Name: exploring the effects of atmospheric forcings on evaporation: experimental integration of the atmospheric boundary layer and shallow subsurface
Uploaded: 2015-06-08T13:00:00
Description: Watch this Scientific Journal Video about Exploring the Effects of Atmospheric Forcings on Evaporation: Experimental Integration of the Atmospheric Boundary Layer and Shallow Subsurface at JoVE.com

This research was funded by the U. S. Army Research Office Award W911NF-04-1-0169, the Engineering Research and Development Center (ERDC) and National Science Foundation grant EAR-1029069. In addition, this research was supported by a Summer Programs in Undergraduate Research grant from Colorado School of Mines. The authors wish to thank Ryan Tolene and Paul Schulte for their contributions.

Note: Laboratory testing is performed using a two-dimensional bench scale tank interfaced with a climate controlled wind tunnel apparatus. Both the bench scale tank and wind tunnel are instrumented with various sensor technologies. The following protocol will first discuss the construction and preparation of the soil tank, followed by a discussion of the wind tunnel and the instrumentation of both. The tank dimensions, wind tunnel dimensions, number of sensors, and sensor technology type presented can be modified to suit...

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Hydrol. 131 (1-4), 63-104 (1992).</li><li>Lehmann, P., Assouline, S., Or, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characteristic+lengths+affecting+evaporative+drying+of+porous+media.">Characteristic lengths affecting evaporative drying of porous media.</a> Phys. Rev. E. 77 (5 Pt 2), 056309 (2008).</li><li>Trautz, A. C., Smits, K. M., Schulte, P., Illangasekare, T. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sensible+heat+balance+and+heat-pulse+method+applicability+to+in+situ+soil-water+evaporation.">Sensible heat balance and heat-pulse method applicability to in situ soil-water evaporation.</a> Vadose Zone J. 13, (2014).</li></ol>

The purpose of this protocol was to develop an experimental apparatus and associated procedures for the generation of high spatial and temporal resolution data required for studying land-atmospheric interactions with respect to heat and mass transfer processes. The experimental apparatus described consisted of a soil tank and a small wind tunnel, both of which were outfitted with an array of sensors for the measurement of pertinent soil and atmospheric variables (e.g., wind speed, relative humidity, soil and air...

Understanding the interaction between the land and atmosphere is paramount to our understanding of many current world problems such as leaking of geologically-sequestered carbon dioxide in soil, climate change, water and food supply, the accurate detection of landmines, and the remediation of ground water and soil. In addition, the primary exchanges of heat and water that drive global and regional meteorological conditions occur at the Earth&#8217;s surface. Many weather and climate phenomena (e.g., hurricanes, El Ni&#38;#241;o, droughts, etc.) are principally driven by processes associated with atmospheric-land surface interactions1. As more than half of the land surface on the Earth is arid or semiarid2-4, accurately describing the water cycle in these regions on the basis of heat and water exchanges between the atmospheric air and the soil surface is critical to improving our understanding of the aforementioned issues, particularly in regions vulnerable to extended drought and desertification. However, despite decades of research, there still remain many knowledge gaps in the current understanding of how the shallow subsurface and atmosphere interact5.
Transport processes involving liquid water, water vapor, and heat in soil are dynamic and strongly coupled with respect to interactions with the soil and enforced boundary conditions (i.e., temperature, relative humidity, thermal radiation). Numerical heat and mass transfer models commonly oversimplify or overlook a number of these complexities due in part to a lack of testing and refinement of existing theories resulting from a paucity of high temporal and spatial resolution data. Datasets developed for model validation are oftentimes lacking critical atmospheric or subsurface information to properly test the theories, resulting in numerical models that do not properly account for important processes or depend on the use of poorly understood parameters that are adjusted or fitted in the model. This approach is widely used due to its simplicity and ease of use and has in some applications shown much merit. However, this approach can be improved upon by better understanding the physics behind these &#8220;lumped parameterizations&#8221; by performing well controlled experiments under transient conditions that are capable of testing heat and water transfer theory6.
Careful experimentation in the laboratory allows precision datasets to be generated that can subsequently be used to validate numerical models. Data available from field sites are often incomplete and costly to obtain, and the degree of control needed to obtain a fundamental understanding of processes and to generate data for model validation could be considered inadequate in some cases. Laboratory experimentation of natural phenomena such as soil evaporation allows atmospheric conditions (i.e., temperature, relative humidity, wind speed) and soil conditions (i.e., soil type, porosity, packing configuration) to be carefully controlled. Many laboratory techniques used to study soil evaporation and soil thermal and hydraulic properties use destructive sampling7-10. Destructive sampling methods require that a soil sample be unpacked to obtain point data, preventing the measurement of transient behavior and disrupting soil physical properties; this approach introduces error and uncertainty to the data. Nondestructive measurements, like the method presented here, allow for more accurate determination and study of the interdependency of soil properties and processes11.
The goal of this work is to develop a soil tank apparatus and associated protocol for the generation of high spatial and temporal resolution data pertaining to the effects of changes in atmospheric and subsurface conditions on bare-soil evaporation. For this work, a small wind tunnel capable of maintaining a constant wind speed and temperature is interfaced with a soil tank apparatus. The wind tunnel and soil tank are instrumented with a suite of state of the art sensor technologies for autonomous and continuous data collection. Wind speed is measured using a stainless steel pitot-static tube attached to a pressure transducer. Temperature and relative humidity are monitored in the atmosphere using two types of sensors. Relative humidity and temperature are also monitored at the soil surface. Sensors in the subsurface measure soil moisture and temperature. Weight measurements of the tank apparatus are used to determine evaporation through a water mass balance. To demonstrate the applicability of this experimental apparatus and protocol, we present an example of bare-soil evaporation under varying wind speed conditions. The soil tank, packed homogeneously with a well characterized sand, was initially fully saturated and allowed to evaporate freely under carefully controlled atmospheric conditions (i.e. temperature, wind speed).

