J. A. Kirchner, D. S. Kimes, and J. E. McMurtrey, "Variation of directional reflectance factors with structural changes of a developing alfalfa canopy," Appl. Opt. 21, 3766-3774 (1982)
Directional reflectance factors of an alfalfa canopy were determined and related to canopy structure, agronomic variables, and irradiance conditions at four periods during a cutting cycle. Nadir and off-nadir reflectance factors decreased with increasing biomass in Thematic Mapper band 3 (0.63–0.69 μm) and increased with increasing biomass in band 4 (0.76–0.90 μm). The sensor view angle had less impact on perceived reflectance as the alfalfa progressed from an erectophile canopy of stems after harvest to a near planophile canopy of leaves at maturity. Studies of directional reflectance are needed for testing and upgrading vegetation canopy models and to aid in the complex interpretation problems presented by aircraft scanners and pointable satellites where illumination and viewing geometries may vary widely. Distinct changes in the patterns of radiance observed by a sensor as structural and biomass changes occur are keys to monitoring the growth and condition of crops.
You do not have subscription access to this journal. Cited by links are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
You do not have subscription access to this journal. Figure files are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
You do not have subscription access to this journal. Article tables are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
Date, Mean Solar Zenith Angle, and Mean of the Corrected, Spectral, Direct/Global Ratios of the 24 Measurement Periods
Date
Solar zenith angle (in parentheses) and the red and IR direct/global ratios, respectively
7/2/81
(17)
(31)
(39)
(48)
0.76
0.78
0.83
0.81
0.80
0.83
0.85
0.83
7/9/81
(17)
(21)
(30)
(42)
(54)
0.83
0.82
0.78
0.76
0.74
0.86
0.85
0.82
0.81
0.79
7/15/81
(17)
(20)
(34)
(48)
(57)
0.84
0.82
0.85
0.77
0.81
0.86
0.84
0.85
0.85
0.83
7/24/81
(34)
(58)
(65)
(69)
0.86
0.79
0.76
0.53
0.86
0.78
0.73
0.74
7/31/81
(21)
(34)
(47)
(57)
(65)
(71)
0.50
0.66
0.67
0.66
0.64
0.59
0.55
0.69
0.69
0.68
0.66
0.62
Table II
Agronomic Characteristics of Alfalfa. Bounds on the Error of Estimation (Two Times the Standard Error) are Followed by the Sample Size in Parentheses
Date
Description
Height (cm)
Percent cover
Wet biomass g per meter square
Dry biomass g per meter square
Leaf area index (one-sided)
Visual percent chlorosis
7/2/81
One day after harvest, green stems only, essentially no leaves
10.3 ± 0.7 (25)
24 ± 5 (300 pts)
153 (1)
67 (1)
—
0
7/9/81
New leaves beginning to grow, 5% weed cover
10.7 ± 1.5 (25)
42 ± 6 (300 pts)
261 (1)
83 (1)
—
0
7/15/81
5% weed cover
20.1 ± 1.1 (60)
80 ± 5 (300 pts)
659 ± 192 (3)
143 ± 50 (3)
0.91 ± 0.19 (3)
5
7/24/81
7% weed cover
41.5 ± 1.1 (70)
88 ± 4 (300 pts)
975 ± 100 (3)
208 ± 14 (3)
1.88 ± 0.37 (3)
5
7/31/81
10% weed cover
51.4 ± 1.3 (70)
92 ± 3 (300 pts)
1483 ± 87 (3)
351 ± 3 (3)
3.00 ± 0.58 (3)
7
Table III
Nadir Reflectance Factors a of an Alfalfa Canopy and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir in TM3 and TM4 bands on Four Dates During a Cutting Cycle
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum and change from nadir for reflectance factors at off-nadir view angles of
15–30°(%)
15–45°(%)
TM3 (0.63–0.69 μm)
7/9/80
42
30
0.084
−29
+29
7/15/80
80
34
0.038
+47
+47
7/24/80
88
34
0.028
+21
+46
7/31/80
92
34
0.026
+23
+50
TM4 (0.76–0.90 μm)
7/9/80
42
30
0.236
+28
+39
7/15/80
80
34
0.373
+15
+30
7/24/80
88
34
0.427
+15
+30
7/31/80
92
34
0.452
+11
+31
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.
Table IV
Nadir Reflectance Factors of an Alfalfa Canopy in Band TM3 and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir Value as a Function of Solar Zenith Angle on Two Dates
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum % change from nadir for reflectance factors at off-nadir view angles of
15–30°(%)
15–45°(%)
7/9/80
42
17
0.095
−24
−32
21
0.091
+25
−32
30
0.084
−29
−29
42
0.073
+27
+53
54
0.059
+56
+86
7/31/80
92
21
0.028
+21
+29
34
0.026
+23
+50
47
0.027
+26
+59
57
0.029
+24
+62
65
0.036
+19
+42
71
0.050
+18
+50
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.
