The overall goal of this procedure is to install a surface renewal system to measure and process field scale energy flux density data. This is accomplished by first installing the tower data logger enclosure and solar panel in the field. The second step is to fix the sensors to the tower.
Next, the sensor wires are connected to the data logger. The final step is to interface to the data logger with a laptop, upload the program to the data logger and check that the data logger is receiving signals from the sensors. Ultimately, surface renewal is used to measure the evapotranspiration of an ecosystem, providing information that is useful to growers for making irrigation decisions in a variety of agricultural ecosystems.
Although the method provides insight as far as agricultural and urban water use, it also has applications as far as natural ecosystems, for example, in forestry and and in wetlands. The implications of this technique extend towards irrigation management because evapotranspiration is the major cause of water loss from crop production systems. Generally, individuals that are new to this method will struggle because they do not know how to install the equipment or verify it's working properly.
We first had the idea for this method when observations of turbulence data led us to adapt the surface renewal concept from chemical engineering to atmospheric science. This procedure will be demonstrated by our team members, Rick Snyder and PA and postdocs, Arturo Calderone and Tom Chaplin. Begin this experiment by establishing the location of the field station and evaluating the wind direction as described in the text protocol.
Prepare as much of the equipment as possible in the laboratory prior to going to the field. First, attach the sensors to the data logger. Then attach the RS 2 32 cable between a laptop and the data logger port.
Open the interface program to communicate with the data logger and then connect to it. Then upload the turnkey data logger program to the data logger. Check data values for all sensors to see whether everything is working correctly.
Once an appropriate position is identified for the station in the study field, set up the tower, stabilize the feet of the tower using the provided stakes. Pound the long copper grounding rod into the soil with a mini sledgehammer in a location that will not interfere with the soil. Heat flux plates.
Connect the grounding rod to the tower using a thick gauge wire and the provided connectors. Attach the data logger enclosure to the main axis of the tower using the U-bolts provided by the manufacturer. Install the two soil heat flux plates with the white dot up at a length of five centimeters in the inter row space away from drip irrigation emitters.
Connect the heat flux plate wires to the data logger channels three and four as differential input sensors. Install the soil thermocouple to span the volume of soil above the ground heat flux plate to account for the change in heat storage above the plates. Connect the soil thermocouple to the data logger on channel five as a differential sensor.
Install the net radiometer on the cross arm. Boom above the canopy and pointing south. Connect the net radiometer to the data logger on channel two.
As a differential sensor. Install the fine wire thermocouple at one end of the cross arm and above the canopy. Air temperature will be sampled at 10 hertz thermocouples.
Used for these measurements are typically a 76 micron diameter. Connect the fine wire thermocouple to the data logger on channel one as a differential sensor where the purple wire goes to one H, the red wire to one L and the clear wire to the signal ground. Install the sonic anemometer at the other end of the cross arm using the new rail joint fitting to measure the three dimensional wind velocities and sonic temperature at 10 hertz.
For calculating the Eddy co variance sensible heat flux. Install the sonic anemometer so the center of the measurement area is located at the same height as the fine wire thermocouple. Next, attach the power supply to the data logger.
Connect a laptop to the data logger using the RS 2 32 cable port on the front of the data logger. Using the turnkey data logger program. Check data values for all sensors to ensure everything is working correctly.
Unscrew the new rail joint and lower the main vertical mast that holds the net radiometer. Bend the insulated thermocouple wire at its junction with the metal cylinder. Carefully dip the exposed thermocouple wires into a jar of lemon juice, making sure not to hit the exposed wires against the jar.
Clean off the lemon juice from the sensor by dipping the wires into a jar of deionized water. Re straighten the thermocouple wire at its junction with the metal. Spray the net radiometer with deionized water and dry it with absorbent wipes.
Raise the vertical mast up to its original position. Re-tighten the new rail set screws and level the net radiometer. An example temperature trace measured with a fine wire thermocouple is represented here.
This trace shows the need for mathematical analysis to extract the signal from this raw data. Visual inspection of this data reveals three or four primary ramps ending at approximately 12 35, 46 and 72 seconds. All of the energy balance components follow a similar diurnal pattern with peak values occurring in the middle of the day.
Net radiation is positive during the day as the surface receives more radiation than it loses and negative at night as the surface loses more radiation than it receives. Exhibiting the diurnal curve expected for sunny springtime days in Northern California. Soil heat flux density also follows a diurnal pattern.
It is positive during the day as energy is conducted from the surface into the ground and negative at night. As more energy is conducted from below to the cooler surface, the latent heat flux follows the expected diurnal curve over a crop with adequate water. During fair weather, net radiation is the dominant energy source for evapotranspiration.
So the latent heat flux density values track the changes in net radiation during the daytime over an actively transpiring crop. It is expected that the majority of the available energy during positive net radiation conditions is partitioned into latent heat flux density rather than sensible heat flux density and soil heat flux density. During windy nights, the latent heat flux was near zero, whereas it was negative during calm nights possibly indicating condensation.
Nevertheless, the daytime contribution to the latent heat flux greatly outweighs the nighttime contribution. So uncertainties in nighttime values are interesting, but relatively unimportant. The daily cumulative evapotranspiration values agree well for the surface renewal station and a weighing lysimeter situated side by side.
In a field of wheat weighing lysimeters are considered a gold standard for evapotranspiration estimates from crop surfaces. On most days, the cumulative evapotranspiration from the Surface renewal station was slightly higher than values from the Lysimeter.Tower. Measurements represent fluxes from a broader area than the lysimeter measurements, and at the time of these measurements, the wheat in the lysimeter was approximately 10 centimeters shorter and less dense than the plants and the remainder of the field.
So higher evapotranspiration values from the flux tower were expected While installing the equipment. It's important to check that the sensors are returning values to the logger before you leave the site Following this procedure. Other measurements like leaf water potential can be used in order to answer other questions like plant water status After its development.
This technique paved the way for researchers in the field of agriculture to explore field scale water use in a variety of crops.