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>ECH2O EC-5 Soil Moisture Sensor (25)</td>
			<td>Decagon Devices Inc. Decagon.com</td>
			<td>40593</td>
			<td>For specifics visit: http://www.decagon.com/products/soils/volumetric-water-content-sensors/ec-5-soil-moisture-small-area-of-influence/. Sampling frequency on 10 minute intervals, accuracy is &#177;3%, and collect data using the Em50 dataloggers</td>
		</tr>
		<tr>
			<td>ECT Soil/Air Temperature Sensor (19)</td>
			<td>Decagon Devices Inc. Decagon.com</td>
			<td>40651</td>
			<td>For specifications visit http://www.decagon.com/products/canopy-atmosphere/temperature/ect-air-temperature/. Sampling frequency on 10 minute intervals, accuracy is &#177;0.5&#176;C, Measure within a temperature of 5 and 40&#176;C, and collect data using the Em50 dataloggers</td>
		</tr>
		<tr>
			<td>EHT Relative Humidity and Temperature Sensor (5)</td>
			<td>Decagon Devices Inc. Decagon.com</td>
			<td>N/A</td>
			<td>Sampling Frequency on 10 minute intervals, accuracy is &#177;3% between 5 and 100% relative humidity, and collect data using Em50 data loggers. For more information visit decagon.com</td>
		</tr>
		<tr>
			<td>Em50 Data Logger (10)</td>
			<td>Decagon Devices Inc. Decagon.com</td>
			<td>40800</td>
			<td>For specifics visit http://www.decagon.com/products/data-management/data-loggers/em50-digital-analog-data-logger/. ECH2O decagon devices, pulls data from the ECT, EC-5, and EHT sensors, and each data logger has 5 sensor connections and a com port that connects from the logger to USB to computer</td>
		</tr>
		<tr>
			<td>Sartorius Weighing Scale (1)</td>
			<td>Sartorius Corporation</td>
			<td>11209-95</td>
			<td>Sartorius Model 11209-95, Range = 65kg, Resolution = &#177;1g</td>
		</tr>
		<tr>
			<td>Infrared SalamandernCeramic Radiative Heater (1)</td>
			<td>Mor Electric Heating Assoc., Inc. http://www.morelectricheating.com/</td>
			<td>FTE 500-240</td>
			<td>5 heaters needed, adjust to ge thte right ambient/free-flow temperature</td>
		</tr>
		<tr>
			<td>2104 Temperature Control System (1)</td>
			<td>Chromalox</td>
			<td>2104</td>
			<td>Controls the heaters</td>
		</tr>
		<tr>
			<td>Infrared Temperature Sensor Regulator (1)</td>
			<td>Exergen Corporation</td>
			<td>N/A</td>
			<td>Monitors the heaters temperatures</td>
		</tr>
		<tr>
			<td>Stainless Steel Pitot-Static Tube (1)</td>
			<td>Dwyer Instruments, Inc. http://www.dwyer-inst.com/</td>
			<td>Series 160</td>
			<td>For specifics visit http://www.dwyer-inst.com/Product/%20TestEquipment/PitotTubes/Series160. Sensor sampling frequency is every 10 minutes, must be connected to differential pressure transducer and anemometer, and convert the pressure data collected into win velocities using Bernoulli&#39;s equation.</td>
		</tr>
		<tr>
			<td>1/2 inch Acrylic (1)</td>
			<td>Colorado Plastics http://www.coloradoplastics.com/</td>
			<td>N/A</td>
			<td>Specific heat of 1464 J kg^-1K^-1, thermal conductivity of 0.2 W m^-1K-1, and a density of 1150 kg m_-3</td>
		</tr>
		<tr>
			<td>Galvanized Steel Ducting Material (1)</td>
			<td>Home Depot</td>
			<td>N/A</td>
			<td>Material used to build wind- tunnel, and both round and rectangular ducting were used in construction and connected using square-to-round reducer duct</td>
		</tr>
		<tr>
			<td>Variable Speed Controller Connected to an In-Line Duct Fan (1)</td>
			<td>Suncourt, Inc. http://www.suncourt.com/</td>
			<td>VS200</td>
			<td>15.3 cm in Diameter Placed in-line with round duct</td>
		</tr>
		<tr>
			<td>Galvanized Steel Damper (1)</td>
			<td>Home Depot</td>
			<td>N/A</td>
			<td>Used to control/reduce speeds in the wind tunnel for low velocity data</td>
		</tr>
		<tr>
			<td>Accusand #30/40 (1)</td>
			<td>Unimin Corporation http://www.unimin.com/</td>
			<td>N/A</td>
			<td>This sand is silica sand and is 99.8% quartz, its grain shape is classified as rounded, the uniformity coefficient is approximately 1.2, and the grain density is 2.66 g/cm3.</td>
		</tr>
	</tbody>
</table>

