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Table 7 Quantitative relationship between spectral indices and NO3–N content

From: Estimation of nitrate nitrogen content in cotton petioles under drip irrigation based on wavelet neural network approach using spectral indices

Type of reflectivity Vegetation indices Functional model Regression equation R2 RMSE (g/L)
First derivative RD Linear y = − 8116.9x + 13,150 0.71 1.77
Exponential y = 17,860e−1.306x 0.73 2.01
Quadratic y = 3178.2x2 − 13,332x + 15,120 0.72 1.88
CIred-edge Linear y = − 76,288x − 66,089 0.80 0.92
Exponential y = 0.067e−12.01x 0.78 0.98
Quadratic y = − 169,389x2 − 400,646x − 221,261 0.80 0.93
NDRE Linear y = − 27,424x − 18,368 0.63 1.14
Exponential y = 147.75e−4.115x 0.56 1.23
Quadratic y = 126,872x2 + 207,536x + 89,944 0.67 1.18
ND705 Linear y = − 18,527x + 3265 0.82 1.52
Exponential y = 3707.4e−2.893x 0.78 1.35
Quadratic y = − 29,935x2 − 30,687x + 2342.8 0.83 1.69
NIR Linear y = − 85,125x + 10,408 0.39 1.45
Exponential y = 11,137e−12.9x 0.35 1.55
Quadratic y = − 2E + 06x2 + 47,910x + 8644.1 0.44 1.44
RI-1 dB Linear y = − 19,373x + 22,583 0.52 2.37
Exponential y = 78,853e−3.076x 0.52 2.91
Quadratic y = 126,872x2 + 207,536x + 89,944 0.59 2.83
Original RD Linear y = − 8564.7x + 25,826 0.65 1.50
Exponential y = 131,902e−1.36x 0.65 1.57
Quadratic y = − 10,788x2 + 39,287x − 26,804 0.67 1.47
CIred-edge Linear y = − 3415.6x + 14,222 0.54 1.69
Exponential y = 19,774e−0.516x 0.49 1.81
Quadratic y = − 357.75x2 − 1914.2x + 12,720 0.54 1.67
NDRE Linear y = − 28,351x + 17,684 0.59 1.70
Exponential y = 34,790e−4.394x 0.56 1.81
Quadratic y = − 202,849x2 + 124,311x − 10,381 0.63 1.60
ND705 Linear y = − 2745.6x + 16,233 0.58 2.38
Exponential y = 29,232e−0.441x 0.59 2.41
Quadratic y = − 35.761x2 − 2493x + 15,800 0.58 2.34
NIR Linear y = − 28,744x + 38,765 0.62 1.66
Exponential y = 946,939e−4.488x 0.60 1.81
Quadratic y = − 180,524x2 + 369,826x − 180,634 0.66 1.53
RI-1 dB Linear y = − 10,960x + 28,560 0.59 1.58
Exponential y = 199,238e−1.729x 0.58 1.67
Quadratic y = − 26,273x2 + 92,468x − 72,663 0.64 1.52
  1. E stands for scientific counting; e is the base of natural logarithm