Table V
Nadir Reflectance Factors a of an Alfalfa Canopy in Band TM4 and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir Value as a Function of Solar Zenith Angle on Two Dates
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum percent change from nadir for reflectance factors at off-nadir view angles of:
15–30°(%)
15–45°(%)
7/9/80
42
17
0.264
+9
+20
21
0.236
+36
+36
30
0.236
+28
+39
42
0.227
+25
+54
54
0.202
+48
+70
7/31/80
92
21
0.471
−11
+15
34
0.452
+11
+31
47
0.438
+20
+39
57
0.434
+16
+40
65
0.432
+15
+35
71
0.468
+17
+37
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.
Tables (5)
Table I
Date, Mean Solar Zenith Angle, and Mean of the Corrected, Spectral, Direct/Global Ratios of the 24 Measurement Periods
Date
Solar zenith angle (in parentheses) and the red and IR direct/global ratios, respectively
7/2/81
(17)
(31)
(39)
(48)
0.76
0.78
0.83
0.81
0.80
0.83
0.85
0.83
7/9/81
(17)
(21)
(30)
(42)
(54)
0.83
0.82
0.78
0.76
0.74
0.86
0.85
0.82
0.81
0.79
7/15/81
(17)
(20)
(34)
(48)
(57)
0.84
0.82
0.85
0.77
0.81
0.86
0.84
0.85
0.85
0.83
7/24/81
(34)
(58)
(65)
(69)
0.86
0.79
0.76
0.53
0.86
0.78
0.73
0.74
7/31/81
(21)
(34)
(47)
(57)
(65)
(71)
0.50
0.66
0.67
0.66
0.64
0.59
0.55
0.69
0.69
0.68
0.66
0.62
Table II
Agronomic Characteristics of Alfalfa. Bounds on the Error of Estimation (Two Times the Standard Error) are Followed by the Sample Size in Parentheses
Date
Description
Height (cm)
Percent cover
Wet biomass g per meter square
Dry biomass g per meter square
Leaf area index (one-sided)
Visual percent chlorosis
7/2/81
One day after harvest, green stems only, essentially no leaves
10.3 ± 0.7 (25)
24 ± 5 (300 pts)
153 (1)
67 (1)
—
0
7/9/81
New leaves beginning to grow, 5% weed cover
10.7 ± 1.5 (25)
42 ± 6 (300 pts)
261 (1)
83 (1)
—
0
7/15/81
5% weed cover
20.1 ± 1.1 (60)
80 ± 5 (300 pts)
659 ± 192 (3)
143 ± 50 (3)
0.91 ± 0.19 (3)
5
7/24/81
7% weed cover
41.5 ± 1.1 (70)
88 ± 4 (300 pts)
975 ± 100 (3)
208 ± 14 (3)
1.88 ± 0.37 (3)
5
7/31/81
10% weed cover
51.4 ± 1.3 (70)
92 ± 3 (300 pts)
1483 ± 87 (3)
351 ± 3 (3)
3.00 ± 0.58 (3)
7
Table III
Nadir Reflectance Factors a of an Alfalfa Canopy and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir in TM3 and TM4 bands on Four Dates During a Cutting Cycle
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum and change from nadir for reflectance factors at off-nadir view angles of
15–30°(%)
15–45°(%)
TM3 (0.63–0.69 μm)
7/9/80
42
30
0.084
−29
+29
7/15/80
80
34
0.038
+47
+47
7/24/80
88
34
0.028
+21
+46
7/31/80
92
34
0.026
+23
+50
TM4 (0.76–0.90 μm)
7/9/80
42
30
0.236
+28
+39
7/15/80
80
34
0.373
+15
+30
7/24/80
88
34
0.427
+15
+30
7/31/80
92
34
0.452
+11
+31
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.
Table IV
Nadir Reflectance Factors of an Alfalfa Canopy in Band TM3 and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir Value as a Function of Solar Zenith Angle on Two Dates
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum % change from nadir for reflectance factors at off-nadir view angles of
15–30°(%)
15–45°(%)
7/9/80
42
17
0.095
−24
−32
21
0.091
+25
−32
30
0.084
−29
−29
42
0.073
+27
+53
54
0.059
+56
+86
7/31/80
92
21
0.028
+21
+29
34
0.026
+23
+50
47
0.027
+26
+59
57
0.029
+24
+62
65
0.036
+19
+42
71
0.050
+18
+50
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.
Table V
Nadir Reflectance Factors a of an Alfalfa Canopy in Band TM4 and Maximum Percent Deviation of Off-Nadir Reflectance Factors from the Nadir Value as a Function of Solar Zenith Angle on Two Dates
Date
Percent cover (%)
Solar zenith angle (deg)
Nadir reflectance factor
Maximum percent change from nadir for reflectance factors at off-nadir view angles of:
15–30°(%)
15–45°(%)
7/9/80
42
17
0.264
+9
+20
21
0.236
+36
+36
30
0.236
+28
+39
42
0.227
+25
+54
54
0.202
+48
+70
7/31/80
92
21
0.471
−11
+15
34
0.452
+11
+31
47
0.438
+20
+39
57
0.434
+16
+40
65
0.432
+15
+35
71
0.468
+17
+37
Reflectance factors are flagged as less than (−) or greater than (+) the nadir reflectance factor.