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

The overall goal of this procedure is to develop a soil tank apparatus and associated protocol to experimentally study the effects of atmospheric forcings on evaporation. This is accomplished by first constructing a two dimensional bench scale soil tank. The second step is to construct a climate controlled wind tunnel apparatus that interfaces with the soil tank.

Next, the wind tunnel and soil tank are instrumented with various sensor technologies used to measure soil, moisture, temperature, relative humidity, and wind speed. The final step is to pack the soil tank with soil and water, determine the desired atmospheric forcings and start the experiment. Ultimately, 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.

The main advantage of this procedure over existing methods is that it allows for the generation of precision data sets that can be used to validate numerical models and study the interdependency of soil properties and processes. Data available from field sites are often incomplete and costly to obtain, and the degree of control needed to obtain and understanding of processes and to generate data for modeled validation could be considered inadequate. This procedure allows atmospheric conditions like temperature, relative humidity, wind speed, and soil conditions to be carefully controlled.

Though this method can provide insight into evaporation as demonstrated here, it can also be applied to other issues that require an understanding of land atmospheric interactions, such as the leaking of geologically sequestered carbon dioxide and soil climate change, food and water supplies, the accurate detection of land mines, and the remediation of groundwater and soil. Along with my graduate student, Andrew Trouts, Victoria Egan, an undergraduate student from my laboratory will be demonstrating the procedure To begin cut a large piece of 1.2 centimeter thick acrylic glass into five individual pains. Acrylic glass allows processes in the subsurface to be visually observed.

Next, draw a five by five grid. That is 25 centimeters by 25 centimeters on each of the two large glass panes. Ensure that each square within the grid has an area of 25 square centimeters.

The grid will be used to properly space. The sensors within the soil tank on one of the large glass panes drill a total of 25 1 0.9 centimeter diameter holes for the soil moisture sensors drill each hole in the center of every square in the grid so that the centers of the hole of two abutting squares are five centimeters apart. The first set of holes is 2.5 centimeters below the top of the tank.

Use appropriately sized taps to cut threads into each of the newly created holes. The five centimeter spacing between sensors ensures that each sensor is outside of the sampling volume of the next closest sensor. In the same manner, drill and tap a total of 25 0 0.635 centimeter diameter holes in the center of each grid box on the other glass pane on the acrylic pain used as the bottom of the tank.

Drill and tap a single half inch diameter hole in the middle of the pane. Glue a mesh screen over the hole on the internal side of the glass on the external side of the bottom plane. Install a 90 degree elbow that is attached to flexible tubing with an adjustable valve.

This valve and tubing is used to drain water from the tank at the termination of an experiment or as a way to install constant head devices for maintaining constant water. Table depths. Use marine grade glue or similar water resistant polymer adhesive to attach and seal the tank together.

Allow the adhesive to cure for one day to raise the tank off of the ground and make room for the 90 degree elbow. Attach two additional pieces of 1.2 centimeter thick acrylic glass with a length of 12 centimeters and a height of five centimeters to the bottom of the tank. Construct the 215 centimeter long upstream portion of the wind tunnel out of rectangular galvanized steel ducting material that has a width of 8.5 centimeters and a height of 26 centimeters.

Surround the outside of the duct with insulation. Next, install five ceramic infrared heating elements positioned in parallel within a reflector along the length of the upstream portion of the wind tunnel. Connect the infrared heating elements to a temperature control system regulated by an infrared temperature sensor.

Construct the midsection of the wind tunnel out of two 1.2 centimeter thick acrylic panels with a length of 25 centimeters and a height of 26 centimeters. Secure the acrylic panels to the top of the soil tank side walls using a strong adhesive tape ensuring that the wind tunnel and soil tank panels sit flush with one another. Drill two 0.635 centimeter diameter holes in one of the midsection panels to insert the temperature and or relative humidity.

