I just ran the 3 ECHO probes used during SCP through a beaker of sand with various amounts of moisture (manually mixed in). The results are:
Variable | Qsoil.g | Qsoil.c | Qsoil.c2 |
---|---|---|---|
S/N | ECHO 001 | ECHO 002 | ECHO 012 |
ID | 28 | 29 | 2A |
0ml/2700ml = 0% | -3.14 | -2.95 | -2.33 |
200/2800 = 7.1% | 3.71 | 3.91 | 4.17 |
400/2900 = 13.8% | 15.87 | 16.00 | 16.30 |
600/2900 = 20.7% | 21.05 | 22.21 | 24.02 |
800/2800 = 28.6% | 25.33 | 25.78 | 29.12 |
This suggests that there are small gain/bias errors that have probe 001 read lower that probe 002 which is lower than 012 -- in other words, an order of 001/002/012
However, the one set of gravimetric samples showed that probe 001 was within 1% of its gravimetric sample, whereas probe 002 was 2% high and probe 012 was 3% low. This would imply that the order was 012/001/002.
Furthermore, the data consistently show an order 012/002/001, though 001 being larger undoubtedly is real.
I am forced to conclude that 002 reading higher than 012 in the field is not a calibration difference. Thus, this difference must have been due to spatial inhomogeneity in the soil.
For my records, here is code to plot this:
ref = c(0,7.1,13.8,20.7,28.6)
e01 = c(-3.14,3.71,15.87,21.05,25.33)
e02 = c(-2.95,3.91,16.00,22.21,25.78)
e12 = c(-2.33,4.17,16.3,24.02,29.12)
matplot(ref,cbind(e01,e02,e12),xlim=c(-5,30),ylim=c(-5,30))
abline(h=0,v=0)
abline(0,1)