CoAgMET Crop ET Coefficients

We compute crop ET with the following algorithm. For each day of the season:

1. Start with stage = 1 and etr_sum = 0.
2. Add the day's ETr to etr_sum.
3. If etr_sum is greater than the stage's threshold, go to the next stage.
4. Use the stage's coefficients to calculate the crop coefficient k:
   1. If the stage is 1, k is stage 2's K min
   2. Calculate frac_sum = etr_sum / threshold * 100
   3. Otherwise, k = frac_sum * C3³ + frac_sum * etr_sum * C2² + frac_sum * C1 + C0
   4. If k exceeds K min or K max, clamp it to one of those values.
5. ET = k * ETr

The following are the coefficients, thresholds, etc. we use with the above algorithm. The crop coefficients used to generate crop ET reports were developed for the Penman-Kimberly model.

Alfalfa

0.33748, -0.0024345, 0.00026031, -0.0000016861
1.0, 0.0, 0.0, 0.0
0.35, 1.00, 1.0, 1.0
4, 24
0.0, 6.0, 6.0

Corn

0.18684, 0.0023555, 0.00016093, -0.0000010779
0.98014, 0.0043027, -0.00013037, 0.00000011096
0.2, 0.96, 0.22, 0.96
4, 20
2.4, 18.8, 17.7

Drybeans

0.19715, 0.00088840, 0.00017947, -0.0000010136
1.0503, -0.00010989, -0.0000037637, 0.000000024275
0.20, 1.00, 0.68, 1.00
5, 31
1.0, 12.9, 11.0

GrassHay

0.28138, 0.021053, -0.000065, 0.0
0.87, 0.0, 0.0, 0.0
0.28, 0.87, 0.87, 0.87
3, 15
0, 2, 43.3

Smallgrn

0.13877, 0.0058077, 0.00018732, -0.0000015644
1.0247, 0.0038453, -0.00017294, 0.00000054111
0.20, 1.00, 0.22, 1.00
3, 23
1.0, 10.9, 13.6

Sgrbeets

0.19291, 0.0018086, 0.000080571, -0.00000015235
1.0038, 0.0016168, -0.000046807, -0.000000034258
0.20, 1.00, 0.66, 1.00
4, 8
1.4, 22.7, 21.0

Potatoes

0.19731, 0.00019446, 0.00017014, -0.0000010004
0.9000, 0.0000, 0.0000, 0.0000
0.20, 0.90, 0.90, 0.90
6, 3
1.3, 16.7, 8.1

Onion

0.39042, -0.00064841, 0.00015373, -0.0000010567
0.88965, 0.0011674, 0.0000092657, -0.00000064297
0.40, 0.80, 0.45, 0.80
3, 22
1.1, 27.0, 10.1

WntrWheat

0.1276, 0.0161, -0.0000228, -0.000000479
1.0263, 0.00373, -0.000182, 0.000000647
0.20, 1.0, 0.22, 1.0
3, 1
0.0, 10.5, 13.0