Simple Analysis of the Two-way Attenuation by Crop Canopy

In this chapter, we treat the total backscattering as the sum of the surface scattering caused by the bare soil and the volume scattering caused by the crop canopy or the crop residues without considering the attenuation effect. This is primarily because the attenuation is relatively weak at the early growing stage. A simple analysis of the two- way attenuation caused by the vegetation and corn residues is performed in this section. The Michigan Microwave Canopy Scattering Model (MIMICS) developed by Ulaby in 1990 (Ulaby et al., 1990) is suitable for vegetation covered areas where the agents responsible for scattering have discrete configurations (Toure et al., 1994). They include wheat, corn residue, soybean and corn; and many studies have adapted this model to describe the scattering of crops such as wheat and soybean (Toure et al., 1994; De Roo et al., 2001). The MIMICS offers an efficient way and is employed for different crop attenuation analysis in this section. However, to apply the MIMICS for attenuation

analysis, some assumptions are required for each crop as the MIMICS was originally developed for the forest areas. In the MIMICS model, the trunk height is considered much larger than the wavelength in order to simplify the computation of the trunk’s scattering matrix. However, in agricultural fields, particularly for corn, soybean and wheat where the stem heights are of the order of the wavelength for the C band earlier in the growing stage (Toure et al., 1994). Therefore, it is reasonable that we assume the wheat in tillering stage consisting of small-sized leaves without stems at the early growing stage from the end of April to the middle of May. The corn residue standing in the soybean field can be treated as a very dry primary branch with a vertical distribution without any leaves as they are slightly larger than the C-band radar wavelength. It should

also be noted that on June 21st, the soybean had emerged but very small and the effects on

the backscattering by their stems can be ignored. In terms of the corn, it can be assumed as consisting of a primary trunk with some broad leaves within its canopy. In addition, the soil conditions are also required in the MIMICS model. Although the soil conditions are variable in different fields, this chapter is focusing on the canopy attenuation analysis. Therefore, we treat all crop fields as having the same ground conditions. For attenuation analysis, crop parameters at different growing stages on May 5th 2014 (S01), May 18th 2014 (S02), June 4th 2014 (S03), and June 21st 2014 (S04) are listed in Table 4-7 with their corresponding two-way attenuation percentage shown in Figure 4-17. It should be noted that some parameters such as the height, leaf density and gravimetric moisture content are measured during field work while other parameters such as the dry density of leaf material or stem material are either referred to Toure et al. (1994) or using default values given by the MIMICS for a simple analysis. Finally, because the RADARSAT-2 data we adopted in this chapter has four modes with incidence angles of 24, 29, 35, and 39 degrees respectively, the analysis of the attenuation of these crops is performed on these four different incidence angles as the crop grows.

Table 4-7. Crop parameters of different agricultural fields in study area 2014 at different growing stages for C-band RADARSAT-2 data with its frequency of 5.405 GHZ. S01: 2014 May 5th; S02: 2014 May 18th; S03: 2014 June 04th; S04: 2014 June 21st.

Crops Structure Parameters S01 S02 S03 S04

Ground Soil Moisture (%) 20 RMS height (cm) 1 Correlation Length (cm) 10 Soil Texture Clay (%) 25 Sand (%) 15 Wheat Leaf

Gravimetric Moisture Content (%) 80 80 75 - Dry density of leaf material (0-1) 0.1 0.1 0.1 - Thickness (cm) 0.02 0.02 0.02 - Length (cm) 6 11 16 - Width (cm) 0.4 0.9 1.5 - Density (N/m3) 1000 0 7500 4500 - Canopy Thickness (m) 0.10 0.15 0.25 Corn Residue Stem

Gravimetric moisture content (%) 10 - - - Dry density of stem material (0-1) 0.3 - - -

Density (N/m3) 15 - - - Diameters (cm) 2.5 - - - Length (m) 0.4 - - - Canopy Thickness (m) 0.40 Corn Stem

Gravimetric moisture content (%) - - 70 70 Dry density of stem material (0-1) - - 0.3 0.3

Density (N/m3) - - 350 140

Diameters (cm) - - 0.5 0.8

Length (m) - - 0.02 0.05

Leaf

Gravimetric moisture content (%) - - 80 80 Dry density of leaf material (0-1) - - 0.1 0.1

Thickness (cm) - - 0.03 0.03 Length (cm) - - 6 15 Width (cm) - - 1.5 4 Density (N/m3) - - 170 120 Canopy Thickness (m) 0.08 0.25 Soybean Leaf

Gravimetric moisture content (%) - - - 80 Dry density of leaf material (0-1) - - - 0.1

Thickness (cm) - - - 0.05

Length (cm) - - - 3

Width (cm) - - - 2.5

Density (N/m3) - - - 360

(a) (b) (c) (d) (e) (f)

(g)

Figure 4-17. Two-way attenuation coefficients of different crops at different growing

stages under different incident angles. (a) wheat field on May 5th 2014 (b)

wheat field on May 18th 2014 (c) wheat field on June 04 2014 (d) soybean

field on May 5th (e) corn field on June 4th 2014 (f) corn field on June 21st 2014. (g) soybean field on June 21st.

Generally, in corn, soybean and wheat fields, as the incidence angle increases, the two- way attenuation becomes more significant as shown in Figure 4-17. This is because the large incidence angle increases the path length through the vegetation which will cause an increase in the extinction coefficient, which is composed of both absorption and scattering losses. In addition, Figure 4-17 shows that the two-way attenuation of wheat is not significant until June 4th with its two-way attenuation being approximately greater

than 30% for the V polarization and greater than 22% for the H polarization. On May 5th

and May 18th, both the H and V polarizations have small two-way attenuation

coefficients with values less than 4% and 15% respectively. For the corn residues left in the soybean field, they are very dry, with gravimetric water content being approximately 10%. This will cause a weak two-way attenuation with values less than 17% for V polarization and less than 15% for the H polarization when the incidence angle is less than 40 degrees. When the corn emerged on June 4th with a very few leaves and small stems, its two-way attenuation for the V polarization is less than 3% while for the H

polarization it is less than 1.5%. As the corn continues to grow until June 21st, the two-

polarization less than 13% and V polarization less than 20%. In terms of the soybean, on

June 21st only very small leaves were observed and their stems can be ignored. Hence, it

seems apparent for both H and V polarization with their two-way attenuation much less than 1%. The proposed model in this chapter is focusing on the crops at the early growing stage; hence, it is reasonable to ignore the attenuation caused by the crop canopy during the early growing stages.