Temperature sensors then construct the first 50 centimeters of the downstream portion of the wind tunnel out of the same size rectangular ducting material as before On the terminating side, reduce the rectangular ducting material to a 15.3 centimeter diameter round duct with a length of 170 centimeters. Install a galvanized steel damper used to adjust wind speeds at the far downstream end of the round duct for aid in wind speed control. Next, install an inline duct fan in the middle of the round duct oriented to expel air from the downstream portion of the wind tunnel.

Interface the fan with a variable speed controller for more precise control of rotational frequency, and as a result, wind speed prior to installation within the soil tank. Secure each soil moisture and temperature sensor within a threaded NPT housing and seal with flashing sealant to prevent moisture intrusion. Do not use silicone-based sealant products as they can interfere with the electronics within some sensors.

Cure the sensors for approximately one week. Wrap the threads of each NPPT housing with plumber's tape prior to installation in the tank to help provide a better seal between the NPT threading and acrylic glass. Then install a total of 25 soil moisture and temperature sensors each horizontally through the walls of the tank.

At the grid locations. Twist the sensor cables in sync with the NPT housing so as not to damage the internal wiring within the cables. Do not over torque the MPTs so as to prevent the glass from cracking once in place, connect the soil moisture sensors and temperature sensors to their designated data loggers.

Install a P toe static tube directly downstream of the soil tank through the hole drilled in the top of the downstream wind tunnel section. Hold the P toe static tube at the desired height from the floor of the section. Then connect the tube to a differential pressure transducer prior to packing the tank with soil.

Test its integrity by performing a leak test. Fill the tank with water and wait for four to six hours to ensure that no leaks in the structure or sensors have developed. After performing the leak, leak, test and draining the tank.

Obtain dry soil to pack the soil tank. Characterize the hydraulic and thermal properties of the selected soil separately in accordance with previously published methods. Carefully wet pack the soil tank using soil and deionized water to wet pack the soil tank.

First, pour approximately five centimeters of water into the tank using a scoop slowly add dry soil to the water in the tank. In 2.5 centimeter depth increments, record the weight of the sand added during each lift so the porosity of the soil packing can be calculated upon completion of each layer. Repeatedly tap the tank walls 100 to 200 times using a rubber mallet to obtain a uniform bulk density throughout.

While tapping, avoid contact with the sensors and sensor wires. The use of vibratory devices should be avoided, so as not to damage the network of sensitive sensors. Place the tank on a weighing scale to monitor cumulative water loss, which can in turn be used to calculate evaporation rate.

Once the setup is complete, determine the desired atmospheric conditions. Ensure that the data loggers and other data acquisition systems are turned on and set to the correct sampling intervals. Start the fan and temperature control system.

Allow the climate conditions to equilibrate before removing the plastic cover on the surface of the soil tank. Finally, run the experiment for the desired length of time. The combined wind tunnel and soil tank apparatus was used to perform a series of experiments in which different boundary conditions at the soil surface were applied.

Although four experiments at different wind speeds and temperatures were performed, the majority of experimental results presented here are for a wind speed of 1.22 meters per second. The relative humidity of the soil surface decreases over time, while the temperature of the soil surface shows an increasing trend over time before stabilizing. These trends were observed in all four experiments and can be explained in terms of the soil drying.

Surface temperature and wind are less influential on local temperatures at greater depths, showing no effect at depths below 12.5 centimeters. Saturation can be related to the different stages of evaporation, which are defined by differences in evaporation rates. Location of the drying front and dominant transport mechanisms.

Increasing wind speeds leads to an increased evaporation rate and shortened stage one evaporation duration. Increasing wind speed beyond three meters per second, however shows little additional impact on stage one evaporation. Stage two evaporation governed primarily by properties of the porous medium, appears to be independent of or only slightly influenced by wind speed.

This experimental protocol is applicable to a variety of environmental conditions to include changes in soil, climate, boundary, or subsurface conditions. The dimensions and sensor layout of the described apparatus can be modified to address the needs of different experiments. In addition, the packing procedure can similarly be modified to account for different packing configurations, such as varying porosity conditions and soil heterogeneity.