exploring the effects of atmospheric forcings on evaporation: experimental integration of the atmospheric boundary layer and shallow subsurface

Evaporation is directly influenced by the interactions between the atmosphere, land surface and soil subsurface. This work aims to experimentally study evaporation under various surface boundary conditions to improve our current understanding and characterization of this multiphase phenomenon as well as to validate numerical heat and mass transfer theories that couple Navier-Stokes flow in the atmosphere and Darcian flow in the porous media. Experimental data were collected using a unique soil tank apparatus interfaced with a small climate controlled wind tunnel. The experimental apparatus was instrumented with a suite of state of the art sensor technologies for the continuous and autonomous collection of soil moisture, soil thermal properties, soil and air temperature, relative humidity, and wind speed. This experimental apparatus can be used to generate data under well controlled boundary conditions, allowing for better control and gathering of accurate data at scales of interest not feasible in the field. Induced airflow at several distinct wind speeds over the soil surface resulted in unique behavior of heat and mass transfer during the different evaporative stages.

The objective of the experiment presented here was to study the effect of wind speed on evaporation from bare soil. Key properties of the test soil used in the present study are summarized in Table 2. A series of experiments were performed in which different boundary conditions at the soil surface (i.e., wind speed and temperature) were applied (Table 3). Although four experiments at different wind speeds and temperatures were performed, the majority of experimental results pres...

Watch this Scientific Journal Video about Exploring the Effects of Atmospheric Forcings on Evaporation: Experimental Integration of the Atmospheric Boundary Layer and Shallow Subsurface at JoVE.com

Exploring the Effects of Atmospheric Forcings on Evaporation: Experimental Integration of the Atmospheric Boundary Layer and Shallow Subsurface

A protocol for the design and construction of a soil tank interfaced to a small climate controlled wind tunnel to study the effects of atmospheric forcings on evaporation is presented. Both the soil tank and wind tunnel are instrumented with sensor technologies for the continuous in situ measurement of environmental conditions.

Evaporation is directly influenced by the interactions between the atmosphere, land surface and soil subsurface. This work aims to experimentally study ...

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