Reports

In the summer of 2019 from June 20th to August 21st multiple measurements of canopy height and phenology were made during the growing season of corn and the Boulding Corn site.  These measurements included:

Vegetation Height

Sonic range finder: DF Robot mounted on the radiation boom at 4m above the ground.  This data was measured every 10sec and stored as 30min averages.  The 30min averages were still very noisy (see Fig 1) so were aggregated into daily average and had a five day running mean applied to smooth of the signal.  This sensor only sees a small patch of corn near the tower, however this corn field is amazingly homogeneous.

Turbulent zveg: A veg height calculated from the eddy turbulence data.  This calculation depends on strong turbulence so even though it can be calculated for each 30min it is usually very noisy.  To clean it up it was filter for times when u-star was greater than 0.3 and then averaged daily and had a five day running mean applied similar to the sonic range finder data.  This measurement's footprint is the same as the flux footprint and usually at the field scale.

Manual VegHT: A manual measurement using a tape measure done approximately every two weeks during field visits.  For this period the veg height is the average of 5 to 10 individual measurements.

Phenology

GCC: Green Chromatic Coordinate calculated from images from a Stardot camera.  The 90th percentile value was calculated from usually 14 images during full light taken at 30min intervals.  Then a five day running mean was applied to further smooth out the signal.

NDVI: NDIV was measured using Decagon SRS sensors.  This data was measured every 10 seconds and saved as 30min averages.  Then only five or six readings centered on noon were averaged for a daily value.

Gap Fraction, LAI:

 

Figure 1. Smoothing out the noisy sonic range data

Fig 2 compares the different veg height and phenology methods.  The turbulent zveg matches the manual veg height measurements amazingly well.  The sonic range seems to be struggling in the early season when the corn is short but seems better in the later season.  NDVI and GCC roughly correlate with the veg height though GCC seems to be more variable than NDVI.  The sonic range, turbulent zveg and GCC all seem to pick up a slow down or dip in early July that isn't apparent in the NDVI or manual veg height.

Figure 2. Veg height and phenology measurements.