content distributions in small soil cores [2]. The development of a vaporization front close to the soil surface can be imaged and linked to a decrease in evaporation or drying rate over time. Drawbacks of conventional magnetic resonance scanners are that they are expensive and immobile. Therefore, we tested the potential of portable MRI scanners that use single sided open U-shaped permanent magnets [3] to measure vaporization fronts in the field. Evaporation leads to an enrichment in heavier water stable isotopes. Soil water isotope profiles can therefore also provide information about evaporation rates and the depth of vaporization fronts. Methods to monitor soil water isotope profiles using gas permeable tubing [4] are currently tested in the lab by comparing them with MRI measurements which are sensitive to the total water content. Another method that can be used to derive the moisture content of the soil surface layer is off-ground ground penetrating radar. Using electromagnetic waves with wavelengths that are smaller than the scales over which soil moisture changes close to the soil surface, the depth and shape of vaporization fronts can be resolved [5]. To monitor evaporation fluxes at the field plot scale, eddy covariance (EC) methods can be used. When combined with footprint models and soil water flow models, spatial variability of the evaporation fluxes within a field plot which was attributed to variations in soil texture and the depth of the groundwater table can be derived [6]. By comparing the diurnal dynamics of the EC measured and simulated evaporation fluxes, the relevance of processes like molecular and turbulent diffusion of vapor in the surface soil layer and the enhancement of vapor diffusion due to thermal gradients could be investigated under natural outdoor conditions.